Hi All, Thanks for all on the list who have given me a hand with getting the RTCM/voter boards running. I currently have 1/3 of my system installed at the sites. It's as follows: A TX only site - running a VOTER board, latest "Chuck" firmware. 9.6MHz OCXO replacing the onboard crystal and a 10MHz OCXO to keep the transmitter stable all on 53MHz. On my bench at home I have an identical setup except with a receiver in addition. Both radios are the same, audio injected in the same spot. I've been testing various frequency offsets to avoid the cancelling of the signal due to the inherent propagation effects at 6 metres. I've been able to run my bench radio into a dummy load and place it so that another radio nearby connected to an external antenna is receiving them almost the same (i.e. overlap). Currently 1-2Hz sounds the best. I have been encoding tone, however there is a very audible hum, as if the CTCSS tone from each transmitter is not in phase. The tone is 91.5Hz. I've checked/readjusted deviation levels etc, also checked the 9.6MHz OCXO etc but I cannot seem to null it out. I've also tested the Simulcast Launch Delay.The main site is about 13 miles away, so I put a value of 330 in there to represent a delay of 66uS on the bench test radio. This makes a little bit of improvement, but not an overall huge improvement. Does anyone have an ideas that I've missed or more information on the usage of the Simulcast Launch delay? Hayden
Wow; Great project. I suggest documenting everything and permanently posting for others to review the system design as a reference. My background is in Public Safety LMR design so I have dabbled a bit in VHF, UHF and 800 MHz simulcast both analog and digital. I might be able to do some computer modelling for you. Here is an example, picture two sites x1 and x2 spaced exactly 13 miles apart. First: They both need to launch the audio waveform at exactly the same time T=0 in same phase. This also means that there may need to be _bulk delay_ to ensure that the most distant (with respect to the IP back haul network) transmitter site, receives the entire data payload before it is transmitted. I am just learning about the RTCM so I believe that bulk delay is inherent to the process. Assuming both sites are exactly synced T=0, then where the wave fronts meet midway will be in exactly sync even though 35.1us (6.5 miles x 5.4 us = 35.1 us) has passed due to propagation delay. The wave fronts will also form a pair of theoretical opposing hyperbolic curves that meet at the center of the two sites like this: x1 )( x2 with the x representing each site. As a mobile travels away from center and along or behind the hyperbolic curves )(, the time differential will increase, creating time differential interference (TDI), or intersymbol interference (ISI) in a digital system. This can be calculated at any point location by using a map and a mileage scale. A mobile very close to or behind either site (paced 13 miles) will experience the delay of 70.2 us or more. This is somewhat mitigated by the capture effect of the closest site and the capture ratio afforded by the modulation index. TDI at 70.2 us is not too bad for analog FM. 135 us, the distortion might be intolerable. Adding a third site will require consideration that its signal may arrive earlier or later than the sites described above, In the event the third site is closer than 6.5 miles, the adjustment would be to increase the launch delay for the third site to meet with sites 1 and 2. If it is further than 6.5 miles, the delay of sites 1 and 2 could be increased equally by the amount required for site 3 to travel. A variation of this scheme, is one where a central site is predominant and is ringed by remote sites. In one configuration, the central site has an omni directional antenna and the remote sites have directional antennas facing outward. The central site would be timed to transmit first, and the remote sites would be delayed until the wavefront reaches them. If the distances are unequal, the remote sites would be timed +/- the time differential of the central site reaching them. There are a couple very proprietary computer programs for modeling this, however it can be worked out by hand. Some thoughts. 1) Don't worry about frequency offset until you are sure the timing and other parameters are correct. Set everything dead on with GPS to start. In commercial systems using GPS disciplined reference frequency offset is usually not performed. 2) Make 100% sure that the modulators are wired for the same polarity. It makes no sense to have one transmitter deviating +4 KHz while another is going -4KHz. Same goes for voice and CTCSS modulation. 3) Are your transmitters the exact same make/model and version? This can be a non starter. You can get away with different RF amps, filters etc, but if you have a Micor exciter and a Mastr II exciter it is going to be funky. 4) The audio chain needs to be identical. Any filters, amps etc, in the transmitter audio chain must be same at each site. 5) Adjust the modulation limiting such that the transmitter audio is never in limiting. For Part 97 work, you could get away with the limiter adjusted beyond system deviation and set the RTCM so that deviation is equal to 5 KHz. Voice deviation for each transmitter must be identical, same with CTCSS. For my commercial projects I recommend the entire simulcast system be staged in a warehouse so that all these parameters can be set at same time with same test equipment in a controlled environment. Any errors must track the same direction. You can do this remotely, but noisy signals will make the setting of values less than ideal. If you reach a level of frustration, bring it all home and tweak it. I hope this helps! On 3/13/2017 7:15 PM, Hayden Honeywood wrote:
Hi All, Thanks for all on the list who have given me a hand with getting the RTCM/voter boards running.
I currently have 1/3 of my system installed at the sites. It's as follows:
A TX only site - running a VOTER board, latest "Chuck" firmware. 9.6MHz OCXO replacing the onboard crystal and a 10MHz OCXO to keep the transmitter stable all on 53MHz.
On my bench at home I have an identical setup except with a receiver in addition. Both radios are the same, audio injected in the same spot.
I've been testing various frequency offsets to avoid the cancelling of the signal due to the inherent propagation effects at 6 metres. I've been able to run my bench radio into a dummy load and place it so that another radio nearby connected to an external antenna is receiving them almost the same (i.e. overlap).
Currently 1-2Hz sounds the best. I have been encoding tone, however there is a very audible hum, as if the CTCSS tone from each transmitter is not in phase. The tone is 91.5Hz. I've checked/readjusted deviation levels etc, also checked the 9.6MHz OCXO etc but I cannot seem to null it out.
I've also tested the Simulcast Launch Delay.The main site is about 13 miles away, so I put a value of 330 in there to represent a delay of 66uS on the bench test radio. This makes a little bit of improvement, but not an overall huge improvement. Does anyone have an ideas that I've missed or more information on the usage of the Simulcast Launch delay?
Hayden
_______________________________________________ App_rpt-users mailing list App_rpt-users@lists.allstarlink.org http://lists.allstarlink.org/cgi-bin/mailman/listinfo/app_rpt-users
To unsubscribe from this list please visit http://lists.allstarlink.org/cgi-bin/mailman/listinfo/app_rpt-users and scroll down to the bottom of the page. Enter your email address and press the "Unsubscribe or edit options button" You do not need a password to unsubscribe, you can do it via email confirmation. If you have trouble unsubscribing, please send a message to the list detailing the problem.
-- Joe Leikhim Leikhim and Associates Communications Consultants Oviedo, Florida JLeikhim@Leikhim.com 407-982-0446 WWW.LEIKHIM.COM
Hayden; How are you generating CTCSS tone? On 3/13/2017 9:41 PM, Joe Leikhim wrote:
Wow;
Great project. I suggest documenting everything and permanently posting for others to review the system design as a reference.
My background is in Public Safety LMR design so I have dabbled a bit in VHF, UHF and 800 MHz simulcast both analog and digital. I might be able to do some computer modelling for you.
Here is an example, picture two sites x1 and x2 spaced exactly 13 miles apart.
First: They both need to launch the audio waveform at exactly the same time T=0 in same phase. This also means that there may need to be _bulk delay_ to ensure that the most distant (with respect to the IP back haul network) transmitter site, receives the entire data payload before it is transmitted. I am just learning about the RTCM so I believe that bulk delay is inherent to the process.
Assuming both sites are exactly synced T=0, then where the wave fronts meet midway will be in exactly sync even though 35.1us (6.5 miles x 5.4 us = 35.1 us) has passed due to propagation delay.
The wave fronts will also form a pair of theoretical opposing hyperbolic curves that meet at the center of the two sites like this: x1 )( x2 with the x representing each site.
As a mobile travels away from center and along or behind the hyperbolic curves )(, the time differential will increase, creating time differential interference (TDI), or intersymbol interference (ISI) in a digital system. This can be calculated at any point location by using a map and a mileage scale.
A mobile very close to or behind either site (paced 13 miles) will experience the delay of 70.2 us or more. This is somewhat mitigated by the capture effect of the closest site and the capture ratio afforded by the modulation index. TDI at 70.2 us is not too bad for analog FM. 135 us, the distortion might be intolerable.
Adding a third site will require consideration that its signal may arrive earlier or later than the sites described above, In the event the third site is closer than 6.5 miles, the adjustment would be to increase the launch delay for the third site to meet with sites 1 and 2. If it is further than 6.5 miles, the delay of sites 1 and 2 could be increased equally by the amount required for site 3 to travel.
A variation of this scheme, is one where a central site is predominant and is ringed by remote sites. In one configuration, the central site has an omni directional antenna and the remote sites have directional antennas facing outward. The central site would be timed to transmit first, and the remote sites would be delayed until the wavefront reaches them. If the distances are unequal, the remote sites would be timed +/- the time differential of the central site reaching them.
There are a couple very proprietary computer programs for modeling this, however it can be worked out by hand.
Some thoughts.
1) Don't worry about frequency offset until you are sure the timing and other parameters are correct. Set everything dead on with GPS to start. In commercial systems using GPS disciplined reference frequency offset is usually not performed.
2) Make 100% sure that the modulators are wired for the same polarity. It makes no sense to have one transmitter deviating +4 KHz while another is going -4KHz. Same goes for voice and CTCSS modulation.
3) Are your transmitters the exact same make/model and version? This can be a non starter. You can get away with different RF amps, filters etc, but if you have a Micor exciter and a Mastr II exciter it is going to be funky.
4) The audio chain needs to be identical. Any filters, amps etc, in the transmitter audio chain must be same at each site.
5) Adjust the modulation limiting such that the transmitter audio is never in limiting. For Part 97 work, you could get away with the limiter adjusted beyond system deviation and set the RTCM so that deviation is equal to 5 KHz. Voice deviation for each transmitter must be identical, same with CTCSS.
For my commercial projects I recommend the entire simulcast system be staged in a warehouse so that all these parameters can be set at same time with same test equipment in a controlled environment. Any errors must track the same direction. You can do this remotely, but noisy signals will make the setting of values less than ideal. If you reach a level of frustration, bring it all home and tweak it.
I hope this helps!
On 3/13/2017 7:15 PM, Hayden Honeywood wrote:
Hi All, Thanks for all on the list who have given me a hand with getting the RTCM/voter boards running.
I currently have 1/3 of my system installed at the sites. It's as follows:
A TX only site - running a VOTER board, latest "Chuck" firmware. 9.6MHz OCXO replacing the onboard crystal and a 10MHz OCXO to keep the transmitter stable all on 53MHz.
On my bench at home I have an identical setup except with a receiver in addition. Both radios are the same, audio injected in the same spot.
I've been testing various frequency offsets to avoid the cancelling of the signal due to the inherent propagation effects at 6 metres. I've been able to run my bench radio into a dummy load and place it so that another radio nearby connected to an external antenna is receiving them almost the same (i.e. overlap).
Currently 1-2Hz sounds the best. I have been encoding tone, however there is a very audible hum, as if the CTCSS tone from each transmitter is not in phase. The tone is 91.5Hz. I've checked/readjusted deviation levels etc, also checked the 9.6MHz OCXO etc but I cannot seem to null it out.
I've also tested the Simulcast Launch Delay.The main site is about 13 miles away, so I put a value of 330 in there to represent a delay of 66uS on the bench test radio. This makes a little bit of improvement, but not an overall huge improvement. Does anyone have an ideas that I've missed or more information on the usage of the Simulcast Launch delay?
Hayden
_______________________________________________ App_rpt-users mailing list App_rpt-users@lists.allstarlink.org http://lists.allstarlink.org/cgi-bin/mailman/listinfo/app_rpt-users
To unsubscribe from this list please visithttp://lists.allstarlink.org/cgi-bin/mailman/listinfo/app_rpt-users and scroll down to the bottom of the page. Enter your email address and press the "Unsubscribe or edit options button" You do not need a password to unsubscribe, you can do it via email confirmation. If you have trouble unsubscribing, please send a message to the list detailing the problem.
-- Joe Leikhim
Leikhim and Associates
Communications Consultants
Oviedo, Florida
JLeikhim@Leikhim.com
407-982-0446
WWW.LEIKHIM.COM
_______________________________________________ App_rpt-users mailing list App_rpt-users@lists.allstarlink.org http://lists.allstarlink.org/cgi-bin/mailman/listinfo/app_rpt-users
To unsubscribe from this list please visit http://lists.allstarlink.org/cgi-bin/mailman/listinfo/app_rpt-users and scroll down to the bottom of the page. Enter your email address and press the "Unsubscribe or edit options button" You do not need a password to unsubscribe, you can do it via email confirmation. If you have trouble unsubscribing, please send a message to the list detailing the problem.
-- Joe Leikhim Leikhim and Associates Communications Consultants Oviedo, Florida JLeikhim@Leikhim.com 407-982-0446 WWW.LEIKHIM.COM
RTCM's need rebooting too for the delay to take effect. At least my RTCM's seemed to require it when I set them. Also when I was experimenting I set the launch delay on the RTCM under test to the propagation time from the remote side to the site I was at. Then I put a dummy load on it (which I could hear from ~20 ft away). I could enable the remote site and listen to the two signals, see how the sounded and make sure that the settings were doing what was expected. Once set properly I could run audio through the remote site and key/unkey the local site under test with no noticeable distortion. Of course this only works if theres a decent RF path from one site to the other. Cheers, Jesse
On Mar 13, 2017, at 6:41 PM, Joe Leikhim <rhyolite@leikhim.com> wrote:
Wow; Great project. I suggest documenting everything and permanently posting for others to review the system design as a reference.
My background is in Public Safety LMR design so I have dabbled a bit in VHF, UHF and 800 MHz simulcast both analog and digital. I might be able to do some computer modelling for you. Here is an example, picture two sites x1 and x2 spaced exactly 13 miles apart. First: They both need to launch the audio waveform at exactly the same time T=0 in same phase. This also means that there may need to be bulk delay to ensure that the most distant (with respect to the IP back haul network) transmitter site, receives the entire data payload before it is transmitted. I am just learning about the RTCM so I believe that bulk delay is inherent to the process. Assuming both sites are exactly synced T=0, then where the wave fronts meet midway will be in exactly sync even though 35.1us (6.5 miles x 5.4 us = 35.1 us) has passed due to propagation delay. The wave fronts will also form a pair of theoretical opposing hyperbolic curves that meet at the center of the two sites like this: x1 )( x2 with the x representing each site. As a mobile travels away from center and along or behind the hyperbolic curves )(, the time differential will increase, creating time differential interference (TDI), or intersymbol interference (ISI) in a digital system. This can be calculated at any point location by using a map and a mileage scale. A mobile very close to or behind either site (paced 13 miles) will experience the delay of 70.2 us or more. This is somewhat mitigated by the capture effect of the closest site and the capture ratio afforded by the modulation index. TDI at 70.2 us is not too bad for analog FM. 135 us, the distortion might be intolerable. Adding a third site will require consideration that its signal may arrive earlier or later than the sites described above, In the event the third site is closer than 6.5 miles, the adjustment would be to increase the launch delay for the third site to meet with sites 1 and 2. If it is further than 6.5 miles, the delay of sites 1 and 2 could be increased equally by the amount required for site 3 to travel.
A variation of this scheme, is one where a central site is predominant and is ringed by remote sites. In one configuration, the central site has an omni directional antenna and the remote sites have directional antennas facing outward. The central site would be timed to transmit first, and the remote sites would be delayed until the wavefront reaches them. If the distances are unequal, the remote sites would be timed +/- the time differential of the central site reaching them. There are a couple very proprietary computer programs for modeling this, however it can be worked out by hand.
Some thoughts.
1) Don't worry about frequency offset until you are sure the timing and other parameters are correct. Set everything dead on with GPS to start. In commercial systems using GPS disciplined reference frequency offset is usually not performed. 2) Make 100% sure that the modulators are wired for the same polarity. It makes no sense to have one transmitter deviating +4 KHz while another is going -4KHz. Same goes for voice and CTCSS modulation.
3) Are your transmitters the exact same make/model and version? This can be a non starter. You can get away with different RF amps, filters etc, but if you have a Micor exciter and a Mastr II exciter it is going to be funky.
4) The audio chain needs to be identical. Any filters, amps etc, in the transmitter audio chain must be same at each site. 5) Adjust the modulation limiting such that the transmitter audio is never in limiting. For Part 97 work, you could get away with the limiter adjusted beyond system deviation and set the RTCM so that deviation is equal to 5 KHz. Voice deviation for each transmitter must be identical, same with CTCSS. For my commercial projects I recommend the entire simulcast system be staged in a warehouse so that all these parameters can be set at same time with same test equipment in a controlled environment. Any errors must track the same direction. You can do this remotely, but noisy signals will make the setting of values less than ideal. If you reach a level of frustration, bring it all home and tweak it. I hope this helps!
On 3/13/2017 7:15 PM, Hayden Honeywood wrote: Hi All, Thanks for all on the list who have given me a hand with getting the RTCM/voter boards running.
I currently have 1/3 of my system installed at the sites. It's as follows:
A TX only site - running a VOTER board, latest "Chuck" firmware. 9.6MHz OCXO replacing the onboard crystal and a 10MHz OCXO to keep the transmitter stable all on 53MHz.
On my bench at home I have an identical setup except with a receiver in addition. Both radios are the same, audio injected in the same spot.
I've been testing various frequency offsets to avoid the cancelling of the signal due to the inherent propagation effects at 6 metres. I've been able to run my bench radio into a dummy load and place it so that another radio nearby connected to an external antenna is receiving them almost the same (i.e. overlap).
Currently 1-2Hz sounds the best. I have been encoding tone, however there is a very audible hum, as if the CTCSS tone from each transmitter is not in phase. The tone is 91.5Hz. I've checked/readjusted deviation levels etc, also checked the 9.6MHz OCXO etc but I cannot seem to null it out.
I've also tested the Simulcast Launch Delay.The main site is about 13 miles away, so I put a value of 330 in there to represent a delay of 66uS on the bench test radio. This makes a little bit of improvement, but not an overall huge improvement. Does anyone have an ideas that I've missed or more information on the usage of the Simulcast Launch delay?
Hayden
_______________________________________________ App_rpt-users mailing list App_rpt-users@lists.allstarlink.org http://lists.allstarlink.org/cgi-bin/mailman/listinfo/app_rpt-users
To unsubscribe from this list please visit http://lists.allstarlink.org/cgi-bin/mailman/listinfo/app_rpt-users and scroll down to the bottom of the page. Enter your email address and press the "Unsubscribe or edit options button" You do not need a password to unsubscribe, you can do it via email confirmation. If you have trouble unsubscribing, please send a message to the list detailing the problem.
-- Joe Leikhim
Leikhim and Associates
Communications Consultants
Oviedo, Florida
JLeikhim@Leikhim.com
407-982-0446
WWW.LEIKHIM.COM _______________________________________________ App_rpt-users mailing list App_rpt-users@lists.allstarlink.org http://lists.allstarlink.org/cgi-bin/mailman/listinfo/app_rpt-users
To unsubscribe from this list please visit http://lists.allstarlink.org/cgi-bin/mailman/listinfo/app_rpt-users and scroll down to the bottom of the page. Enter your email address and press the "Unsubscribe or edit options button" You do not need a password to unsubscribe, you can do it via email confirmation. If you have trouble unsubscribing, please send a message to the list detailing the problem.
Joe's statement about launch delay is critical and that unfortunately is the problem with RTCMs in simulcast. The problem is the DAC used in the RTCM does not start at the exact time every time. I remember Jim Dixon working on this issue and was unable to workaround the problem that is build into the silicon. The problem is particularly noticeable when humming tone on the TX. If you can get by with no PL and limit your coverage overlap you can simulcast with RTCM but it's never going to be perfect. On Mon, Mar 13, 2017 at 6:41 PM, Joe Leikhim <rhyolite@leikhim.com> wrote:
Wow;
Great project. I suggest documenting everything and permanently posting for others to review the system design as a reference.
My background is in Public Safety LMR design so I have dabbled a bit in VHF, UHF and 800 MHz simulcast both analog and digital. I might be able to do some computer modelling for you.
Here is an example, picture two sites x1 and x2 spaced exactly 13 miles apart.
First: They both need to launch the audio waveform at exactly the same time T=0 in same phase. This also means that there may need to be *bulk delay* to ensure that the most distant (with respect to the IP back haul network) transmitter site, receives the entire data payload before it is transmitted. I am just learning about the RTCM so I believe that bulk delay is inherent to the process.
Assuming both sites are exactly synced T=0, then where the wave fronts meet midway will be in exactly sync even though 35.1us (6.5 miles x 5.4 us = 35.1 us) has passed due to propagation delay.
The wave fronts will also form a pair of theoretical opposing hyperbolic curves that meet at the center of the two sites like this: x1 )( x2 with the x representing each site.
As a mobile travels away from center and along or behind the hyperbolic curves )(, the time differential will increase, creating time differential interference (TDI), or intersymbol interference (ISI) in a digital system. This can be calculated at any point location by using a map and a mileage scale.
A mobile very close to or behind either site (paced 13 miles) will experience the delay of 70.2 us or more. This is somewhat mitigated by the capture effect of the closest site and the capture ratio afforded by the modulation index. TDI at 70.2 us is not too bad for analog FM. 135 us, the distortion might be intolerable.
Adding a third site will require consideration that its signal may arrive earlier or later than the sites described above, In the event the third site is closer than 6.5 miles, the adjustment would be to increase the launch delay for the third site to meet with sites 1 and 2. If it is further than 6.5 miles, the delay of sites 1 and 2 could be increased equally by the amount required for site 3 to travel.
A variation of this scheme, is one where a central site is predominant and is ringed by remote sites. In one configuration, the central site has an omni directional antenna and the remote sites have directional antennas facing outward. The central site would be timed to transmit first, and the remote sites would be delayed until the wavefront reaches them. If the distances are unequal, the remote sites would be timed +/- the time differential of the central site reaching them.
There are a couple very proprietary computer programs for modeling this, however it can be worked out by hand.
Some thoughts.
1) Don't worry about frequency offset until you are sure the timing and other parameters are correct. Set everything dead on with GPS to start. In commercial systems using GPS disciplined reference frequency offset is usually not performed.
2) Make 100% sure that the modulators are wired for the same polarity. It makes no sense to have one transmitter deviating +4 KHz while another is going -4KHz. Same goes for voice and CTCSS modulation.
3) Are your transmitters the exact same make/model and version? This can be a non starter. You can get away with different RF amps, filters etc, but if you have a Micor exciter and a Mastr II exciter it is going to be funky.
4) The audio chain needs to be identical. Any filters, amps etc, in the transmitter audio chain must be same at each site.
5) Adjust the modulation limiting such that the transmitter audio is never in limiting. For Part 97 work, you could get away with the limiter adjusted beyond system deviation and set the RTCM so that deviation is equal to 5 KHz. Voice deviation for each transmitter must be identical, same with CTCSS.
For my commercial projects I recommend the entire simulcast system be staged in a warehouse so that all these parameters can be set at same time with same test equipment in a controlled environment. Any errors must track the same direction. You can do this remotely, but noisy signals will make the setting of values less than ideal. If you reach a level of frustration, bring it all home and tweak it. I hope this helps!
On 3/13/2017 7:15 PM, Hayden Honeywood wrote:
Hi All, Thanks for all on the list who have given me a hand with getting the RTCM/voter boards running.
I currently have 1/3 of my system installed at the sites. It's as follows:
A TX only site - running a VOTER board, latest "Chuck" firmware. 9.6MHz OCXO replacing the onboard crystal and a 10MHz OCXO to keep the transmitter stable all on 53MHz.
On my bench at home I have an identical setup except with a receiver in addition. Both radios are the same, audio injected in the same spot.
I've been testing various frequency offsets to avoid the cancelling of the signal due to the inherent propagation effects at 6 metres. I've been able to run my bench radio into a dummy load and place it so that another radio nearby connected to an external antenna is receiving them almost the same (i.e. overlap).
Currently 1-2Hz sounds the best. I have been encoding tone, however there is a very audible hum, as if the CTCSS tone from each transmitter is not in phase. The tone is 91.5Hz. I've checked/readjusted deviation levels etc, also checked the 9.6MHz OCXO etc but I cannot seem to null it out.
I've also tested the Simulcast Launch Delay.The main site is about 13 miles away, so I put a value of 330 in there to represent a delay of 66uS on the bench test radio. This makes a little bit of improvement, but not an overall huge improvement. Does anyone have an ideas that I've missed or more information on the usage of the Simulcast Launch delay?
Hayden
_______________________________________________ App_rpt-users mailing listApp_rpt-users@lists.allstarlink.orghttp://lists.allstarlink.org/cgi-bin/mailman/listinfo/app_rpt-users
To unsubscribe from this list please visit http://lists.allstarlink.org/cgi-bin/mailman/listinfo/app_rpt-users and scroll down to the bottom of the page. Enter your email address and press the "Unsubscribe or edit options button" You do not need a password to unsubscribe, you can do it via email confirmation. If you have trouble unsubscribing, please send a message to the list detailing the problem.
-- Joe Leikhim
Leikhim and Associates
Communications Consultants
Oviedo, Florida JLeikhim@Leikhim.com 407-982-0446 <(407)%20982-0446> WWW.LEIKHIM.COM
_______________________________________________ App_rpt-users mailing list App_rpt-users@lists.allstarlink.org http://lists.allstarlink.org/cgi-bin/mailman/listinfo/app_rpt-users
To unsubscribe from this list please visit http://lists.allstarlink.org/ cgi-bin/mailman/listinfo/app_rpt-users and scroll down to the bottom of the page. Enter your email address and press the "Unsubscribe or edit options button" You do not need a password to unsubscribe, you can do it via email confirmation. If you have trouble unsubscribing, please send a message to the list detailing the problem.
-- -- Tim
Thanks Tim! So the CTCSS is starting out of phase. Does this affect voice audio and tones in that bandwidth as well? I am a long time lurker, wishing to put this technology to use someday. I was saddened to learn that Jim Dixon passed away. I hope someone can take the torch and further develop this promising product. As you probably know commercial simulcast is very expensive to deploy and for a single channel system the cost is ridiculous because commercial hardware is scaled for larger networks. So this could have applications in commercial applications as well. Joe On 3/13/2017 11:01 PM, Tim Sawyer wrote:
Joe's statement about launch delay is critical and that unfortunately is the problem with RTCMs in simulcast. The problem is the DAC used in the RTCM does not start at the exact time every time. I remember Jim Dixon working on this issue and was unable to workaround the problem that is build into the silicon. The problem is particularly noticeable when humming tone on the TX. If you can get by with no PL and limit your coverage overlap you can simulcast with RTCM but it's never going to be perfect.
On Mon, Mar 13, 2017 at 6:41 PM, Joe Leikhim <rhyolite@leikhim.com <mailto:rhyolite@leikhim.com>> wrote:
Wow;
Great project. I suggest documenting everything and permanently posting for others to review the system design as a reference.
My background is in Public Safety LMR design so I have dabbled a bit in VHF, UHF and 800 MHz simulcast both analog and digital. I might be able to do some computer modelling for you.
Here is an example, picture two sites x1 and x2 spaced exactly 13 miles apart.
First: They both need to launch the audio waveform at exactly the same time T=0 in same phase. This also means that there may need to be _bulk delay_ to ensure that the most distant (with respect to the IP back haul network) transmitter site, receives the entire data payload before it is transmitted. I am just learning about the RTCM so I believe that bulk delay is inherent to the process.
Assuming both sites are exactly synced T=0, then where the wave fronts meet midway will be in exactly sync even though 35.1us (6.5 miles x 5.4 us = 35.1 us) has passed due to propagation delay.
The wave fronts will also form a pair of theoretical opposing hyperbolic curves that meet at the center of the two sites like this: x1 )( x2 with the x representing each site.
As a mobile travels away from center and along or behind the hyperbolic curves )(, the time differential will increase, creating time differential interference (TDI), or intersymbol interference (ISI) in a digital system. This can be calculated at any point location by using a map and a mileage scale.
A mobile very close to or behind either site (paced 13 miles) will experience the delay of 70.2 us or more. This is somewhat mitigated by the capture effect of the closest site and the capture ratio afforded by the modulation index. TDI at 70.2 us is not too bad for analog FM. 135 us, the distortion might be intolerable.
Adding a third site will require consideration that its signal may arrive earlier or later than the sites described above, In the event the third site is closer than 6.5 miles, the adjustment would be to increase the launch delay for the third site to meet with sites 1 and 2. If it is further than 6.5 miles, the delay of sites 1 and 2 could be increased equally by the amount required for site 3 to travel.
A variation of this scheme, is one where a central site is predominant and is ringed by remote sites. In one configuration, the central site has an omni directional antenna and the remote sites have directional antennas facing outward. The central site would be timed to transmit first, and the remote sites would be delayed until the wavefront reaches them. If the distances are unequal, the remote sites would be timed +/- the time differential of the central site reaching them.
There are a couple very proprietary computer programs for modeling this, however it can be worked out by hand.
Some thoughts.
1) Don't worry about frequency offset until you are sure the timing and other parameters are correct. Set everything dead on with GPS to start. In commercial systems using GPS disciplined reference frequency offset is usually not performed.
2) Make 100% sure that the modulators are wired for the same polarity. It makes no sense to have one transmitter deviating +4 KHz while another is going -4KHz. Same goes for voice and CTCSS modulation.
3) Are your transmitters the exact same make/model and version? This can be a non starter. You can get away with different RF amps, filters etc, but if you have a Micor exciter and a Mastr II exciter it is going to be funky.
4) The audio chain needs to be identical. Any filters, amps etc, in the transmitter audio chain must be same at each site.
5) Adjust the modulation limiting such that the transmitter audio is never in limiting. For Part 97 work, you could get away with the limiter adjusted beyond system deviation and set the RTCM so that deviation is equal to 5 KHz. Voice deviation for each transmitter must be identical, same with CTCSS.
For my commercial projects I recommend the entire simulcast system be staged in a warehouse so that all these parameters can be set at same time with same test equipment in a controlled environment. Any errors must track the same direction. You can do this remotely, but noisy signals will make the setting of values less than ideal. If you reach a level of frustration, bring it all home and tweak it.
I hope this helps!
On 3/13/2017 7:15 PM, Hayden Honeywood wrote:
Hi All, Thanks for all on the list who have given me a hand with getting the RTCM/voter boards running.
I currently have 1/3 of my system installed at the sites. It's as follows:
A TX only site - running a VOTER board, latest "Chuck" firmware. 9.6MHz OCXO replacing the onboard crystal and a 10MHz OCXO to keep the transmitter stable all on 53MHz.
On my bench at home I have an identical setup except with a receiver in addition. Both radios are the same, audio injected in the same spot.
I've been testing various frequency offsets to avoid the cancelling of the signal due to the inherent propagation effects at 6 metres. I've been able to run my bench radio into a dummy load and place it so that another radio nearby connected to an external antenna is receiving them almost the same (i.e. overlap).
Currently 1-2Hz sounds the best. I have been encoding tone, however there is a very audible hum, as if the CTCSS tone from each transmitter is not in phase. The tone is 91.5Hz. I've checked/readjusted deviation levels etc, also checked the 9.6MHz OCXO etc but I cannot seem to null it out.
I've also tested the Simulcast Launch Delay.The main site is about 13 miles away, so I put a value of 330 in there to represent a delay of 66uS on the bench test radio. This makes a little bit of improvement, but not an overall huge improvement. Does anyone have an ideas that I've missed or more information on the usage of the Simulcast Launch delay?
Hayden
_______________________________________________ App_rpt-users mailing list App_rpt-users@lists.allstarlink.org <mailto:App_rpt-users@lists.allstarlink.org> http://lists.allstarlink.org/cgi-bin/mailman/listinfo/app_rpt-users <http://lists.allstarlink.org/cgi-bin/mailman/listinfo/app_rpt-users>
To unsubscribe from this list please visithttp://lists.allstarlink.org/cgi-bin/mailman/listinfo/app_rpt-users <http://lists.allstarlink.org/cgi-bin/mailman/listinfo/app_rpt-users> and scroll down to the bottom of the page. Enter your email address and press the "Unsubscribe or edit options button" You do not need a password to unsubscribe, you can do it via email confirmation. If you have trouble unsubscribing, please send a message to the list detailing the problem.
-- Joe Leikhim
Leikhim and Associates
Communications Consultants
Oviedo, Florida
JLeikhim@Leikhim.com <mailto:JLeikhim@Leikhim.com>
407-982-0446 <tel:%28407%29%20982-0446>
WWW.LEIKHIM.COM <http://WWW.LEIKHIM.COM>
_______________________________________________ App_rpt-users mailing list App_rpt-users@lists.allstarlink.org <mailto:App_rpt-users@lists.allstarlink.org> http://lists.allstarlink.org/cgi-bin/mailman/listinfo/app_rpt-users <http://lists.allstarlink.org/cgi-bin/mailman/listinfo/app_rpt-users> To unsubscribe from this list please visit http://lists.allstarlink.org/cgi-bin/mailman/listinfo/app_rpt-users <http://lists.allstarlink.org/cgi-bin/mailman/listinfo/app_rpt-users> and scroll down to the bottom of the page. Enter your email address and press the "Unsubscribe or edit options button" You do not need a password to unsubscribe, you can do it via email confirmation. If you have trouble unsubscribing, please send a message to the list detailing the problem.
-- -- Tim
_______________________________________________ App_rpt-users mailing list App_rpt-users@lists.allstarlink.org http://lists.allstarlink.org/cgi-bin/mailman/listinfo/app_rpt-users
To unsubscribe from this list please visit http://lists.allstarlink.org/cgi-bin/mailman/listinfo/app_rpt-users and scroll down to the bottom of the page. Enter your email address and press the "Unsubscribe or edit options button" You do not need a password to unsubscribe, you can do it via email confirmation. If you have trouble unsubscribing, please send a message to the list detailing the problem. -- Joe Leikhim
Leikhim and Associates Communications Consultants Oviedo, Florida JLeikhim@Leikhim.com 407-982-0446 WWW.LEIKHIM.COM
I have two sites on UHF 18 miles apart simulcasting with the RTCM's. use the 9.6 MHz OCXO's for the clock in the RTCM improves their simulcast usability greatly but I do have some CTCSS wobble in some areas between the sites. It does not cause any issues with the intelligibility of the audio but you can hear it is there in non capture overlap areas. This is the first I am hearing of the DAC issue which certainly explains it. I tailored the system for minimum overlap and my CTCSS tones are deviating at 400 Hz to further reduce the audio of the CTCSS wobble. 400 Hz is more than enough CTCSS level for any modern radio to decode almost right down to the noise. Anyway this is what I have done and so far its been working very well. On Mon, Mar 13, 2017 at 11:15 PM, Joe Leikhim <rhyolite@leikhim.com> wrote:
Thanks Tim!
So the CTCSS is starting out of phase. Does this affect voice audio and tones in that bandwidth as well?
I am a long time lurker, wishing to put this technology to use someday. I was saddened to learn that Jim Dixon passed away. I hope someone can take the torch and further develop this promising product.
As you probably know commercial simulcast is very expensive to deploy and for a single channel system the cost is ridiculous because commercial hardware is scaled for larger networks. So this could have applications in commercial applications as well.
Joe
On 3/13/2017 11:01 PM, Tim Sawyer wrote:
Joe's statement about launch delay is critical and that unfortunately is the problem with RTCMs in simulcast. The problem is the DAC used in the RTCM does not start at the exact time every time. I remember Jim Dixon working on this issue and was unable to workaround the problem that is build into the silicon. The problem is particularly noticeable when humming tone on the TX. If you can get by with no PL and limit your coverage overlap you can simulcast with RTCM but it's never going to be perfect.
On Mon, Mar 13, 2017 at 6:41 PM, Joe Leikhim <rhyolite@leikhim.com> wrote:
Wow;
Great project. I suggest documenting everything and permanently posting for others to review the system design as a reference.
My background is in Public Safety LMR design so I have dabbled a bit in VHF, UHF and 800 MHz simulcast both analog and digital. I might be able to do some computer modelling for you.
Here is an example, picture two sites x1 and x2 spaced exactly 13 miles apart.
First: They both need to launch the audio waveform at exactly the same time T=0 in same phase. This also means that there may need to be *bulk delay* to ensure that the most distant (with respect to the IP back haul network) transmitter site, receives the entire data payload before it is transmitted. I am just learning about the RTCM so I believe that bulk delay is inherent to the process.
Assuming both sites are exactly synced T=0, then where the wave fronts meet midway will be in exactly sync even though 35.1us (6.5 miles x 5.4 us = 35.1 us) has passed due to propagation delay.
The wave fronts will also form a pair of theoretical opposing hyperbolic curves that meet at the center of the two sites like this: x1 )( x2 with the x representing each site.
As a mobile travels away from center and along or behind the hyperbolic curves )(, the time differential will increase, creating time differential interference (TDI), or intersymbol interference (ISI) in a digital system. This can be calculated at any point location by using a map and a mileage scale.
A mobile very close to or behind either site (paced 13 miles) will experience the delay of 70.2 us or more. This is somewhat mitigated by the capture effect of the closest site and the capture ratio afforded by the modulation index. TDI at 70.2 us is not too bad for analog FM. 135 us, the distortion might be intolerable.
Adding a third site will require consideration that its signal may arrive earlier or later than the sites described above, In the event the third site is closer than 6.5 miles, the adjustment would be to increase the launch delay for the third site to meet with sites 1 and 2. If it is further than 6.5 miles, the delay of sites 1 and 2 could be increased equally by the amount required for site 3 to travel.
A variation of this scheme, is one where a central site is predominant and is ringed by remote sites. In one configuration, the central site has an omni directional antenna and the remote sites have directional antennas facing outward. The central site would be timed to transmit first, and the remote sites would be delayed until the wavefront reaches them. If the distances are unequal, the remote sites would be timed +/- the time differential of the central site reaching them.
There are a couple very proprietary computer programs for modeling this, however it can be worked out by hand.
Some thoughts.
1) Don't worry about frequency offset until you are sure the timing and other parameters are correct. Set everything dead on with GPS to start. In commercial systems using GPS disciplined reference frequency offset is usually not performed.
2) Make 100% sure that the modulators are wired for the same polarity. It makes no sense to have one transmitter deviating +4 KHz while another is going -4KHz. Same goes for voice and CTCSS modulation.
3) Are your transmitters the exact same make/model and version? This can be a non starter. You can get away with different RF amps, filters etc, but if you have a Micor exciter and a Mastr II exciter it is going to be funky.
4) The audio chain needs to be identical. Any filters, amps etc, in the transmitter audio chain must be same at each site.
5) Adjust the modulation limiting such that the transmitter audio is never in limiting. For Part 97 work, you could get away with the limiter adjusted beyond system deviation and set the RTCM so that deviation is equal to 5 KHz. Voice deviation for each transmitter must be identical, same with CTCSS.
For my commercial projects I recommend the entire simulcast system be staged in a warehouse so that all these parameters can be set at same time with same test equipment in a controlled environment. Any errors must track the same direction. You can do this remotely, but noisy signals will make the setting of values less than ideal. If you reach a level of frustration, bring it all home and tweak it. I hope this helps!
On 3/13/2017 7:15 PM, Hayden Honeywood wrote:
Hi All, Thanks for all on the list who have given me a hand with getting the RTCM/voter boards running.
I currently have 1/3 of my system installed at the sites. It's as follows:
A TX only site - running a VOTER board, latest "Chuck" firmware. 9.6MHz OCXO replacing the onboard crystal and a 10MHz OCXO to keep the transmitter stable all on 53MHz.
On my bench at home I have an identical setup except with a receiver in addition. Both radios are the same, audio injected in the same spot.
I've been testing various frequency offsets to avoid the cancelling of the signal due to the inherent propagation effects at 6 metres. I've been able to run my bench radio into a dummy load and place it so that another radio nearby connected to an external antenna is receiving them almost the same (i.e. overlap).
Currently 1-2Hz sounds the best. I have been encoding tone, however there is a very audible hum, as if the CTCSS tone from each transmitter is not in phase. The tone is 91.5Hz. I've checked/readjusted deviation levels etc, also checked the 9.6MHz OCXO etc but I cannot seem to null it out.
I've also tested the Simulcast Launch Delay.The main site is about 13 miles away, so I put a value of 330 in there to represent a delay of 66uS on the bench test radio. This makes a little bit of improvement, but not an overall huge improvement. Does anyone have an ideas that I've missed or more information on the usage of the Simulcast Launch delay?
Hayden
_______________________________________________ App_rpt-users mailing listApp_rpt-users@lists.allstarlink.orghttp://lists.allstarlink.org/cgi-bin/mailman/listinfo/app_rpt-users
To unsubscribe from this list please visit http://lists.allstarlink.org/cgi-bin/mailman/listinfo/app_rpt-users and scroll down to the bottom of the page. Enter your email address and press the "Unsubscribe or edit options button" You do not need a password to unsubscribe, you can do it via email confirmation. If you have trouble unsubscribing, please send a message to the list detailing the problem.
-- Joe Leikhim
Leikhim and Associates
Communications Consultants
Oviedo, Florida JLeikhim@Leikhim.com 407-982-0446 <%28407%29%20982-0446> WWW.LEIKHIM.COM
_______________________________________________ App_rpt-users mailing list App_rpt-users@lists.allstarlink.org http://lists.allstarlink.org/c gi-bin/mailman/listinfo/app_rpt-users To unsubscribe from this list please visit http://lists.allstarlink.org/cgi-bin/mailman/listinfo/app_rp t-users and scroll down to the bottom of the page. Enter your email address and press the "Unsubscribe or edit options button" You do not need a password to unsubscribe, you can do it via email confirmation. If you have trouble unsubscribing, please send a message to the list detailing the problem.
-- -- Tim
_______________________________________________ App_rpt-users mailing listApp_rpt-users@lists.allstarlink.orghttp://lists.allstarlink.org/cgi-bin/mailman/listinfo/app_rpt-users
To unsubscribe from this list please visit http://lists.allstarlink.org/cgi-bin/mailman/listinfo/app_rpt-users and scroll down to the bottom of the page. Enter your email address and press the "Unsubscribe or edit options button" You do not need a password to unsubscribe, you can do it via email confirmation. If you have trouble unsubscribing, please send a message to the list detailing the problem.
-- Joe Leikhim
Leikhim and Associates
Communications Consultants
Oviedo, Florida JLeikhim@Leikhim.com 407-982-0446 <(407)%20982-0446> WWW.LEIKHIM.COM
_______________________________________________ App_rpt-users mailing list App_rpt-users@lists.allstarlink.org http://lists.allstarlink.org/cgi-bin/mailman/listinfo/app_rpt-users
To unsubscribe from this list please visit http://lists.allstarlink.org/ cgi-bin/mailman/listinfo/app_rpt-users and scroll down to the bottom of the page. Enter your email address and press the "Unsubscribe or edit options button" You do not need a password to unsubscribe, you can do it via email confirmation. If you have trouble unsubscribing, please send a message to the list detailing the problem.
First: They both need to launch the audio waveform at exactly the same time T=0 in same phase. This also means that there may need to be bulk delay to ensure that the most distant (with respect to the IP back haul network) transmitter site, receives the entire data payload before it is transmitted. I am just learning about the RTCM so I believe that bulk delay is inherent to the process.
Assuming both sites are exactly synced T=0, then where the wave fronts meet midway will be in exactly sync even though 35.1us (6.5 miles x 5.4 us = 35.1 us) has passed due to propagation delay.
That's not correct (the part about "the wave fronts"). Simulcast audio/PL launch delay only guarantees AF synchronicity to the extent practical/possible (e.g. within a few microseconds of each other), but certainly not at the same RF phase. A microsecond at 150 MHz is 150 cycles.
The wave fronts will also form a pair of theoretical opposing hyperbolic curves that meet at the center of the two sites like this: x1 )( x2 with the x representing each site.
Again, you can't look at it in the RF domain; AF sync is what you're working hard to achieve; you'll never get it at RF. Also keep in mind that sync at the midpoint isn't necessarily the goal. In a perfect world with no terrain, identical antenna patterns, identical ERP, etc. it might be, but not in the real world. You need to do coverage models for each of the sites (using actual ERP's and antenna patterns), see where the most-problematic areas are (i.e. the areas where capture effect isn't going to hide potential sins), and make decisions as to where to optimize and where to tolerate degradation, and then set delays accordingly.
1) Don't worry about frequency offset until you are sure the timing and other parameters are correct.
Agree. Until the delays are correct (and again, that doesn't necessarily mean equal at the midpoint), there's no sense in adding another variable to the equation. Offsetting carrier frequency only helps mitigate cancellation nulls, it does nothing to mitigate delay errors. --- Jeff WN3A --- This email has been checked for viruses by Avast antivirus software. https://www.avast.com/antivirus
Jeff is correct for pointing out the need for coverage modeling. This saves a great deal of time, effort and guess work. Terrain makes a huge difference when it comes to where you have overlap and also where you have overlap and lack capture of any one transmitter site. One thing many people get wrong with simulcast is that more power can be detrimental to achieving your goal. Modeling the coverage from each site and choosing the particular antenna and pattern for where you want to cover in respect to the other transmitters is key. In my humble opinion and it has worked for me, my main goal when creating a simulcast system is not how to get the audio sounding good in overlap but how to reduce the overlap areas with no capture to a minimum. This has to do mainly with the terrain causing issues where you audio is not arriving to the field unit(s) with the calculated delay due to anomalies in the terrain. You also need to pay attention to particular antenna characteristics such as gain, electrical down tilt, mechanical down tilt and so on. I have found normally the site within a simulcast system that has the greatest coverage is often the site with one of the lowest ERP's. Once the RF coverage side of the house is right, the rest is adjustable. This is coming from a commercial simulcast standpoint. On Tue, Mar 14, 2017 at 12:34 AM, Jeff DePolo <jd0@broadsci.com> wrote:
First: They both need to launch the audio waveform at exactly the same time T=0 in same phase. This also means that there may need to be bulk delay to ensure that the most distant (with respect to the IP back haul network) transmitter site, receives the entire data payload before it is transmitted. I am just learning about the RTCM so I believe that bulk delay is inherent to the process.
Assuming both sites are exactly synced T=0, then where the wave fronts meet midway will be in exactly sync even though 35.1us (6.5 miles x 5.4 us = 35.1 us) has passed due to propagation delay.
That's not correct (the part about "the wave fronts"). Simulcast audio/PL launch delay only guarantees AF synchronicity to the extent practical/possible (e.g. within a few microseconds of each other), but certainly not at the same RF phase. A microsecond at 150 MHz is 150 cycles.
The wave fronts will also form a pair of theoretical opposing hyperbolic curves that meet at the center of the two sites like this: x1 )( x2 with the x representing each site.
Again, you can't look at it in the RF domain; AF sync is what you're working hard to achieve; you'll never get it at RF.
Also keep in mind that sync at the midpoint isn't necessarily the goal. In a perfect world with no terrain, identical antenna patterns, identical ERP, etc. it might be, but not in the real world. You need to do coverage models for each of the sites (using actual ERP's and antenna patterns), see where the most-problematic areas are (i.e. the areas where capture effect isn't going to hide potential sins), and make decisions as to where to optimize and where to tolerate degradation, and then set delays accordingly.
1) Don't worry about frequency offset until you are sure the timing and other parameters are correct.
Agree. Until the delays are correct (and again, that doesn't necessarily mean equal at the midpoint), there's no sense in adding another variable to the equation. Offsetting carrier frequency only helps mitigate cancellation nulls, it does nothing to mitigate delay errors.
--- Jeff WN3A
--- This email has been checked for viruses by Avast antivirus software. https://www.avast.com/antivirus
_______________________________________________ App_rpt-users mailing list App_rpt-users@lists.allstarlink.org http://lists.allstarlink.org/cgi-bin/mailman/listinfo/app_rpt-users
To unsubscribe from this list please visit http://lists.allstarlink.org/ cgi-bin/mailman/listinfo/app_rpt-users and scroll down to the bottom of the page. Enter your email address and press the "Unsubscribe or edit options button" You do not need a password to unsubscribe, you can do it via email confirmation. If you have trouble unsubscribing, please send a message to the list detailing the problem.
Jeff; Yes True; It is indeed the _audio phasing_ that needs to be synced at midpoint. I was trying to state in as few words as possible a very complex topic. And yes, in the "real world" the reception will have exaggerated multi-path effects due to the frequency error which are unavoidable. I worked on some of the early crude analog 800 MHz FM simulcast and my ears hurt listening to them. Going from two transmitters to three or four (or more) becomes very complex in short order. I have used computer software to simulate all the effects to timing offset signal strength (reliability) and signal capture. EIA/TIA TSB-88D describes monte carlo power weighted methodology used in the computer models. Some software models can do automatic iterations of timing to choose best fit. Joe On 3/14/2017 12:34 AM, Jeff DePolo wrote:
First: They both need to launch the audio waveform at exactly the same time T=0 in same phase. This also means that there may need to be bulk delay to ensure that the most distant (with respect to the IP back haul network) transmitter site, receives the entire data payload before it is transmitted. I am just learning about the RTCM so I believe that bulk delay is inherent to the process.
Assuming both sites are exactly synced T=0, then where the wave fronts meet midway will be in exactly sync even though 35.1us (6.5 miles x 5.4 us = 35.1 us) has passed due to propagation delay. That's not correct (the part about "the wave fronts"). Simulcast audio/PL launch delay only guarantees AF synchronicity to the extent practical/possible (e.g. within a few microseconds of each other), but certainly not at the same RF phase. A microsecond at 150 MHz is 150 cycles.
The wave fronts will also form a pair of theoretical opposing hyperbolic curves that meet at the center of the two sites like this: x1 )( x2 with the x representing each site. Again, you can't look at it in the RF domain; AF sync is what you're working hard to achieve; you'll never get it at RF.
Also keep in mind that sync at the midpoint isn't necessarily the goal. In a perfect world with no terrain, identical antenna patterns, identical ERP, etc. it might be, but not in the real world. You need to do coverage models for each of the sites (using actual ERP's and antenna patterns), see where the most-problematic areas are (i.e. the areas where capture effect isn't going to hide potential sins), and make decisions as to where to optimize and where to tolerate degradation, and then set delays accordingly.
1) Don't worry about frequency offset until you are sure the timing and other parameters are correct. Agree. Until the delays are correct (and again, that doesn't necessarily mean equal at the midpoint), there's no sense in adding another variable to the equation. Offsetting carrier frequency only helps mitigate cancellation nulls, it does nothing to mitigate delay errors.
--- Jeff WN3A
--- This email has been checked for viruses by Avast antivirus software. https://www.avast.com/antivirus
_______________________________________________ App_rpt-users mailing list App_rpt-users@lists.allstarlink.org http://lists.allstarlink.org/cgi-bin/mailman/listinfo/app_rpt-users
To unsubscribe from this list please visit http://lists.allstarlink.org/cgi-bin/mailman/listinfo/app_rpt-users and scroll down to the bottom of the page. Enter your email address and press the "Unsubscribe or edit options button" You do not need a password to unsubscribe, you can do it via email confirmation. If you have trouble unsubscribing, please send a message to the list detailing the problem.
-- Joe Leikhim Leikhim and Associates Communications Consultants Oviedo, Florida JLeikhim@Leikhim.com 407-982-0446 WWW.LEIKHIM.COM
participants (6)
-
Hayden Honeywood -
Jeff DePolo -
Jesse Lloyd -
Joe Leikhim -
Joe Moskalski -
Tim Sawyer