Gurus, What are the limitations with audio-driving a 50MHz FM Transmit VCO directly? Sorry that's a really open-ended question. I suppose it will vary per each chan_ driver as well, as well as the emphasis settings and so on. So dealing with the issues of emphasis, compression, (audio)bandwidth limiting and so on - how much can be taken out of the radio and handled by the chan_ drivers, and how different are they each? It was suggested elsewhere, that everything over 12KHz be not permitted anywhere near an FM modulator stage, but how much does that actually happen with the CM119 chipset? I ask because on a transmit-only site, there hardly seems to be any point to having a "transceiver" there at all, when a clock generator, buffer, PA, and filtering, is a fairly simple unit. Cheers, Steve
If you are talking about connecting a fob/URI directly to a modulator input of a direct FM radio, then you can set up USBradio or maybe simpleusb, to set txprelim=yes then it will preemphasize and limit the audio. Not sure where your 12 KHz limitation is reference to but if you are looking to limit at 25 Khz(5KHz), then set TX audio with a 1K tone @ 3K deviation. If you intend to limit at 12.5 KHz(2.5KHz), then set deviation with a 1K tone with 1.5 KHz deviation. I have several radios I feed into direct mod with voice and tone. Jon VA3RQ On 5/31/2016 3:40 AM, Steve Wright wrote:
Gurus,
What are the limitations with audio-driving a 50MHz FM Transmit VCO directly?
Sorry that's a really open-ended question. I suppose it will vary per each chan_ driver as well, as well as the emphasis settings and so on.
So dealing with the issues of emphasis, compression, (audio)bandwidth limiting and so on - how much can be taken out of the radio and handled by the chan_ drivers, and how different are they each?
It was suggested elsewhere, that everything over 12KHz be not permitted anywhere near an FM modulator stage, but how much does that actually happen with the CM119 chipset?
I ask because on a transmit-only site, there hardly seems to be any point to having a "transceiver" there at all, when a clock generator, buffer, PA, and filtering, is a fairly simple unit.
Cheers, Steve
_______________________________________________ App_rpt-users mailing list App_rpt-users@ohnosec.org http://ohnosec.org/cgi-bin/mailman/listinfo/app_rpt-users
To unsubscribe from this list please visit http://ohnosec.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.
It was suggested elsewhere, that everything over 12KHz be not permitted anywhere near an FM modulator stage, but how much does that actually happen with the CM119 chipset? Simple - Sampling frequency.
While there are audio filters built into the audio chain of the DSP routines, this audio is in a digital form, and must be converted to analog before being applied to the modulator. This is done inside of the CM119's DAC/Codec. Unfortunately, along with the frequency/amplitude limited audio signals, the output of the CM119 contains the audio sampling frequency. If you look at the output of a "naked" CM119, it's easy to see this distortion, it looks like stair steps. Since these stair steps are square waves, they contain harmonics. It's these harmonics and sampling frequency that the audio filtering found on the RIM, RIM-Lite, and URI remove. These filters roll off the top-end frequency response and therefore remove the stair steps. The tricky part is to place the "knee" of the filter at a frequency that sufficiently rolls off the sampling frequency and harmonics, yet lets enough high-frequency voice audio though to achieve good fidelity. In the case of the RIM, this frequency is set for 10KHz. In teh case of the URI, it's MUCH lower. You may ask, "Why should I care if there are high-frequency audio components being fed directly to my modulator?" Well, *YOU* may not care, but your adjacent channel neighbor and other services sure will!! High frequency harmonics in your audio modulation chain create spurs and sideband noise in your transmitter. This is an issue even if you are deploying a "transmit only" site. While YOUR equipment at a particular site may be only a transmitter, someone else may be trying to use a receiver at that site. There are MUCH better explanations for the necessity of transmitter audio bandwith limiting found elsewhere on the web. The purpose of this explanation was to illustrate that when feeding a modulator DIRECTLY, you should try to keep your modulating audio clean to be a good neighbor. You don't have to worry about this when feeding audio into a radio ahead of the mic/audio processor since it takes care of all of this filtering for you. Scott Scott Zimmerman Amateur Radio Call N3XCC 474 Barnett Road Boswell, PA 15531 On 5/31/2016 3:40 AM, Steve Wright wrote:
Gurus,
What are the limitations with audio-driving a 50MHz FM Transmit VCO directly?
Sorry that's a really open-ended question. I suppose it will vary per each chan_ driver as well, as well as the emphasis settings and so on.
So dealing with the issues of emphasis, compression, (audio)bandwidth limiting and so on - how much can be taken out of the radio and handled by the chan_ drivers, and how different are they each?
It was suggested elsewhere, that everything over 12KHz be not permitted anywhere near an FM modulator stage, but how much does that actually happen with the CM119 chipset?
I ask because on a transmit-only site, there hardly seems to be any point to having a "transceiver" there at all, when a clock generator, buffer, PA, and filtering, is a fairly simple unit.
Cheers, Steve
_______________________________________________ App_rpt-users mailing list App_rpt-users@ohnosec.org http://ohnosec.org/cgi-bin/mailman/listinfo/app_rpt-users
To unsubscribe from this list please visit http://ohnosec.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.
Steve, Are you wanting to start from scratch building a transmitter, or start from an existing radio, trimming away unneeded modules? For VHF-LO FM, there is a overflow of dirt cheap surplus commercial hardware available worldwide. I highly recommend using surplus radios for several reasons. One of those reasons is if you have a hardware failure, you can have 10 spare 100 watt transmitters on the shelf waiting, rather than just being down while waiting for repairs. As for directly modulating a VCO, that is a complex subject, assuming you need to stay within frequency and bandwidth tolerances, at is typically required. If you have a lab full of test equipment available, and are willing to put in the time designing a proper PLL filter, pulling this off is very do-able, and many have. Only once the homebrew exciter is working properly with signals from a clean audio signal generator should you at attach it to AllStar. The output spectrum directly from a CM1xy, running at 48KHz sampling rate is pretty dirty. When you plug a headset into the CM1xy and listen to music, etc., you'll never notice this noise--the human ear isn't good enough! However, this noise will modulate a VCO, generating spurs and wide-band trash; hence the reason that URI and RIM hardware include audio LP filters at their outputs. For a review of the CM1xy output spectrum WITHOUT filtering, take a look at this review. Of particular interest are the SMPTE and CCIF IMD performance tests, where you'll see loud DAC "image" related spurs between 30KHz and 96KHz. There are also more harmonically-related spurs even further out, at higher frequencies, which can't be seen on these Spectrum Analyzer plots: http://nwavguy.blogspot.com/2011/11/c-media-cm119-dac.html Okay, now that the basic transmitter is working, fast forward to AllStar. You'll have to use the usbradio channel driver, since simpleusb doesn't implement an output level limiter and perhaps other needed features (like CTCSS tone generation). Once all the levels are set and checked out, and the exciter spectrum is clean, you should be ready. So, from my perspective, this is a basic take on what's needed. All do-able, if you've got hours and hours of time, determination, test equipment and the engineering skills needed. This is what the ham radio homebrewing spirit is about, of course, and I'm sure it would be a fun project. 73, David KB4FXC On Tue, 31 May 2016, Steve Wright wrote:
Gurus,
What are the limitations with audio-driving a 50MHz FM Transmit VCO directly?
Sorry that's a really open-ended question. I suppose it will vary per each chan_ driver as well, as well as the emphasis settings and so on.
So dealing with the issues of emphasis, compression, (audio)bandwidth limiting and so on - how much can be taken out of the radio and handled by the chan_ drivers, and how different are they each?
It was suggested elsewhere, that everything over 12KHz be not permitted anywhere near an FM modulator stage, but how much does that actually happen with the CM119 chipset?
I ask because on a transmit-only site, there hardly seems to be any point to having a "transceiver" there at all, when a clock generator, buffer, PA, and filtering, is a fairly simple unit.
Cheers, Steve
Assuming the VCO is part of a PLL, the PLL will tend to "correct" the modulation, especially those low frequency (example CTCSS and DCS signalling ) components that fall within the loop filter. Most modern land mobile transmitters have modulation compensation where a small amount of modulation is fed to the Reference Oscillator in order to fool it into ignoring the "error' it would see due to the modulation of the VCO. So to answer your question, you should insert modulation where it can feed both the VCO and the Reference Oscillator Modulation Compensation circuitry other wise the result will be distorted. The matters of preemphasis/deemphasis and channel BW modulation mask are another important set of topics and beg the question, why do you want to drive the VCO directly without employing the existing filtering? On 5/31/2016 3:40 AM, Steve Wright wrote:
Gurus,
What are the limitations with audio-driving a 50MHz FM Transmit VCO directly?
Sorry that's a really open-ended question. I suppose it will vary per each chan_ driver as well, as well as the emphasis settings and so on.
So dealing with the issues of emphasis, compression, (audio)bandwidth limiting and so on - how much can be taken out of the radio and handled by the chan_ drivers, and how different are they each?
It was suggested elsewhere, that everything over 12KHz be not permitted anywhere near an FM modulator stage, but how much does that actually happen with the CM119 chipset?
I ask because on a transmit-only site, there hardly seems to be any point to having a "transceiver" there at all, when a clock generator, buffer, PA, and filtering, is a fairly simple unit.
Cheers, Steve
_______________________________________________ App_rpt-users mailing list App_rpt-users@ohnosec.org http://ohnosec.org/cgi-bin/mailman/listinfo/app_rpt-users
To unsubscribe from this list please visit http://ohnosec.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 (5)
-
David McGough -
Joe Leikhim -
Jon Rorke -
Scott Zimmerman -
Steve Wright