[App_rpt-users] Simulcasting with RTCM/voter

[Joe Leikhim]

Hayden;

Your timing for bench testing, by delaying the transmitter on the bench 
to match the 13 mile distant TX makes perfect sense from the perspective 
of testing. Not sure what model repeater station you have but it sounds 
like you have modded the VCO significantly so comparing frequency 
response of all the exciters may be in order. Also some transmitters 
have a two port modulation scheme so that CTCSS tones do not get 
cancelled by the VCO PLL loop correction. Your CTCSS shortfall may be 
indicative of the PLL loop trying to cancel the CTCSS waveform.

 From Tim's comments about the DAC phase problem,  there may be factors 
outside your control, in which case using directional antennas on the 
remote TX's may be a solution, but not ideal, especially at 6M. If this 
problem were entirely limited to the CTCSS tone, I would suggest 
generating CTCSS locally at each site from a GPS locked generator. 
However that might require a separate delay block for CTCSS, where delay 
is required. If the DAC affects voice as well, I wouldn't bother with 
that solution.

Personally I hope the developers get the bugs worked out as I see some 
commercial potential for the RTCM.

Joe

On 3/13/2017 11:02 PM, Hayden Honeywood wrote:
> Thanks for the information Joe.
>
> The RTCM generates the CTCSS in the main audio path with an entry in 
> voter.conf.
>
> There is no separate deviation adjustment on the board, you set a 
> global level in that config file.
> OK on the delays between sites etc. I think I understand whats going 
> on there. As I mentioned beforehand I think a lot of my issue is that 
> the 2nd site radio is on the bench and the delay from the main site TX 
> is causing distortion problems. I'm hoping to document a lot of the 
> simulcast information on my blog in time.
>
> In answer to some of your points -
>
> 1) I've played with the offset mainly because at low band 50MHz due to 
> the propagation effects, complete cancellations happen in overlap when 
> both signals from the transmitters are approx the same. Whereas on UHF 
> I believe this is not to much of an issue due to reflections, doppler 
> etc. Only really an issue with fixed stations I believe. Offsetting by 
> a few Hz I can hear the null move around, and cancellation is not so bad.
>
> 2) The exciters have been set the same - same model, same inputs etc.
>
> 3) Yes. The only differences may be is that the VCO of the units is 
> modified to 50MHz from 70 by adding capacitance in various places. The 
> alignment of both VCO's is the same however - set to 3V on a specific 
> test point.
>
> 4) Yep all the same. I've used a CTCSS encoder input which is post 
> pre-emphasis and limiting. Limiting is done by the RTCM.
>
> 5) As above - although I've noticed that the CTCSS's deviation on the 
> bench test transmitter is about 50Hz low when aligning for a 1K = 3K 
> deviation. I've tried turning up the TX level pot to equal it, but it 
> still sounds the same.
>
> Hayden
>
>
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-- 
Joe Leikhim


Leikhim and Associates

Communications Consultants

Oviedo, Florida

JLeikhim at Leikhim.com

407-982-0446

WWW.LEIKHIM.COM

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