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.

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


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