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.
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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Leikhim and Associates
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Oviedo, Florida
407-982-0446