I felt I needed to let those on this mailing list know my findings with
simulcasting with the RTCM units since I have received a great deal of help
and information from people on this list.
For the past week I have had a two (2) site simulcast system up and running
on UHF using the RTCM units. I have done a great deal of testing and
proving on the bench and it is so far working beautifully. The one issue
that was made apparent to me early on was that in order for the RTCM's to
be suitable for simulcast, the processor inside it needed to be clocked to
a more accurate source. This source is supposed to be the Programmable
Clock Generator Module in order to produce a GPS locked 9.6 MHz reference
for the processor. Unfortunately this unit is not currently available.
Knowing this, I started to go down the road of possibly designing my own
but stumbled upon a cheap, plentiful supply of Symmetricom 9.6 MHz sinewave
OCXO's. I took a chance and ordered a few to experiment with to see if the
RTCM would be able to use this in place of the supplied crystal for a
clock. I found they did, with that I decided to modify the RTCM with an SMA
connector just about the audio adjustment pots and use this OCXO. I trimmed
the frequency on the units to match each other and used them to produce the
processor clock.
After a week of testing I have found the results in the overlap areas to be
exactly as I would expect them on a public safety/commercial simulcast
system. While in an overlap region where both transmitters are within 3 dB
of one another, to the ear you hear no audio phase wandering. Of course
this has only been a week of testing so I wouldn't call it case closed yet.
I will say that if this system remains this way after testing for 6 months
to a year than using a 9.6 MHz OXCO is a viable alternative to achieve the
needed precision and accuracy needed for the processor clock to make it
suitable for simulcast.
If anyone has any questions about my system feel free to contact me. i will
more than happy to share everything I have done with anyone.
Thank you to all on the list that contribute.
73's for now
Joe
KC2IRV
