[Discuss-gnuradio] TVRX tests in narrowband applications

[Matteo Campanella]

Hello, I started my testing evening (after ARRL worldwide phone contest,
where I was desperately looking for Vermont to close WAS - are there hams
in Vermont??? :-)) taking care of TVRX.

First thing I did was to attach the TVRX to the tribander collinear I use
for V/UHF local traffic and use the nbfm receiver to listen. Actually,
most of the signals, easily heard on the ic706mkiig were not audible on
the usrp, even at the maximum gain. Only 9+ signals were indeed
demodulated and therefore audible (at the maximum gain). After all the
sensitivity of 706 is 0.18uV, while the datasheet of the microtune unit
says 40dbuV - so at least 40db gain are missing in the tuner - the worst
thing is that the noise figure is 10dB, so the internal amplifier isn't
quite based on GaAs ;-)

Going to the workbench, I injected a 145MHz, 400Hz, 5KHz deviation FM
modulated signal in the ic706 and the address@hidden gain, looking for the point
where the signal was audible - it turned out to be -100dBm for the tvrx
and -112dBm for the ic706 - so we're missing at least 12dBm power gain.
These measurements are indicative and quite empyrical - "audible" is not
very scientific as definition and the tuner is a 75ohm device, quite
mismatched when connected to 50ohms gear.

My final evaluation is that, since I want to use the equipment also for
experimenting in the UHF FSK G3RUH-like field, I will need to add a
serious low noise front end to the tuner - something for example built
around the MGA62563.

Is there anyone that already dealt with the subject and has already
realized and tested some circuitry? Any suggestions?

Matteo iz2eeq