Re: [Discuss-gnuradio] TVRX tests in narrowband applications

[Eric Blossom]

On Mon, Mar 06, 2006 at 11:17:59AM +0100, Matteo Campanella wrote:
> 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 ;-)

Remember that it was designed for receiving TV (most likely off a
cable -- it is a cable modem tuner!) not weak signal anything.  When
it was in production, it cost something on the order of $10.

> 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.

For the 2m stuff, you might want to take a look at the Ramsey
Electronics PR100 preamp kit.  It's got a 3-pole LC filter, about
20 dB of gain, followed by a two stage helical filter.  1 dB noise figure.
http://www.ramseyelectronics.com/cgi-bin/commerce.exe?preadd=action&key=PR100
The schematics are in the manual, which is available for download.

> Is there anyone that already dealt with the subject and has already
> realized and tested some circuitry? Any suggestions?
> 
> Matteo iz2eeq

Good luck getting Vermont!

Eric