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

From: Marcus Leech
Subject: Re: [Discuss-gnuradio] TVRX tests in narrowband applications
Date: Mon, 06 Mar 2006 15:11:32 -0500
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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 ;-)

It's not unusual for cheap TV tuners to have high noise figures, and insufficient gain for weak signal work. Even the IC706, without the pre-amp, has a fairly high noise
 figure (8.5dB--according to some tables I found on the web).

For weak-signal work, it's entirely conventional and usual to use an external LNA, mounted directly at your antenna--putting an LNA next to your receiver is pointless, for any substantial
 length of feedline between your LNA input and antenna.

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.
The underlying tuner is designed for the cable-TV market, where high signal levels are the norm. Down East Microwave makes some excellent LNAs for 432Mhz and 144Mhz. You'll also find commercial VHF/UHF TV "signal booster" amplifiers might
 do it for you, but they have much higher (3dB or more) noise figures.

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.

I strongly recommend buying an external LNA  (HEMT or GaAsFET) and placing
it right at your antenna. Such LNAs have gains of around 15-25dB, and noise figures usually around 0.5-0.8dB. Cable losses for a 100ft run will be around 3-4dB. There's no point putting a low-noise amplifier on the front of your TV-RX, unless the feedline run to the antenna is very short (less than 1ft or so).

Marcus Leech                            Mail:   Dept 1A12, M/S: 04352P16
Security Standards Advisor        Phone: (ESN) 393-9145  +1 613 763 9145
Strategic Standards
Nortel Networks                          address@hidden

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