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Re: [Discuss-gnuradio] TVRx and BasicRx gain questions

From: Matt Ettus
Subject: Re: [Discuss-gnuradio] TVRx and BasicRx gain questions
Date: Wed, 25 Feb 2009 23:22:48 -0800
User-agent: Thunderbird (X11/20090105)

Sebastiaan Heunis wrote:
Hi all

Can someone please provide me with some guidance?  We are working on a
passive radar project and the aim is to implement a pcl radar receiver
on the USRP1 using TVRx daughterboards.  I have the Microtune 7901.

I did some tests to see what the effects of the RF and IF amplifiers
in the tuner are (linear region,etc.).  I noticed that the maximum
output of the TVRx is -15dBm regardless of the input signal and the
gains.  For this measurement, I've got the test circuit that they have
in the Microtune datasheet with the Minicircuits RF transformer to
match the 200 ohm output of the TV tuner to 50 ohms.  Also, I captured
a signal from the TVRx and looked at it in Matlab.  The maximum I
value is 17 and the maximum Q value is 16, which means that we are not
close to using the whole dynamic range.

There are a couple of things to note here. First, the 200 ohm output of the microtune is plenty to drive the ADCs which have an input impedance close to 1K. So using a transformer to step the impedance down to 50 ohms gives you a loss of 6dB (2x voltage).

Second, At 0dB of PGA gain, the maximum input to the ADCs is 2Vpp differential, which is 10dBm in a 50 ohm system. The programmable gain amplifier gives you 20dB of gain, so with the gain at max, the input needed to clip would be -10dBm, which is about right for the max output from the TVRX.

I also did a test with the BasicRx db.  This time I injected a 13dBm
signal into RX-A.  The maximum value in Matlab is 7919 which looks
much better.

13dBm will clip the ADC.  10dBm is the real max full scale value.

Now I have 2 questions:
-Does anyone know anything about the TVRx and why I only get an output
of -15dBm?  The Microtune datasheet is a bit vague.  It only says that
the signal level for 1dB RF gain compression is 72dBuV (-113dBm) with
AGC deactivated.  Maybe the TVRx wasn't designed to utilise the entire
dynamic range of the ADC??

With the PGA, it uses most of the dynamic range.


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