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

[Matt Ettus]

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.

Matt