Forum: GNU Radio TVRx and BasicRx gain questions

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998dbc3ac73d5487c59c0ee0f5baeeb1?d=identicon&s=25 Sebastiaan Heunis (Guest)
on 2009-02-25 15:57
(Received via mailing list)
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.

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.

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??
-What is the maximum signal level after the DDC+filtering,etc?  My
guess is around 2047*8 because of the bit-shifting at the ADC
interface.

Sebastiaan

--
Sebastiaan Heunis
Radar and Remote Sensing Group, University of Cape Town
Tel:  +27 83 305 5667
3596cfe1d579c65b9babd35e8787977c?d=identicon&s=25 Matt Ettus (Guest)
on 2009-02-26 08:23
(Received via mailing list)
Sebastiaan Heunis wrote:
> 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
998dbc3ac73d5487c59c0ee0f5baeeb1?d=identicon&s=25 Sebastiaan Heunis (Guest)
on 2009-03-06 13:45
(Received via mailing list)
Matt

Sorry to bother.  Please tell me why I lose 6dB when measuring the
signal at pins 17 and 18?  I have the RF transformer from Minicircuits
that is supposed to match the 200ohm output of the TVRX to a 50ohm so
that I can measure it on a spectrum analyser.  Because it is matched,
I should not have a 6dB loss?

I believe the input impedance of the ADC is around 220ohms?  How does
the BasicRx work?  The input is 50ohms, so is there an RF transformer
on the BasicRx that matches this to the ADC?

Thanks in advance.

Sebastiaan
3596cfe1d579c65b9babd35e8787977c?d=identicon&s=25 Matt Ettus (Guest)
on 2009-03-08 08:22
(Received via mailing list)
Sebastiaan Heunis wrote:
> Matt
>
> Sorry to bother.  Please tell me why I lose 6dB when measuring the
> signal at pins 17 and 18?  I have the RF transformer from Minicircuits
> that is supposed to match the 200ohm output of the TVRX to a 50ohm so
> that I can measure it on a spectrum analyser.  Because it is matched,
> I should not have a 6dB loss?

While the transformer does not lose power, you have to understand that
the ADC is a voltage input device.   A transformer that converts 200
ohms to 50 ohms divides the voltage in half.  When we say that the full
scale level is 10 dBm, it actually means 2V peak to peak.  Once you put
it into the transformer, you'll only have 1V peak to peak.


  > I believe the input impedance of the ADC is around 220ohms?  How
does
> the BasicRx work?  The input is 50ohms, so is there an RF transformer
> on the BasicRx that matches this to the ADC?

It is a voltage input.  We match the input with a 50 ohm terminating
resistor.

Matt
D868bf0bf60b372a6f5dd6f0b85a89bc?d=identicon&s=25 Kees Jeronimus (Guest)
on 2009-03-08 19:44
(Received via mailing list)
Which can easily be seen if you realize that in ideal
Passive transformer no power is added or lost.

Remember : P = (U*U)/R

No let's use it for the transformer:

Pin =  (Uin*Uin)/200
Pout = (Uout*Uout)/50

Passive no loss transformer:

Pin = Pout

So:

(Uin*Uin)/200 = (Uout*Uout)/50

<=>

(Uout*Uout) = (Uin*Uin) * (50/200) = (Uin*Uin)/4

So:

Uout = Uin/2

There you have the factor 0.5!

Kees (W1/PE1LDW)
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