Digital amplitude in benchmark_tx.py

GNU Radio
1

Hi,

I was trying out some experiments using USRP1 and the files
benchmark_tx.py
and benchmark_rx.py in the digital folder. I wanted to know few things.

(i) I saw the option of digital amplitude in benchmark_tx.py. Could
someone
tell me the gain of which component on the USRP board is controlled by
this
digital amplitude.

(ii) Also I tried to connect two USRP1 in a wired fashion and put one
to
transmit using benchamrk_tx.py and the other to receive using
benchmark_rx.py with the default gain of 45. I noticed that at digital
amplitudes (at the transmitter) of 0.64 to 1, the BER suddenly increases
inspite of wired connection. At values 0.63 and below, the bit errors
are
zero as expected. I was wondering if someone could explain this
behaviour.

(iii) In addition, I measured power levels at each digital amplitude in
increments of 0.1 (i.e. 0.1, 0.2, 0.3, …, 1) and I observed the
following values using a spectrum analyzer:

Digital Amp Power(dBm)

0.1 -8 to -4 0.2 -2 to 2 0.3 1 to 5 0.4 4 to 7 0.5 5 to 8 0.6 5 to
9
0.7 6 to 10 0.8 7 to 9 0.9 8 to 11 1 8 to 11

As can be seen, at larger digital amplitudes, there is not much
difference
in power levels between adjacent values of digital amplitudes. It seems
like
something at the transmitter is saturating, not allowing more than a
particular amount of power to be transmitted. Could someone please
elaborate
on this behaviour and its reasons?

Thanks in advance.

Regards,
Neha.

2

On Mon, Nov 30, 2009 at 07:53:11PM -0600, neha kochar wrote:

(i) I saw the option of digital amplitude in benchmark_tx.py. Could someone
tell me the gain of which component on the USRP board is controlled by this
digital amplitude.

None is. Have a closer look at transmit_path.py, and you’ll see this
controls the signal level before the ADC. When you set amp == 1, the
full ADC scale will be used – however, this will also cause the
saturation effects (when you run the ADC at full scale, you will get
spurious signals, which is why you should back off a bit). I guess that
also explains the rest…

Cheerio
MB

values using a spectrum analyzer:
0.6 5 to 9
there is not
not allowing
reasons?

Thanks in
advance.

Regards,

Neha.

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