Re: [Discuss-gnuradio] Digital amplitude in benchmark_tx.py

[Martin Braun]

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


> (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.
> 

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-- 
Karlsruhe Institute of Technology (KIT)
Communications Engineering Lab (CEL)

Dipl.-Ing. Martin Braun
Research Associate

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Fax: +49 721 608-6071
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KIT -- University of the State of Baden-Württemberg and
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