Forum: GNU Radio Maximum value of samples

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Sebastiaan H. (Guest)
on 2009-03-10 13:23
(Received via mailing list)
Hi

Can anyone please suggest a setup that I can use to test the maximum
value of samples?  I know that we have 16bit signed values.  Looking
at the ADC interface on the USRP, we sign extend each 12-bit sample
and add 3 zeros to the right.  This should give us a maximum value of
16376 for samples?

I tried using a BasicRX daughterboard.  I have a function generator,
inputting a 20MHz, 10dBm signal into the db.  In my app I mix this
down to 100kHz using the cordic.  I also decimate by 16.  The PGA gain
is set to 20dB.  The FPGA build is the standard 2rx/2tx with hbf.  The
maximum sample value that I get with this is 8635, so I'm using about
half of the ADC's dynamic range.

Sebastiaan

--
Sebastiaan H.
Radar and Remote Sensing Group, University of Cape Town
Tel:  +27 83 305 5667
Firas A. (Guest)
on 2009-03-10 14:31
(Received via mailing list)
Hi,

> From: Sebastiaan H. <removed_email_address@domain.invalid>
>
> Can anyone please suggest a setup that I can use to test
> the maximum value of samples?
>
> Sebastiaan


The quickest way is to run grc, connect usrp to number sink and run the
graph.

Regards,

Firas
Karim El Defrawy (Guest)
on 2009-03-10 14:35
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Hi,
I am using a FLEX2400 transceiver daughterboard and was wondering what's
the minimum step size that I can use to set the center frequency.

I tried steps down to 500KHz and I could tell the difference between the
generated carrier frequency of a sine wave on an oscilloscope.

When I tried lower steps I was not able to see any difference in the
generated frequency.
Thanks,
Karim
Don W. (Guest)
on 2009-03-10 16:25
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Sebastiaan H. wrote:

> maximum sample value that I get with this is 8635, so I'm using about
> half of the ADC's dynamic range.

You also need to consider the inherent gain of the CORDIC algorithm
(approx.
1.647/2 in the USRP) and the effect of filtering out the negative
frequency
components of your signal.  Because of these, you can use the full
dynamic
range of the ADC and never see sample values close to 16376.  In fact,
at
10dBm input and 20dB PGA gain I expect your ADC is clipping.  Have you
looked at a plot of your recieved waveform?

-- Don W.
Markus F. (Guest)
on 2009-03-10 18:26
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Firas A. schrieb:
> The quickest way is to run grc, connect usrp to number sink and run the graph.
>
I tried the Number Think, but does it show the current values or the
maximum values ?

I noticed that the values differ from the values shown in the FFT Plot.
How to interprete the values in the Number Sink ?

In the FFT-Plot it is used dBFS as i think.
Josh B.:
"USRP values are signed 16 bit values, so ┬▒ 32768 range."

How can i compare the value from my FFT-Plot with this value ?

Regards Markus
Eric B. (Guest)
on 2009-03-10 19:17
(Received via mailing list)
On Tue, Mar 10, 2009 at 05:33:36AM +0100, Karim El Defrawy wrote:
> Karim
Which interface are you using to set the center frequency?

If you use the "tune" method, it adjusts both the VCO on the
daughterboard and the digital up/down converter giving a minimum step
size of about 0.01 Hz.

Eric
Josh B. (Guest)
on 2009-03-10 20:41
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Markus F. wrote:
>> The quickest way is to run grc, connect usrp to number sink and run
>> the graph.
>>
> I tried the Number Think, but does it show the current values or the
> maximum values ?
>

Numbersink updates slowly, its best used with signals that change
slowly, like the average, rms, some of the SNR probe blocks.

The scopesink will also show the current values.

> I noticed that the values differ from the values shown in the FFT Plot.
> How to interprete the values in the Number Sink ?
>

Thee FFT plot is in decibels, its all relative. So if your signal is
40dB above the noise floor in the fft plot, than you calculate SNR:
http://en.wikipedia.org/wiki/Signal-to-noise_ratio... and
life is good.

If you really want to know how many volts, how many dBm, how much is a
count, then you must calibrate your receiver. Feed a signal of known
amplitude into your receiver, look at the levels in scope and fft plot.

If it helps, this shows how the fft plot in dB is calculated from the
fft:
http://gnuradio.org/trac/browser/gnuradio/trunk/gn...

-Josh
Sebastiaan H. (Guest)
on 2009-03-10 21:03
(Received via mailing list)
Don

> In fact, at
> 10dBm input and 20dB PGA gain I expect your ADC is clipping.  Have you
> looked at a plot of your recieved waveform?

I agree with you on this.  According to the ADC datasheet, we need a
2Vp-p signal at the ADC to utilise the full dynamic range.  One of the
graphs on the datasheet also shows that the input impedance of the
ADC, when the input signal is between 20MHz and 60MHz, varies between
220 and 230 ohms.

This means that anything above around 7dBm should cause the signal to
get clipped.  With input=10dBm and PGA gain = 20dB, I captured a
signal and plotted it in Matlab.  It's just a normal sine wave with
its peak around 8600.  No clipping, which is surely wrong?

I just need to figure out how efficiently the TVRx uses the dynamic
range of the ADC, so I want to compare the maximum sample value that I
get using the TVRx with the maximum possible sample value.

Thanks for the help.

Sebastiaan
Markus F. (Guest)
on 2009-03-11 01:23
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Josh B. schrieb:
>
> If it helps, this shows how the fft plot in dB is calculated from the
> fft:
> 
http://gnuradio.org/trac/browser/gnuradio/trunk/gn...
>
Thank you, :-)

i am not sure which solution is the best one, but there are 2 further
solutions i think.

The second is to set up the <ref_scale> in the FFT Plot and
the third is to set an offset in the Usrp Source i think.

But the last one i didn't see any example. Maybe the last one would
be the best ?

Regards Markus
Markus F. (Guest)
on 2009-03-11 01:29
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Josh B. schrieb:
>
> Thee FFT plot is in decibels, its all relative. So if your signal is
> 40dB above the noise floor in the fft plot, than you calculate SNR:
> http://en.wikipedia.org/wiki/Signal-to-noise_ratio... and
> life is good.
I am not sure whether the Power of the Noise value will be measured ?
Because of the heavy changing noise floor.

Would it be better to calculate it or to measure the noise floor ?
http://www.sengpielaudio.com/calculator-noise.htm

Regards Markus
Karim El Defrawy (Guest)
on 2009-03-12 00:31
(Received via mailing list)
> Which interface are you using to set the center frequency?

The "tune" method.


 > If you use the "tune" method, it adjusts both the VCO on the
 > daughterboard and the digital up/down converter giving a minimum step
 > size of about 0.01 Hz.

OK, thanks. That's good to know.
Marco Bottino (Guest)
on 2009-03-18 16:05
(Received via mailing list)
Hello,

I was doing some test with a 6Volts battery with the USRP in order to be
able to use my USRP outdoors.
(Sonnenschein A506/4.2 s http://www.sonnenschein.org/A500.htm)
Everything was fine when I unintentionally inverted the polarity of the
battery.
At the moment, connecting the power converter to the usrp, the led
doesn't blink anymore and the fan doesn't work too.
I'm really not familiar with the electronic so I don't know what can be
happened inverting the plus with the minus...
Is there any hope to bring back to life my USRP? Can the fuse have saved
the board?
Any help will be very happreciated.

Thank you,

Marco
Eric B. (Guest)
on 2009-03-18 18:25
(Received via mailing list)
On Wed, Mar 18, 2009 at 10:04:27AM -0400, Marco Bottino wrote:
> happened inverting the plus with the minus...
> Is there any hope to bring back to life my USRP? Can the fuse have saved
> the board?
> Any help will be very happreciated.
>
> Thank you,
> Marco

The fuse might have saved it.  Try replacing it.  It's a 3 amp fuse,
size 0603.

Eric
Marco Bottino (Guest)
on 2009-03-18 20:21
(Received via mailing list)
Selon Eric B. <removed_email_address@domain.invalid>:

> > I'm really not familiar with the electronic so I don't know what can be
>
> Eric
>

Ok, it should be fine...
Thank you,

Marco
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