Using sig_source_c on a USRP2

GNU Radio
1

Hello,

I have been playing around with the USRP2, mainly to
get a feeling for how much data I can pass across the
Ethernet.

I have noticed something odd

When I use sig_source_c with a sample rate of 20 Ms/s
and a frequency of 1 MHz with the WBX board set to
1 GHz and the interpolation set to 20 I would have
expected the output on the HP 8592L to be
at 1.001 GHz but it is at 1 GHz. I have tried various
combinations of sample rate and frequency and can make
little sense of what is happening.

When I use the audio sink and use sig_source_f it works
exactly how I expected.

What am I doing wrong ?

  • Charles
2

On Sun, Feb 21, 2010 at 1:13 PM, Charles B. [email protected]
wrote:

be at 1.001 GHz but it is at 1 GHz. I have tried various
combinations of sample rate and frequency and can make
little sense of what is happening.

When I use the audio sink and use sig_source_f it works exactly how I
expected.

What am I doing wrong ?

You should definitely be able to see something at 1.001GHz, but any
residual DC offset will also cause a spike at 1.000GHz when using a
vector modulator.

What’s your resolution bandwidth set at on your spectrum analyzer?
Can you post a link to a screen shot or two?

Brian

3

Hello Brian,

Sorry it is not possible to get a screen shot at the moment mainly
because I
have turned everything off for the day and not having a GPIB i/f on the
analyser I would have to find my video digitiser and take a snapshot
from the video out.

I can see the residual at 1 GHz it is just that the main signal is not
where
I was expecting it. The signal is > 30 db above the residual. When I
set the frequency to 10 MHz for example the main signal is at about +2
Mhz
above the residual (from memory).

I thought I might be doing something wrong or using something that
was not supported. The documentation is fairly sparse and even more
so on the USRP2. The USRP2 sinks don’t have the same naming format
as those for the USRP1 i.e sink_32fc vs sink_c which confuses me.

  • Charles

----- Original Message -----
From: “Brian P.” [email protected]
To: “Charles B.” [email protected]
Cc: [email protected]
Sent: Sunday, February 21, 2010 7:55 PM
Subject: Re: [Discuss-gnuradio] using sig_source_c on a USRP2

4

On Mon, Feb 22, 2010 at 08:38, Charles B. [email protected]
wrote:

Well here is the screen shot.
http://www.chbrain.dircon.co.uk/usrp2_signal.jpg

The sample rate was set to 20 M, the frequency to 5 MHz
and the interpolation to 20. The WBX carrier was set to 1 GHz

Can you post the exact command line you are using? Also, can you try
again with interpolation 10 instead of 20 to see if there is a
difference?

Johnathan

5

Hello again Brian,

Well here is the screen shot.

The sample rate was set to 20 M, the frequency to 5 MHz
and the interpolation to 20. The WBX carrier was set to 1 GHz

I am sure I have done something wrong

  • Charles

----- Original Message -----
From: “Brian P.” [email protected]
To: “Charles B.” [email protected]
Cc: [email protected]
Sent: Sunday, February 21, 2010 7:55 PM
Subject: Re: [Discuss-gnuradio] using sig_source_c on a USRP2

6

The problem has now been solved. I had assumed the USRP2’s
DAC sample rate of 400 MS/s was the rate I had to interpolate to.
Not so, it should be 100/s MS/s.

Thanks all

  • Charles