[Discuss-gnuradio] USRP receiver filter response

[Kell, Scott]

Hi,

I've started evaluating the USRP for use in a "wideband radio" application, 
where a wideband front end shifts a 30 MHz chunk of spectrum down to baseband 
(0-30MHz) which is then digitized by the USRP.  The USRP DDCs are then used to 
tune to a channel in that spectrum.

I put a test tone directly into the USRP mother board's receive channel and 
used usrp_fft.py to measure the tone.  In doing this I saw some things which I 
don't understand.  

Center frequency used on the usrp_fft.py: 1250 kHz
Decimation rate used on the usrp_fft.py: 128 (for a span of 500 kHz, 1000 kHz 
to 1500 kHz)

The test tone was 6.1 dBm @ 1245 kHz and was measured by the usrp_fft program 
to be 22.4 dB.  This is near the middle of channel near the carrier.

I then changed the test tone to be 6.1 dBm @ 990 kHz which is outside the 
channel by 10 kHz.  It was aliased to 1490 kHz, just 10 kHz inside the upper 
half of the channel.  This alias component was measured to be -42.5 dB, or 64.9 
dB down from the center of the band.

I then changed the test tone to be 6.1 dBm @ 950 khz.  This tone was aliased to 
1450 kHz and was measured to be -20.0 dB, or only 42.4 dB down from the center.

I then saw the post on the USRP filter response by Firas [1] where he showed 
that for an overall decimation rate of 64 the CIC filter response had better 
than a 55 dB anti-alias rejection. [2]  But I'm seeing only 42 dB (overall 
response).

Does anyone have any ideas that explain this 13 dB discrepancy in the filter 
response?  Or am I missing something obvious...

Thanks!
Scott


[1] http://lists.gnu.org/archive/html/discuss-gnuradio/2007-05/msg00191.html 
[2] 
http://www.nabble.com/file/8500/%280%20-%2032%29MHz%29%20CIC%20Frequency%20Response%20%28CIC%20decimation%20%3D32%29.JPG
 

Scott Kell
Technical Director           
Signalscape, Inc
919-678-6590