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Re: [Discuss-gnuradio] Troubleshooting GPS on DBSRX


From: Peter Monta
Subject: Re: [Discuss-gnuradio] Troubleshooting GPS on DBSRX
Date: Wed, 04 Apr 2007 09:29:34 -0700
User-agent: Thunderbird 1.5.0.10 (X11/20070302)


However, I cannot acquire using a USRP source_c. Please share any tips you have.

Are you sure the antenna is getting power?  (Are you using an
active patch antenna?)  You would need to short J101 on the
dbs_rx to complete the 5V bias tee.

Incidentally, with the 5V in place, the board cannot survive
shorts at the antenna SMA connector---a small inductor acts
as a fuse and gets toasted.  Replacing it results in good operation
again, though.



3. Signal generator
Thinking I was improperly tuning the USRP, I connected a signal generator to it and verified that L1 was being brought to baseband. However I was frightened to see some artifacts on the USRP FFT that do not appear on the HP spectrum analyzer. They appear out of the noise for strong input signals (-75dBm).

I also see some pretty strong spurs, usually at multiples of 4 MHz,
but this hasn't hurt signal quality too much.



Q: Does my hack do what I think it does?

It sounds okay, but you could also try just giving a frequency of
1575.6, which will result in the same R and N.


Q: What does set_gain() do on the USRP with a DBSRX board? What should I set it to?

I've been using a gain of 92, which gives a signal strong
enough so that sending only the most-significant 8 bits
over USB still results in a good signal.  You might try
several different gain values; if you're not setting the gain
at all, that's probably a problem.

Have you tried the usrp_rx_cfile.py application to dump a file
similar to KD7LMO's?  For example,

./usrp_rx_cfile.py -g 92 -R a -d 8 -f 1.57542G -N 100M -s -8 >gps.usrp

Since you've successfully acquired satellites with the KD7LMO
files, this may not be an issue, but it might be good to specify
a large Doppler search range for the acquisition step (say +- 50 kHz),
consistent with the 50ppm absolute accuracy of the USRP's LO.

Cheers,
Peter Monta





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