Gregory W Heckler wrote: > I've hit a wall with using the DBSRX to record GPS L1 C/A code data. The > signal path consists of the following: > > Spirent GPS Simulator -> 2 MHz wide SAW @ L1 -> +40 dB Miteq Amp -> > DBSRX -> USRP > > Notes/Settings: > > 1) Spirent Simulator: > Static scenario, 39 deg North, -84.866 deg West, 0.0 meter height > Visible SVs: 9, 17, 21, 8, 23, 1, 3 ,31, 29, 25, 5, 30 > C/N0 of all SVs: 50 dB-Hz > > 2) SAW Filter > 2 MHz bandwidth > Center frequency = 1.57542e9 Hz > .5-.8 dB NF > > 2) Miteq Amp: > +40 dB gain > .2-.8 dB NF > > 3) DBSRX: > Target LO frequency: 1.57542e9 - 604000 Hz = 1.574816 Hz > Actual LO frequency: 1.5748125e9 Hz > Resulting IF frequency: 607500 Hz > Refclck_divisor = 16 > N = 25197 > R = 64 > GC1 Gain: 30 dB > GC2 Gain: 0 dB > Baseband filter 3 dB cutoff: 4 MHz > > 4) USRP > Decimation: 16 > PGA Gain: 0 dB > > > I've noticed that the DLL of my software receiver settles to +15 Hz, and > the true IF is +24 kHz from the predicted IF. This would indicate that > the 64 MHz board clock is ~1 kHz from its spec value. This, in itself is > not a problem, but I was wondering if this was within tolerances of the > onboard XO? > > The real problem lies in the fact that the carrier tracking loop (a 3rd > order PLL) of my software receiver cannot achieve phase lock. The phase > jitter looks high, and the LO frequency drifts so much it dominates over > the Doppler derived from satellite motion. Maybe you could inject a stable frequency near the wanted RX frequency. Say a few Mhz away from the 1.57542e9 you want to receive. Then you could use this in the output to remove the jitter and LO drift.
for example: inject 1600000000 Mhz (=25 harmonic of 64MHz) at the input (after the saw filter) This will result in 24.58 Mhz +jitter +LOdrift at the input of the USRP (after dbs_rx downmix) Use a second channel in the USRP to get this 24.58 Mhz to the host at around 0 + jitter + LOdrift. Mix (multiply) the conjugate of this with the actual signal on channel 0 to remove jitter and LO-drift. You might need to low-pass filter it first or even use a second PLL. If the stability of the 64Mhz clock of the usrp is the problem, then you need an external stable source. If only the jitter of the dbs_rx is the problem, then you can use the 24th or 25th harmonic of the 64Mhz usrp clock. Greetings, Martin Greetings, Martin > > If anyone would like any GPS IF data I would be happy to email it to > your personal email address (indicate how many seconds of data you would > like). Thanks! > > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > http://lists.gnu.org/mailman/listinfo/discuss-gnuradio > _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio