[Discuss-gnuradio] GPS with DBSRX, Almost There

[Gregory W Heckler]

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.

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!