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[Discuss-gnuradio] GPS with DBSRX, Almost There

From: Gregory W Heckler
Subject: [Discuss-gnuradio] GPS with DBSRX, Almost There
Date: Tue, 06 Mar 2007 15:37:03 -0500
User-agent: Thunderbird 2.0b2 (Windows/20070116)

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


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

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

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!

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