Re: [Discuss-gnuradio] GPS with DBSRX, Almost There

[Martin Dvh]

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!
> 
> 
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