Chris: I was referring to the Doppler estimate output by your PLL over time (traditional tracking), whereas you indicated that you were looking at repeated acquisition attempts. If the jump is visible in all satellites than I agree with your suspicion of the DBSRX card. Note that the ref frequency you pass from the mainboard to the DBSRX daughtercard cannot be too high, I made that same mistake a long while back. The ref clock divisor must 16 or greater (ie the reference frequency is 4 MHz or lower).
-----Original Message----- From: Chris Stankevitz [mailto:[EMAIL PROTECTED] Sent: Mon 6/2/2008 6:34 PM To: Heckler, Gregory W. (GSFC-596.0) Cc: 'discuss-gnuradio@gnu.org' Subject: Re: [Discuss-gnuradio] DBSRX used for GPS: cycle slip Gregory W Heckler wrote: > Chris: > > So you are not tracking the signal in the traditional sense, only > looking at the frequency and delay estimate of your FFT acquisition. How > far spaced are your Doppler bins in the FFT acquisition? If you are > using a 1 ms integration time, and hence a 500 Hz bin spacing, if the > signal's true Doppler is towards the middle of a bin it is ambiguous > which bin the peak power will reside in due to noise. I have observed > that the USRP's clock is jittery (PLL tracking is only possible by > opening up the bandwidth beyond the recommend 18 Hz for a 3rd order > PLL), however I have never observed a 500 Hz instantaneous jump in > frequency (which would cause all of the channels to dump). Do you have > tracking data with multiple SVs which also point to a 500 Hz frequency > jump? Greg, 1. Our acq doppler bins are spaced 100Hz apart. The sudden jump is 1000Hz (10 bins). As I said earlier, this problem only occurs on one of my DBSRX boards. On the "faulty" DBSRX board, the problem exists for all satellites. If it turns out the DBSRX board is not faulty, it could indicate that the parameters I'm passing to the DBSRX/MAX211x are on the "hairy edge" of being stable for the chip. 2. I have also noticed USRP's jittery clock. However, my 2nd order PLL is able to track it. One thing that concerns me about your setup is that you are sampling at 2Msps complex -- the bare minimum according to Nyquist. FYI I am using 4Msps. 3. I do not know what you mean by this: "So you are not tracking the signal in the traditional sense, only looking at the frequency and delay estimate of your FFT acquisition." -- perhaps you can rephrase? Chris
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