RE: [Discuss-gnuradio] DBSRX used for GPS: cycle slip

[Heckler, Gregory W. (GSFC-596.0)]

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:address@hidden]
Sent: Mon 6/2/2008 6:34 PM
To: Heckler, Gregory W. (GSFC-596.0)
Cc: 'address@hidden'
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