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[Discuss-gnuradio] GPS with DBSRX, Breakthough?

From: Gregory W Heckler
Subject: [Discuss-gnuradio] GPS with DBSRX, Breakthough?
Date: Wed, 07 Mar 2007 19:37:39 -0500
User-agent: Thunderbird 2.0b2 (Windows/20070116)

To all concerned parties:

I think I've discovered the problem. My "tune" routine chose the R and N dividers to minimize the difference between the command and desired LO frequencies. For L1 this ended up being 64 and 25197. The refclk was set at 4 MHz, producing an R divider frequency of 62500 Hz. For a sanity check I enabled the debug output of the Max2118 chip, in order to probe the comparison frequency on the CNTOUT pin. Probing the pin produced a nice square wave at 31250 Hz. The inconsistency bothered me, and I double checked my driver was writing the correct values to the Max2118 registers, it checked out as ok. On a lark I decided to sacrifice the LO error for a greater comparator frequency. After changing the R value to 8 and N to 3150 I recorded some new data and crunched it with my software receiver. To my amazement the phase jitter went away. So, two questions:

1) Why the inconsistency with the R divider debug output on the CNTOUT pin? An R divider value of 8 produced a square wave at 250 kHz, again 2X lower than the values of R and reclk should produce. I am positive the value being written to the register complies with the spec sheet (R = 2*2^(R_register))

2) The Max2118 spec sheet quotes an XTAL input from 4 to 27 MHz. Obviously the low comparison frequency was effecting the stability of the PLL in the Max2118, why did the Python DB-SRX driver default the refclk divide to 16, placing the refclk at 4 MHz, rather than placing the refclk frequency towards the middle (16 MHz) of the spec? Was this done for some other technical reason?

I will try to quantify the C/N0 loss tomorrow. Additionally, I'd like to thank everyone very much for the help that has been given to date.

-Greg Heckler

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