[Discuss-gnuradio] GPS with DBSRX, Breakthough?

[Gregory W Heckler]

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