Re: [Discuss-gnuradio] GPS with DBSRX, Breakthough?

[Matt Ettus]

Gregory W Heckler wrote:
> 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:

One other observation --


If you properly tune the loop filter in both cases, there should be an 
18dB phase noise difference (div by 8 better than div by 64) inside the 
loop bandwidth for the 2 cases you list.  Since we aren't changing the 
loop filters (they are passive components on the board), it can be even 
worse.  The components on the board are optimized for a 1 MHz compare 
frequency.

These 2 factors can easily account for the performance differences you 
are seeing, and going to a div-by-4 would probably improve it more.  
There is very little cost to using a very coarse frequency step in the 
PLL since we have very fine tuning capability ( ~ 14 millihertz) in the 
digital downconverter.  Thus, tuning the LO exactly on-frequency is of 
no benefit, and actually makes things worse.  This is why we use 4 or 8 
MHz steps in the RFX-series boards.

Matt