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

From: Matt Ettus
Subject: Re: [Discuss-gnuradio] GPS with DBSRX, Breakthough?
Date: Wed, 07 Mar 2007 17:34:50 -0800
User-agent: Thunderbird (X11/20070302)

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.


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