Re: [Discuss-gnuradio] grGPS Preliminary Schematic

[David Bengtson]

Martin Dvh wrote:
> David Bengtson wrote:
> > After far to long, I've updated the information on the Gnu Radio GPS
> > board. I've finished a preliminary schematic, and I'd appreciate any
> > comments that anyone might have.
> Why don't you use the usrp to generate your 16.384 or 32.768 MHz refclock.
> Most daugterboards (except the tvrx) use this feature (They all use a 4Mhz 
> refclock on io pin 0.
> The only difference with your design is that you need another frequency.
> The cyclone fpga in the usrp has internal PLLs which can generate all kinds of 
> frequencies.
> (They are not used at the moment)

(Sorry, there is a typo, the reference frequencies are 16.368/32.736 MHz)
That might work. 16.368 MHz/4 MHz is 1023/250, or a comparison frequency 
of 16 kHz. From a frequency generation point of view, I could probably 
get the right frequency from the USRP. I'm concerned about jitter and 
noise on that line though. The reference is used directly for the 
comparison frequency in the on-chip PLL, to generate a LO that is 
1571.328 MHz. A 16 kHz spurious on this would really cause problems.

Because of this, I'd like to use a a clean reference clock for the 
reference. I could change the layout to bring a signal over from the 
connector to the reference input to check that though.  That would make 
it possible to try this approach.

Re-Using the reference clock is why I brought the buffered clock signal 
over to the connector, so that it will be available to the FPGA.

Dave