David Bengtson wrote:
Martin Dvh wrote:
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
The generation of 1571.328 from this source is almost surely a deal
killer because of the phase noise. That LO must be accurate, stable,
and as phase noise free as possible. We are trying to work with a
signal of only a few dB dynamic range signal and it is weak, right at
the noise floor in most systems. Cascaded LNA's will bring the signal
up but also lower the IP3 and the dynamic range and greatly increase the
front end susceptibility to overload, interference, and collapse. This
particular oscillator and the attendant mixer cannot be scrimped on. At
least do a serious noise budget analysis to make sure the system will
actually be usable after built.
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
--
Robert W. McGwier, Ph.D.
Center for Communications Research
805 Bunn Drive
Princeton, NJ 08540
(609)-924-4600
_______________________________________________
Discuss-gnuradio mailing list
[email protected]
http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
