Re: [Discuss-gnuradio] Suggestions regarding a frequency reference

[David Bengtson]

Matt Ettus wrote:

> David P. Reed wrote:
>  
>
> > In interfacing a  new 5 GHz up/downconverter to the USRP (my summer
> > project), I find that I need a 40 MHz frequency reference to drive a PLL
> > in the chip.   My first prototype will interface using the Basic Rx/Tx
> > daughterboards, and it occurs to me that I can probably use one of the
> > DAC clocks for that.
> >
> > Should this work? Any other suggestions (at this stage I'd prefer to use
> > an existing, programmable source on the USRP, rather than another chip
> > and crystal)?
> >    
>
> Does it need to be 40 MHz?  40 is going to be hard to produce cleanly on
> the board.  128 divided by any integer is much easier.
>
> Matt
>
>
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>  
Sounds like a nice summer project :-) 40 MHz seems to be a standard TCXO 
frequency, based on a quick perusal of the Digikey catalog. It would be 
a single part to add to the board. If this is used as a reference 
oscillator for a PLL, then you might be better off from  a performance 
point of view by adding the part and getting a clean reference. Your 
spurious emission's specification's will thank you.

Digitally, you could multiply the 128 by 5 to get 640 MHz, and then 
divide by 16 to get 40 MHz, but I don't think you could do this in the 
FPGA.

Dave