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Re: [Discuss-gnuradio] grGPS Preliminary Schematic

From: Daniel O'Connor
Subject: Re: [Discuss-gnuradio] grGPS Preliminary Schematic
Date: Thu, 6 Apr 2006 16:08:00 +0930
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On Thursday 06 April 2006 15:11, Marcus Leech wrote:
> > Also, I think getting 32.768 MHz from 4Mhz with a PLL would be pretty
> > difficult (if not impossible)
> Given that you haven't looked into the PLLs on the FPGA, it's rather too
> early
>   to conclude that you couldn't synthesize 32.768Mhz from the 4Mhz
> reference clock that's already on the USRP.
> Given reasonable dividers for both reference and VFO, I can't see why
> synthesizing
>   32.768Mhz would be all that difficult.  I can understand why you might
> want to
>   use a dedicated oscillator--you don't have to muck with the FPGA.
> But asserting that
>   it's "difficult (if not impossible)" is hard to support if you haven't
> looked into it.

According to the datasheet the PLL can only do m/n where m is between 1 and 32 
and n is between 1 and 4.

Therefore I don't see how it's possible. I was only guessing before, but I 
just checked the datasheet and it seems impossible.

You could get close with a Stratix II.. 4 * 498/62 = 32.129 MHz

Whereas a Cyclone can only do m/n where m is 1 to 32 and n is 1 to 4.

Daniel O'Connor software and network engineer
for Genesis Software - http://www.gsoft.com.au
"The nice thing about standards is that there
are so many of them to choose from."
  -- Andrew Tanenbaum
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