[time-nuts] time-nuts Digest, Vol 59, Issue 97

Dave Baxter dave at uk-ar.co.uk
Tue Jun 30 11:21:31 EDT 2009


> Message: 3
> Date: Tue, 30 Jun 2009 00:20:50 +0200
> From: Alberto di Bene <dibene at usa.net>
> Subject: Re: [time-nuts] GPSDO project (SDR)
> To: Discussion of precise time and frequency measurement
> 	<time-nuts at febo.com>
> Message-ID: <4A493E42.4010704 at usa.net>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
> 
> Dave Baxter wrote:
> > 
> > Personally, I wouldn't bother with any high accuracy 
> clock/oscillator 
> > for the SDR, unless you can also synch the AD sampling clock in the 
> > soundcard.
> 
> The SDR-IQ,  SDR-14, Qs1r, Perseus and the HPSDR project do 
> not rely on a sound card ADC to sample any signals.....
> The RF signal is directly sampled at RF, with sampling rates 
> up to 122.88 MHz (depending on the model), then it is 
> frequency-shifted, then downsampled by the firmware in the 
> accompanying FPGA. Then the downsampled stream is sent to the 
> PC via the USB 2.0 port.  The software on the PC receives 
> directly digital data, no pesky sound card is needed to digitize it.
> 
> So the _only_ factor affecting the frequency calibration is 
> the clock of the RF ADC.
> 
> 73  Alberto  I2PHD


In that case, it's the RF ADC clock that needs locking.  No doubt that
is controled by the 66.666MHz reference much discussed?   (I do not have
schematics / info on that device.)   Either way, I stil doubt GPS
locking it would be of much great benefit.   With the exception of
checking the accuracy of some Standard Time and Frequency transmissions,
I can't think of any real reason to do it, other than "because we can"?

It's also that much more stuff to potentialy go wrong at a moments
notice.  Thinking of the KISS principle, not exactly in the HPSDR line
of things.  Mind you, you should see my shack, and the over complicated
system I've created to monitor HF beacons!  ;-)   Currently down due to
an overheating CPU in one machine.

73..
Dave G0WBX.

PS:	I think I've got the NTP timekeeping machine configured sorted
and running, now waiting for the replacement 1PPS sourcing GPS rx to
arrive.




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