[time-nuts] Measuring frequency rather than tuning crystal

[Tom Van Baak]

Paul,

Right, there's no need to physically calibrate the LO when you can simply adjust all measurements in software, either inside the instrument or on the PC that's logging the measurements from the instrument.

This technique is used for many high-end measurements and also extends to GPS post-processing for even greater accuracy.

Look in the archives for GPS disciplined frequency counter. 

If you keep the raw data (DUT-REF, REF-GPS) you essentially avoid the entire rats nest of Vref, DAC, EFC, disciplining algorithms, and hard coded tuning constants. It just becomes a correlation in software.

/tvb

> On Feb 26, 2015, at 5:30 AM, Paul Alfille <paul.alfille at gmail.com> wrote:
> 
> I have a couple of HP 5370s with the beaglebone brain transplant. They come
> with a nice 10811 that has a little adjustment screw.
> 
> Testing against a Thunderbolt or KS-24361 the 5370 is off by less than 1Hz.
> 
> I know the traditional method would be to adjust the crystal slowly and
> make careful measurements, but since I have a fancy computer in there, I
> wonder if I could just adjust the frequency in software. 64-bit floating
> point numbers should have sufficient accuracy. All reported measurments
> would be corrected for the actual reference frequency.
> 
> Basically, I'd have a 10000000.226 Hz internal reference.
> 
> In fact, could I connect the beaglebone to a a GPS 1 pps source and make
> this a GPS-disciplined-software-corrected oscillator.
> 
> So my question is is this a known technique? The discipline feedback
> circuit seems a little different, I'd adjusting for drift and offset, but
> not the gain of control-oscillator linkage.
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