[time-nuts] Measuring frequency rather than tuning crystal

Dave Martindale dave.martindale at gmail.com
Fri Feb 27 16:23:19 EST 2015


The TF930/960 does have a calibration procedure that is performed from 
the front panel.  Basically, you feed it a stable input from any known 
source (so both 1 Hz and 10 MHz from a GPSDO should work) and then 
adjust until the displayed frequency agrees with the known input 
frequency.  The resolution of this setting is quite a bit better than 
the stability of the TCXO in the box.

Now, this process could occur either by actually adjusting the frequency 
of a VCTCXO in the box using a DAC, or by changing a calibration 
constant stored in the memory of the device.  I suspect it's actually 
the former, because the instructions say that the adjustment path has a 
low-pass filter that you need to allow to settle.  This wouldn't be 
necessary if the calibration simply changed a stored number.

Dave

On 27/02/2015 15:08, Paul Alfille wrote:
> I don't think your TTi TF930 has a GPS input to calibrate against, based on
> a quick perusal of the data sheet. I would guess that the calibration
> constants are thus fixed from the factory (including temperature
> coefficients).
>
>
> On Thu, Feb 26, 2015 at 3:36 PM, James via time-nuts <time-nuts at febo.com>
> wrote:
>
>> I presume that this is what my TTi TF930 does. Calibration is closed box
>> so I guess the TCXO is free running and the micro inside just uses
>> calibration constants.
>>
>> James
>>
>>
>>
>>
>>
>>
>>
>>
>> -----Original Message-----
>> From: Paul Alfille <paul.alfille at gmail.com>
>> To: Discussion of precise time and frequency measurement <
>> time-nuts at febo.com>
>> Sent: Thu, 26 Feb 2015 19:02
>> Subject: [time-nuts] Measuring frequency rather than tuning crystal
>>
>>
>> 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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