[time-nuts] Measuring frequency rather than tuning crystal

[Dave Martindale]

Here is a quote from the TF930/TF960 Service Guide, in the section on
calibrating the internal TCXO:

"Set the Measurement Time to 1s and repeat the process. The adjustment per
step is now a decade smaller and multiple presses may be needed. The
measurement restarts after each set of key presses. It will take a few
seconds for the reading to stabilise because of the settling time of a
filter on the control voltage. Aim to be within about 0.5Hz and then move
to the next step."

The reference to "the control voltage" strongly suggests that it is
actually adjusting the timebase frequency via the frequency control input,
not adjusting a numerical constant.  We know that the TCXO is actually a
VCTCXO, because it can be phase locked to an external reference.  And we
know it's actually phase locking the internal reference when an ext. ref.
is supplied, not merely substituting the external reference in place of the
internal one, because the manual says that the external reference must be a
high-accuracy 10 MHz signal - it is not possible to use a different
reference frequency.  (There is more detail - see the "External Reference"
section of the 930 or 960 manual).

- Dave

On Fri, Feb 27, 2015 at 4:23 PM, Dave Martindale <dave.martindale at gmail.com>
wrote:

> 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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>