[time-nuts] Re: repair of an HP E1938A OCXO

[Bob Camp]

Hi

If you want to map “some reading” on the thermistor to 103.0 C on the crystal blank, you 
need a way to know the crystal blank temperature. You do a sweep run of some sort. You 
log the thermistor bridge readings vs frequency. You then convert this to temperature based
on the data on that specific crystal. 

The B mode is “linear” over temperature. It has a nice steep slope of frequency vs temperature.
This makes it ideal for checking oven temperature. Since you need a calibrated crystal and 
circuit changes, there are practical limits using it. 

One would assume this was done “way back when” on the HP design. The data probably got 
saved somewhere in the PIC code. It may well have been updated when thermistor vendors
changed. 

If you want to use the labeled temperature on the crystal to set the oven, you need that 
mapping. Why do it this way? It could save you a lot of “fun” in production.

Long long ago folks would do a “turn hunt” on the crystal. They then would ship the part.
Eventually they found that you actually needed to do that turn hunt at multiple temperatures
due to a range of issues. Every OCXO design is different and exactly how this or that one is 
processed will be a “that depends” sort of thing. 

Bob 

> On Jul 1, 2025, at 8:43 AM, Chris Caudle via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> On Thursday, June 26, 2025 3:47:48 PM Central Daylight Time Bob Camp via time-
> nuts wrote:
>> There really is no practical way to get an accurate temperature sensor
>> inside the oven / on the crystal, without upsetting the operation of the
>> device.
> 
> Do you really need to measure the temperature independently?  I don't fully 
> understand the control scheme and the differences between C mode and B mode, 
> but if you get the oven to the right temperature, will that not set the 
> oscillator output back to within spec?  I recall the original description was 
> the measurement was with EFC connected to 0V, so I guess you would need to 
> know what the nominal value of the EFC should be for on-target frequency.
> Am I thinking too simplistically?  Is there a range of oven temperatures where 
> the output is close to nominal but the temperature coefficient is not optimal?
> 
> -- 
> Chris Caudle
> 
> 
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