Is this an OCXO ?

[Wp100]

Hi,

Purchased this supposed unused but old stock 13mhz OCXO from abroad which was connected up as per the datasheet the seller supplied.

However instead of the expected heater current it just used 3.5ma, so believed the heater was shot, though the scope showed a decent enough 13mhz signal.

Assuming it was just acting a a simple oscillator decided to take a look inside and below is what I found.

It looks like the heart of it is a clear device the shape of a crystal can, possibly a heater ? stuck to the top of what looks like a 28 pin smd.
The resistance across the 2 leads of that crystal shaped can is about 500k

Can anyone say if it actually is an OCXO ? - with that low current wondered if it was and early version of a TCXO ?

If it is a OCXO with a failed heater to what extent would the accuracy be affected ? - only going to be used on the 'bench' Ta 21c

thanks

 

[KeepItSimpleStupid]

This https://en.wikipedia.org/wiki/Crystal_oven suggests 1-3 Watts, Notice that some say a transistor is used for the heating element, so it must be polarity sensitive.

I believe there is a controlled warm-up time for the crystal as well. The article references 75C as a temperature.

You have various temperature coefficients here: https://en.wikipedia.org/wiki/Crystal_oscillator Look like up to 300 ppm/deg C,

 

[dr pepper]

A lack of insulation also suggests no heater.

3.5ma isnt enough for an oven, your right, is that right at switch on?

Look like more pins than a conventional oven, I wonder if you need to link or apply power to more than 1 to get the oven working, Ihave an ex mil one that runs from 24v, the logic runs from 5v.

 

[KeepItSimpleStupid]

This https://www.nelfc.com/ocxo/ site is interesting. Applications may require an external potentiometer.

 

[Diver300]

That isn't an oven oscillator. The crystal touching the case would loose too much heat, and would make a lousy oven oscillator.

Check the frequency. If the frequency is 20 - 40 ppm high at room temperature, that would indicate an AT cut crystal oven oscillator. 10 - 30 ppm low at room temperature would indicate an SC cut crystal oven oscillator. I guess that the frequency will be very close to correct.

The crystal is in a glass housing, an HC-26 one. Those are seriously expensive, and along with the obvious complication of the circuit makes me think that it is a TCXO. I can't place the PLCC IC, but I suspect that it might be one that was specifically designed for TCXOs. Plessey, I think it was, made one that used analogue multipliers to create the cubic correction that is needed for AT cut crystals. The coefficients were set digitally for ease of setting, but the multiplication was analogue to avoid switching noise and for ease of setting.

Some TCXOs in that package needed an external potentiometer to trim to the correct frequency. It is very rare to have separate oven and logic supplies on that 5 pin layout.