Re: OCXO/Si5351A Calibration/Testing...Wierd Failure!! #ocxo

Ron Carr

I played around with building and adjusting the OCXO for about 3 weeks.  I was probably overly concerned about drawing too much current and burning out the heater FET's.   I used WSPR drift reports to see how it was working which was a very slow process. 

When turning up R21 by a very small amount,  first there is no change.  Then about 15 seconds later the current will start to rise.  Another 30 seconds and the current will be very high.  Then it will very slowly fall down.

Power off and back on, the current will be very high.   Power off and wait 2 minutes and then back on it will slowly rise.  

In looking at the web page for the kit, in hindsight I think I would try the mod by OZ8PZ, reducing the value of R5.  Also in hindsight if you just adjust for the heater current to be about 100ma, you will be very close to the correct setting.

I also played with shorting C1 where it works as a P controller.   Easy to adjust but I wondered if it would work over the temperature range of my house which would be about 65 in the winter and maybe as high as 95 in the summer.

I also tried using an external resistor to limit the current as is suggested on the web page.  The problem with this approach is that once the heater FET's go into saturation, no heat is generated inside the oven and all the heat is in the external current limiting resistor.

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