[time-nuts] OXCO Issues

Max Robinson max at maxsmusicplace.com
Sat May 11 15:05:52 EDT 2013

I once designed and built an analog proportional crystal oven.  What I 
didn't realize was that my bench power supply had developed a fault and 
wasn't regulating perfectly.  When I connected it to a good power supply it 
went unstable.  Rather than reengineer the control loop I connected a 2 ohm 
resistor in series with the power supply.  All of the electronics was on the 
other side of a 5 volt regulator so it wasn't effected by the voltage 
variations, only the oven heater was.  It has worked fine for years that 


Max.  K 4 O DS.

Email: max at maxsmusicplace.com

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----- Original Message ----- 
From: "ed breya" <eb at telight.com>
To: <time-nuts at febo.com>
Sent: Saturday, May 11, 2013 9:42 AM
Subject: Re: [time-nuts] OXCO Issues

> Why not just run it from a separate, regulated supply? Until you check it 
> with solid voltage applied, you won't know for sure - maybe the OCXO is 
> OK, but the oven circuit can't handle too high an impedance from the 
> supply.
> At low frequencies, the OC part may exhibit negative resistance, making an 
> oscillator under some conditions. To see how this can happen, picture the 
> unit operating in static environmental conditions. After warmup, the whole 
> thing should take constant power - on average - to maintain the XO 
> temperature. If the supply voltage increases, the current must decrease in 
> order for the power to stay the same - again, on average, over some time 
> frame. This is negative resistance, which can provide power gain to form 
> an oscillator.
> Ed
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