[time-nuts] GPS: ADEV or MDEV?

Tom Van Baak tvb at LeapSecond.com
Mon Apr 23 07:43:57 EDT 2007

>> * The Vref output of most OCXO's is from a Zener diode inside the can.  These 
>> typically have aging, thermal sensitivity and very poor voltage  accuracy, 
>> and there are much better monolithic high-precision, low-tempco  voltage 
>> reference available on the market now (Digikey etc). Depending on the  internal 
>> design of the OCXO, it may help the noise and stability to add an  external 100uF 
>> or larger tantalum cap to the Vref pin, since it's voltage is  likely used as 
>> the power supply for the Oscillator etc.
> If you want the lowest tempco and drift available try using an LTZ1000 
> (used in HP 8 1/2 digit DVMs).

Question - does Vref drift matter for a GPSDO? It seems it is
either too low to be of concern or the effect of Vref drift is in
fact indistinguishable from OCXO frequency drift and is thus
transparently compensated by any 3rd order loop?

I don't know for sure, but is 1 ppm/day drift in Vref typical? If
so given an EFC gain of 1 Hz / 10 MHz / 10 V = 1e-6 / V the
net drift per day due to Vref is 1e-12 which is 50x below the
quartz drift spec of say a hp 10811A (5e-10/day).

Is the zener drift plot in Analog Devices AN-713 typical?


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