[time-nuts] 10811 changing frequency when load impedance changes ?

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Dec 25 01:42:46 EST 2010


Bruce Griffiths wrote:
> Mark Spencer wrote:
>> Seasons greetings.
>> I was pleasantly surprised to find that a new to me HP 5328A had an 
>> 10811 in
>> it.    (I was expecting the older OCXO..)     After leaving it 
>> running for a
>> week I adjusted the frequency and while doing so I noticed that the 
>> out put
>> frequency changes by a third of a hz or so if I add or remove a 50 
>> ohm load.   I'm measuring the frequency at the timebase output 
>> connection of the
>> 5328A.  Is this to be expected ?
>>
>> Many thanks
>> Mark Spencer
>>
>>
>>
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> No, that seems to be a couple of orders of magnitude larger than the 
> specs for the 10811A would lead one to believe.
> Maybe there's a problem with the output buffer amp in the 10811A?
>
> Bruce
>
>
The spec for the 10811A actually states that the shift is < 5E-10 (5mHz) 
for a 10% (5 ohm) change in a 50 ohm load and a for a 25% change (250 
ohm) in a 1K load
A naive extrapolation would suggest a shift of around 20mHz for a 50 ohm 
to 1K load change your frequency shift (333mHz) is about 15x this.

However the 10811A is notorious for injection locking if the reverse 
isolation of the external circuitry is inadequate.

Bruce





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