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

[Mark Spencer]

Thanks.    I'll have to do some more experimenting, in one case I was feeding 
the time base output of the 5328A into a 1 meg scope input, and using the scope 
to compare the frequency to my thunderbolt, in the other case I was feeding the 
time base output into my 5370B set to 50 ohms and using the time interval 
function of the counter to compare the frquency to the thunderbolt.

I couldn't figure out why I was seeing different frequency results on the scope 
and the 5370B untill i put a 50 ohm terminator in paralel with the scope input 
then things made sense.    


I'll try altering the load impedance along the lines shown in the specs and see 
what the results are.    I also noticed that the wave form on the scope was very 
dirty (ie the peaks were very hard to see), and it cleaned up to a more or less 
sine wave when I added the 50 ohm terminator, perhaps the 5328A is not properly 
terminating the 10811.    


The 10811 seems stable so long as the load impedance does not change so it may 
still be useable.

I'm also wondering if I was getting some injection locking when I was comparing 
the frequency using the scope, the results seemed very very good, almost to good 
and the cable between the scope and the 5328A was not seeing 50 ohms at either 
end which probably wasn't helping either.   My elcheapo pre made RG58 patch 
cords of un known shielding probably aren't helping either.  I'll have to make 
up some proper patch cords using some double shieled cable I've stashed away.


 


----- Original Message ----
From: Bruce Griffiths <bruce.griffiths at xtra.co.nz>
To: Discussion of precise time and frequency measurement <time-nuts at febo.com>
Sent: Fri, December 24, 2010 10:42:46 PM
Subject: Re: [time-nuts] 10811 changing frequency when load impedance changes ?

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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