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

Magnus Danielson magnus at rubidium.dyndns.org
Sat Dec 25 12:52:06 UTC 2010


Mark,

On 12/25/2010 06:54 AM, 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 ?

Look at the signal with a scope and either change the impedance on the 
scope or by letting it sniff the coax as it goes to whatever can handle it.

Sometimes reflections in cables can cause extra transitions for a 
counter. That's why I seem to like to have a scope around when using 
counters to verify the signal. In practice not all sources in a lab is 
capable of driving 50 Ohm so it is not as simple as always terminating 
correctly.

The board that sticks in between the 5328A motherboard and the 10811A 
has a driver for frequency output which does not give a nice and clean 
output on high-impedance loads, but it gives nice and sharp transitions, 
in fact the falling transition is at 160 MV/s and your best bet.

BTW, I love that my 5335A has slew rate measurement built-in. :)

Cheers,
Magnus



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