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 <[email protected]> To: Discussion of precise time and frequency measurement <[email protected]> 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 >> >> >> >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> > 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 _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
