[time-nuts] About HP10544A

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Mar 7 23:26:21 UTC 2009

Esa Heikkinen wrote:
> I also doubt that there could be something wrong with emitter follower 
> so I also did some tests with "capacitive coupler" build on bottom PCB 
> of the HP10544:
> http://www.amigazone.fi/files/gpsdo/544coupler.jpg
> (1 kohm. terminaton is not visible on the picture)
> Generally it's just a wire going over 10 MHz trace on the PCB and then 
> it's connected to the spectrum. By this way it's possible to measure the 
> output without loading it with any galvanic connection. Of course there 
> is almost 30 dB of loss but it's not any trouble with spectrum analyzer 
> having over -145 dBm noise floor.
> But with that kind of measure the output specrum looks like this:
> http://www.amigazone.fi/files/gpsdo/544-5.png
> So there's less harmonics but 2nd and 3rd are still high if compared 
> with other OCXO's. Hard to say anything of higher harmonics because that 
> kind of coupler takes in all background noise like FM broadcasts etc. so 
> I wouldn't trust these measurements too much (and because of that I 
> didn't even mention them at first).

I've set up a Spice simulation of the 10544A output stage (includes the
1k2 source impedance at the 10544A emitter follower input) plus an
emitter follower biased as you describe.

About the only way to achieve the distortion you observe is if the
capacitance of the wire connecting your emitter follower to the 10544A
output is too large (around 50pF or more).
Can you send a picture of this connection?
Did you use PVC insulated wire for this?


More information about the time-nuts mailing list