[time-nuts] About HP10544A

Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon Mar 9 22:33:54 UTC 2009

Esa Heikkinen wrote:
>> Since retrofitting an improved oscillator circuit isn't really an option
>> you will need to filter the output to reduce the harmonic content.
>> Try a bandpass filter driven by the buffer and terminated in 50 ohms.
> Well I have to decide what to do, get another oscillator or try the 
> filtering. The difference to another oscillators (like tbolt ocxo or 
> LPRO) is so huge that I do not know how hard it would filter the 10544A 
> to the same level, which parts to use and how much it will cost.
>> Should you retrofit an improved oscillator circuit you may as well
>> replace the oven controller to eliminate the oven switching frequency
>> related sidebands.
> Infact the switcher sidebands are now gone:
> http://www.amigazone.fi/files/gpsdo/544-13.png
> http://www.amigazone.fi/files/gpsdo/544-14.png
> It was like thios earlier:
> http://www.amigazone.fi/files/gpsdo/544-1.PNG
> http://www.amigazone.fi/files/gpsdo/544-2.PNG
> It was easier than I expected... When I took the output directly from 
> OSC pins without using the coax connector like you suggested the 
> switcher peaks was gone!
> So I had a closer look to the PCB today and noticed that it has only 2 
> layers and the ground net is too thin and goes around PCB totally wrong 
> way so that switcher current seems to flow via the signal flow. It's PCB 
> layout design fault, I think that with correct layout design this could 
> be done even with 2-layer PCB correctly.
> But this is not a problem of course because I will design my own PCB for 
> the final system anyway, having SMA connectors for 10 MHz and EFC. I'll 
> also create separate power supply for ovens and signalling stages.

The switcher sidebands will still be there, they are just buried in the
spectrum analyser noise floor.

Does the board use the recommended LC filters and regulator for the
oscillator supply as depicted in Figure 3 on the 10544A data sheet?

Reducing the harmonics by 40 dB shouldn't be too difficult.
However any amplifier after the filter will need to be carefully
designed to keep the distortion below -60dBc.

Its better to use a multi section LC filter rather than trying to do it
with a single LC filter.
The filter phase stability will be better with a multi section LC filter.

Alternatively you could use a bandpass filter combined with some series
tuned shunt LC circuits to short out the 2nd and 3rd harmonic components.

The required parts shouldn't be too expensive, however you may need to
wind your own inductors for the series tuned LC circuits.
Air core or powdered iron core inductors should be OK as long as you use
shields between filter sections etc.


More information about the time-nuts mailing list