[time-nuts] About HP10544A

Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon Mar 9 22:48:36 UTC 2009

Bruce Griffiths wrote:
> 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.
> Esa
> 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.
> Bruce
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Another option (which I used several decades ago with a 10544A) was to
use a double tuned critically coupled RF transformer driven by a unity
gain buffer with another unity gain amplifier buffering the output
across the secondary tuning capacitor. NB the Q of the primary and
secondary tuned circuits should be the same (need to insert a resistor
in the secondary to set the Q). The inductors were hand wound and
trimmer caps were used for tuning.


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