[time-nuts] quick and very dirty phase comparator
Bruce Griffiths
bruce.griffiths at xtra.co.nz
Wed Jun 4 04:44:12 EDT 2008
Ulrich Bangert wrote:
> Bruce,
>
> thank you for correcting me. Here I have clearly fooled myself. However
> your posting originated some new ideas: With the GCD becoming THAT low
> an analogue phase lock to a 10 MHz reference will not be easy. But if we
> stop to think about phase locked VCXOs we need not bother anymore about
> odd exotic xtal frequencies
Yes you would need a VCXO with low close in phase noise for the
17.73447MHz source.
That crystal frequency isnt too exotic as RS components have suitable
crystals, if you build your own VCXO.
> at all that may generate us a GCD of 10.
> Instead we are free to choose for example 10000010 Hz for the
> controller's frequency. Which brings us back to a construction of a good
> offset generator.
>
> Until now I have believed that a good (low phase noise, high stability)
> offset generator would involve
>
> a) a number of single sideband mixers (as described in
> www.horology.jpl.nasa.gov/papers/fssa.pdf)
>
> or
>
> b) the well known offset synthesizer circuitry as described by Rick
> Karlquist.
>
> I am sure that both ideas work excellent, although I am unsure whether
> a) can generate an 10 Hz offset. However, both methods involve circuitry
> that I would not call exactly "quick and dirty" and their use would
> overstress the try to make something really simple. On a new internet
> search for "offset generator" I came over this one:
>
>
10Hz offset by method a is trivial (9.99999MHz is just as useful as
10.00001MHz):
1) Use a LSB mixer to mix 10MHz with 10MHz/1000 to generate 9.99MHz
2) Bandpass filter this and then use a USB mixer to mix 9.99MHz with
9.99MHz/1000 to generate 9.99999MHz.
3) Use a PLL to phase lock a low noise VCXO (spare 10811A or similar
detuned mechanically by 10Hz??) to the 9.99999MHz output to remove spurs
etc.
Even a DDS followed by a PLL cleanup loop (10811 plus analog PD etc.)
should work well although with a binary tuning word obtaining an exact
10.00001MHz (or alternatively 9.99999 MHz) output isnt possible.
A DDS has some advantages over a synthesizer using dividers in that
additional noise isnt aliased into the output.
> www.diva-portal.org/diva/getDocument?urn_nbn_se_liu_diva-1838-1__fulltex
> t.pdf
>
> What do you think about that topology? Let the "IF in" be the needed
> offset and the "offset OSC" be our 10 MHz reference. Would that not make
> an really easy way to generate an precise offset with the wanted
> features?
>
>
Not much (can work well with 20kHz offset but not with 10Hz offset),
direct generation mixing 10Hz with 10MHz like this requires a rather low
PLL bandwidth.
Also filtering out the unwanted sideband may be problematic.
> Best regards
> Ulrich Bangert
>
>
Bruce
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