[time-nuts] quick and very dirty phase comparator
bruce.griffiths at xtra.co.nz
Wed Jun 4 04:44:12 EDT 2008
Ulrich Bangert wrote:
> 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
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
> b) the well known offset synthesizer circuitry as described by Rick
> 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
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
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.
> 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
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
Also filtering out the unwanted sideband may be problematic.
> Best regards
> Ulrich Bangert
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