Re: twin crystal oscillator


"Tim Wescott" <tim@xxxxxxxxxxxxxxxxxxxxxxx> wrote in message
news:116aj537n0mkr19@xxxxxxxxxxxxxxxxxxxxx
> Michael Black wrote:
>
> > Tim Wescott (tim@xxxxxxxxxxxxxxxxxxxxxxx) writes:
> >
> >>colin wrote:
> >>
> >>>while im waiting for my OCXOs to arrive and thinking about what makes
one
> >>>OCXO better than others with regard to 1hz phase noise, i thought what
if 2
> >>>crystal/amp stages were used for the oscillator, the bandwidth must be
> >>>reduced quite considerably and hence the close in phase noise too.
> >>>
> >>>not much came up on a search, maybe i will build one and see. i have
read
> >>>that very old crystals are better becuase their ageing has settled down
to a
> >>>low rate and are the larger size gives better Q.
> >>>
> >>>Colin =^.^=
> >>>
> >>>
> >>
> >>I wouldn't try two crystals _in_ an oscillator, but I believe that you
> >>can get good results by following an oscillator with a crystal filter.
> >>
> >
> > Or there's that circuit, I first saw it in an article by Rohde and
> > I thought it was his idea, where you extract the signal from the side
> > of the crystal away from the active device. I can't remember the
> > circuit, or even if it was special, but he moved the side of the
> > crystal that normally was grounded off ground, I think with a
> > resistor, and took the signal from there. The crystal was
> > then acting as a filter between the oscillator and the rest of
> > the world.
> >
> > Michael
> >
> That would certainly reduce phase noise. If it were me I'd look at
> implementing the best oscillator I could (I'd probably check the Butler
> oscillator for starters). If that weren't enough I'd follow it with a
> filter and accept the inevitable tradeoff between filter bandwidth and
> the difficulty of matching the filter passband with the oscillator
> frequency.
>
> There is literature out there on low phase noise oscillators, but
> crystal and otherwise.
>
> --
>
> Tim Wescott

doh, too late done it already, managed to get it working @ 20mhz with 2
bfe520 transistor inverters stages, would rather use series resonant sc cut
crystals rather than the at cut parallel resonance mode crystals wich is all
i seem to be able to get hold of easily. was thinking a 3 stage might be
better still as the phase needed for each stage would more convenient.

I figured a second one inside of the loop will be beter than on the outside
due of the effect of positive feedback, although of course one (or more) on
the outside is a posibility too. ive not tried it in circuit yet as i need
to make a second one, also the colector waveform has a weird kink in it
almost like its trying to oscillate at 2nd harmonic (wich it shldnt) but i
need to multiply it anyway. (and the ocxos have turned up now too :)

Ive seen the circuit refered to in a couple of places, ideal if you need a
clean sinewave. there are many low phase noise circuits out there wich apear
to be as good as the (single) crystal used.
my circuit feedback is similar in nature to a butler type.


Colin. =^.^=


.

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