Re: twin crystal oscillator

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
Wescott Design Services
http://www.wescottdesign.com
.

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