[time-nuts] xtal oscillator phase noise

Rick Karlquist richard at karlquist.com
Sun Dec 30 14:11:55 EST 2007


AFAIK, in a crystal oscillator specifically (not oscillators
in general), the oscillator transistor has almost nothing to
do with either close in or far out phase noise.  The close in
phase noise is typically limited by the crystal's intrinsic
noise and the far out phase noise is limited by the buffer
amplifier, assuming you take the output through the crystal
as is done in the 10811.  Certainly in the case of the 10811,
the oscillator transistor (a selected 2N5179) is not a player
in terms of noise.  BTW, the selection is based on startup
issues and has nothing to do with noise.  The buffer amplifier
transistor contributes shot noise, however this is determined
by physics, not choice of transistor.  So again, choice of
transistor is irrelevent for noise purposes.  You do have to
worry about distortion in the buffer amplifier that can
upconvert 1/f noise.  This can possibly require transistors
with constant beta vs collector current, depending on the
circuit design.

Rick Karlquist N6RK


Grant Hodgson wrote:
> Henk
>
> Two things will dominate if you want such a low phase noise spec.:- the
>   loaded Q of the oscillator circuit, and the flicker corner frequency
> of the sustaining amplifier transistor.  To get a high loaded Q you need
> a crystal with a high unloaded Q - maybe 100 000 or more - this isn't
> difficult to achieve from a good crystal manufacturer, but you can't
> expect any old crystal to work.  And the rest of the oscillator circuit
> should not load the crystal too much, otherwise the loaded Q, and thus
> phase noise, will suffer.  Good crystal manufacturers will provide the
> necessary measurements of series resistance, motional inductance (or
> capacitance, or unloaded Q - doesn't matter which) and static
> capacitance.  Lesser crystal manufacturers - don't.
>
> Also the flicker corner frequency of the transistor needs to be as low
> as possible.  Generally speaking, at offsets below the flicker corner
> frequency you will get 30dB/decade, above the flicker corner frequency
> you should get 20dB/decade, or flat, depending on the level of the phase
> noise floor.  If you can find a transistor with a lower corner
> frequency, the flicker noise will be reduced.  In fact, this is one of
> the dominant parameters when choosing a transistor as an oscillator -
> any old transistor can be made to oscillate, but to do so with a low
> flicker corner frequency is not so easy, and the corner frequency is
> usually a function of bias current.
>
> At 11MHz, most crystal oscillators use parallel resonant crystals,
> although some are series resonant, such as the excellent Driscoll
> oscillator which is capable of the performance you desire with a
> suitable crystal.
>
> Then you have the added problem of the FSUP.  It's a superb instrument,
> but it has it's limitations.  The FSUP data sheet states a phase noise
> spec. of -130dBc at 10Hz offset for a 10MHz signal, which gives a
> resulting sensitivity of -127dBc - 3dB worse than what you are trying to
> achieve.  You would need option B60 (cross correlation) to significantly
> reduce the effect of the internal source by (say) 15db or so.
>
> regards
>
> Grant
>
> Henk wrote :-
> Hello,
>
> Some questions on xtal oscillator phase noise. Attached the
> measurement result of my series resonant xtal oscillator.
> It is a class A, ibias 5 mA, Ixtal 1 mArms. Transistor selected for
> low Rbb' 20 Ohm, Ft 100MHz. Reference voltage 5V from an ADR445,
> filtered with 10uF folie cap. Phase noise target -130dBc at 10Hz.
>
> 1. Is series resonant better or easier to engineer than parallel
> resonant?
>
> 2. Where should I have 20 dB/decade and where 30 B/decade?
>
> 3. Some suggestions for the next 25dB?
>
> 4. Is there more to learn from the attached picture?
>
> regards,
>
> Henk
>
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