[time-nuts] xtal oscillator phase noise

Grant Hodgson grant at ghengineering.co.uk
Sun Dec 30 13:03:10 EST 2007


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.



Henk wrote :-

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

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?



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