[time-nuts] xtal oscillator phase noise

Henk ten Pierick henk at deriesp.demon.nl
Tue Jan 1 12:37:59 UTC 2008


On Dec 30, 2007, at 19:03, 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.

It showed to be very difficult to come lower than what I have now. If  
can be the crystal. How can I decide?

> 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.

I used the BC375 for the low Rbb' and assume that the noise corner  
must be low as a result of that. Is this not true?

> 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.

I was aware that most lower frequency circuits are parallel resonant.  
I used series in class A because I thought is was better, it is  
easier to use the current though the xtal. Is there a fundamental  
difference between parallel ore series w.r.t performance?

> 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.

I do normally not have access to a FSUP but borrowed the instrument  
for two weeks. To my luck it has the B60 option and I used this of  
coarse. There must be a reason for my employer to buy this fantastic  
tool.

Henk





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