[time-nuts] xtal oscillator phase noise

Henk ten Pierick henk at deriesp.demon.nl
Sat Jan 5 16:50:54 EST 2008

On Jan 2, 2008, at 0:06, Bruce Griffiths wrote:

> Henk
> The circuit diagram helps a lot.

Thanks a lot for the comments on the schematic, they explain a lot.

> I will create some circuit schematics for crystal oscillators that
> control the crystal current more directly and use a common base output
> buffer.

thanks in advance

> Meanwhile look at:
> http://www.ko4bb.com/~bruce/LPND.html
> <http://www.ko4bb.com/%7Ebruce/LPND.html>

I did and it was very helpful

> This crystal oscillator circuit is a variant of the low noise  
> oscillator
> proposed by Wenzel for use with fundamental crystals. It uses a common
> base buffer and demonstrates several low noise biasing techniques that
> can be used.

The schematic is easy to understand, but it is not clear to me what  
defines the xtal current. The loading of the oscillator due to the  
base resistor of 10k is surprising. I expected a controlled loop for  
the xtal current in a well designed oscillator.
How high or low is the loaded Q. How is noise matching for the  
oscillator defined?

> The 2nd and third references above the oscillator schematic explain  
> the
> mechanisms for generating AM and PM noise in a BJT RF amplifier. The
> derivation is quite mathematical (statistics and calculus) but the
> conclusions are relatively simple.

Nice reading with clear conclusions. A lot of the cited measures and  
comments from you and other  time nuts are now better understood.

> To drive an HC04 the common base buffer can use a load consisting  
> of an
> inductor shunted by a resistor to develop the drive. The shunt  
> inductor
> reduces the dc gain (from base to collector) of the  buffer stage and
> hence the low frequency noise voltage developed across the collector
> base capacitance. Such noise voltages modulate the output capacitance
> and hence the phase shift of the buffer, increasing the buffer phase
> noise.  Nonlinearities such as hfe variation with current tend to
> increase the buffer output AM noise not the PM noise.

It is funny that I always tried to avoid coils because of sensitivity  
for magnetic fields and now I learn that I have to use coils for the  
lowest phase noise. Why is there not an inductor used i.s.o a  
resistor of 10k in the base of Q104? Avoids the loading with the  
consequence that xtail current control has to be done in an other way.

I assume that the loaded Q of the oscillator is important for a low  
phase noise. The circuit suggests different. Am I missing something?


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