[time-nuts] Low noise frequency multiplication

[Dr Bruce Griffiths]

David I. Emery wrote:
> On Fri, Mar 02, 2007 at 04:02:39AM +1300, Dr Bruce Griffiths wrote:
>
>   
>> Its not just the temperature coefficients, real inductors and capacitors 
>> have inherent phase noise.
>> Silver mica capacitors can be very bad as are ferrite core inductors.
>> Mylar capacitors are good as are NP0/C0G ceramics and X7R is acceptable 
>> for low impedance coupling and decoupling.
>> Air core and iron powder core inductors are good.
>>     
>
> 	This is intriguing, and something I'd really not thought about.
>
> 	Care to elaborate as to magnitudes and nature of such random 
> presumptive variations in capacitance/inductance - does the sideband
> energy induced by  these effects roll off with  rapidly with increasing
> frequency or extend up to tens of  kHz or MHz ?    What causes them ?  
> Are they actual small random changes in capacitance or inductance or a
> more complex non-linear physical effect involving energy storage
> mechanisms ?
>
> 	 On another point I can certainly see the issues with long time
> constant phase shifts through narrow crystal bandpass filters with hi Q
> resonances caused by thermal shifts in component values, but do real
> world crystal "clean-up" filters actually contribute significant short
> term (say 1 Hz and above) phase modulation ?    Many RF applications
> care deeply about absolutely minimized levels of energy say 1 Hz to 100
> kHz or more from the carrier but could care less about absolute phase
> relative to a reference (or Allen Deviation measured with taus in
> hours).  Thus the thermal changes aren't important, but modulation at
> much higher frequencies is.   And yes I can see that vibration makes
> crystal clean up filters a problem... at least for systems subject to
> enough to cause microphonic effects.
>
> 	I had never thought about relative performance issues of using a
> VCXO locked with a really narrow band PLL to a lower frequency reference
> versus  a multiplier with a narrow band cleanup filter at the output...
> other than to realize that unless one uses a more complex PLL design
> really narrow band loops implemented extremely straightforwardly
> (perhaps to the point of idiocy) require the higher frequency VCXO to be
> accurately on frequency within the low pass bandwidth else the loop
> won't capture. This gets a bit dicey if one is talking 1 Hz or less at
> 100 MHz.
>
> 	And of course if the loop bandwidth is wider, then the phase
> noise in that wider bandwidth is more or less the phase noise of the
> reference times the multiplying factor and not just the phase noise of
> the VXCO..   But yes, these days a loop can be designed to handle this
> capture issue with a little more effort...
>
>   
David

Even vibration isolated isothermal constant temperature crystals exhibit 
phase noise.
The components exhibit fluctuations in reactance.
The additional phase noise decreases with frequency offset, but if you 
are multiplying by 100x or so, even the phase noise at an offset of 
100Hz at the reference frequency may be a problem see:

http://tf.nist.gov/timefreq/general/pdf/1244.pdf

Bruce