[time-nuts] Low noise frequency multiplication

[David I. Emery]

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...

-- 
   Dave Emery N1PRE,  die at dieconsulting.com  DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in 
celebration of what could have been, but wasn't and is not to be now either."