[time-nuts] Low Phase-Noise measurements and design (was Re: HP 5071A Electron Multiplier of Cesium Beam Tube)

Magnus Danielson magnus at rubidium.dyndns.org
Sat Sep 12 10:17:40 UTC 2009


Shouldn't this thread have a more appropriate subject?

>   > A better  approach IMHO is to work on pushing the  limits  of what
>   > can be  done  with  homebrew  crystal  oscillators.  The excellent
>   > broadband floor  of Wenzel and similar oscillators is  not  due to
>   > their use of exotic crystals, but to their use of  good oscillator
>   > circuit topologies (and no buffering to speak of).
>   This is  very  interesting news. I thought  it  took  excellent high
>   quality quartz and very good low noise circuitry.
>   Can you tell more about how it is done? Do you happen to know of any
>   schematics? What kind of crystal would be suitable? I would  be very
>   interested in any additional info.

What a good crystal gives you is a high Q value and high purity and 
mounting which gives you low drift. Ovenizing aids to lower the 
frequency deviations due to temperature sensitivity, as usual.

The low noise oscillator and buffering amps makes the next important 
step. The 1/f noise and white noise is of importance. I recommend 
Rubiolas book and do read it through carefully. He drives the points 
down very well. In the end, the concepts are simple and the way 
interaction occurs isn't too complex. For phase-noise, the 1/f noise and 
white noise of the oscillating amplifier becomes crutial, as they 
convert into 1/f³ and 1/f² noise within the bandwidth defined by the 
crystal Q-value. The output buffer amps noise can then hide part of that 
spectrum, but as Rubiola points out, spending too much money on the 
buffer amps and not on the oscillating amps will be wasted as the 1/f² 
and 1/f³ noise rises quicker than the 1/f noise of the buffer amp.

>   > The crystal's  job is stability, not noise, and unlike  low noise,
>   > good stability is relatively cheap and trivial nowadays  thanks to
>   > cheap GPS clocks, rubidiums, and good-quality OCXOs.
>   Yes, I very much agree. GPS solves a lot of problems.
>   >> So the  question  is what kind of tweaking is needed  to  get the
>   >> best performance  in  a regenerative divider,  and  what  kind of
>   >> equipment is  needed to do it? Then, is perfection  really needed
>   >> in order  to  beat  the  Wenzel  ULN?  Maybe  put  up  with lower
>   >> performance in the beginning, then upgrade later.
>   > In practice  many applications for ULN-class  oscillators  put the
>   > broadband floor at risk in other ways. Very few  buffer amplifiers
>   > have a  noise floor below -170 dBc/Hz, for  instance. Fortunately,
>   > apart from  timing metrology, ULNs often end  up  driving high-end
>   > ADCs, where  the  application is likely to be a good  test  bed in
>   > itself.
>   I thought  the  noise in a 50 ohm resistor set  the  lower  limit to
>   -174dBc. Modern  amplifiers are better than that. For example,  a 50
>   ohm resistor  has 0.894nV/sqrt(Hz) noise, but you  can  get wideband
>   amplifiers with 0.7nV/sqrt(Hz) noise, which is equal to the noise in
>   a 30.6 ohm resistor. (Of course, flicker noise is not included)

-174 dBm, but not -174 dBc... the intrinsic noise of the resistor does 
not change with the amplitude of the carrier, it has a fixed amplitude 
with relation to the carrier... unless you consider the heating effect, 
but it needs to be very high to make any larger contribution as you most 
probably start with room temperature for most cases.

>   High speed adcs have very low jitter requirements to  maintain ENOB,
>   so anything that can improve the noise is helpful.
>   >> One trick  I have found that really helps isolate  circuit blocks
>   >> is to  put  them  on their own small island  pcb,  which  is then
>   >> soldered to the main ground plane to hold it in place.  Then find
>   >> the location  of   ground   connections   that   give  the lowest
>   >> crosstalk. A brief description is here.
>   > Yep, totally, and the islands become reusable components  in their
>   > own right.
>   > That's a  valid  thing to do, although I find that  when  I'm that
>   > concerned with  isolation,  I probably want a  full  shield anyway
>   > (hence the use of lots of discrete Hammond boxes).  Sometimes even
>   > this approach  is   self-defeating,   as   when   I  find  that my
>   > tightly-sealed Hammond enclosures make good cavity oscillators.
>   I'm probably  preaching to the choir, but do you find  the waveguide
>   cutoff frequency  for the box? It's pretty easy - you can  do  it in
>   your head. For example, the cutoff frequency is
>   fc = c / 2w, where
>   fc = cutoff in GHz
>   c  = speed of light, 30 cm/ns
>   w  = width in cm
>   So a box 4 inches wide would be
>   fc = 30 / (2 * 10)
>      = 30 / 20 
>      = 1.5 GHz
>   Here's a  calculator  that  gives  the  attenuation  at  any desired
>   frequency below cutoff:
>   http://www.k5rmg.org/calc/waveguide.html
>   Another problem  is  the pcb will resonate at  some  frequency, just
>   like a patch antenna.
>   For example, a 100mm x 50 mm (4 inch x 2 inch) pcb will  resonate at
>   700MHz. But  drop  the size to a 1 inch  square,  and  the resonance
>   moves up  to 2.768 GHz. This is a bit more difficult to  do  in your
>   head, so here's a calculator to help:
>   http://www.emtalk.com/mpacalc.php

There is of course ways to mitigate that resonance by design.

However, for homebrew it may just be simpler to obey the simple rules.


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