[time-nuts] HP 5071A Electron Multiplier of Cesium Beam Tube

[John Miles]

>   That's an  interesting  answer.  Can you explain  what  you  mean by
>   "faster digital noise analysis capabilities"?

The 3048A is relatively cumbersome to use, compared to a modern phase-noise
test set with high dynamic range ADCs.  Conceptually, a software radio with
multiple ADC channels could be used to measure phase noise directly as well
as to perform other timing-related measurements.  The devil's in the
details, though, because the state of the art in digital PN measurement is
down below -170 dBc/Hz, and the front-end requirements (noise, jitter,
channel isolation...) are accordingly strict.  To compete with the better
commercial gear you need to employ cross-correlation and various other
error-cancellation techniques.  It starts to look like real work before
long.

It would be relatively trivial to build a mediocre digital PN test set, but
such an instrument probably wouldn't be useful for characterizing
high-quality crystal oscillators by itself.  It would be more challenging to
build one that could routinely compete with the 3048A's analog front end in
the general case.

>   The reason  this interest me is I'd like to get the low  phase noise
>   of a  Wenzel 100MHz ULN, but I understand the price is  $1,500 which
>   is a bit too high.

Wait by the river, and one will eventually come floating by.  Or...

>   Some guys  at  NIST got very good noise performance  with  a  DRO at
>   10GHz. This  is  interesting, since  MiniCircuits  sells inexpensive
>   low-noise microwave  amplifier  ic's  and  mixers.  So  it  might be
>   possible to get a low noise cavity DRO at 8GHz and  use regenerative
>   dividers to  get down to 1GHz (8 / 2^3), then use  injection locking
>   to get  down  to 10MHz. This could be an inexpensive  solution  to a
>   difficult problem. And you have shown you can put 10GHz on FR4, so a
>   Rogers pcb may not be needed:
>   http://www.thegleam.com/ke5fx/hpll.htm

Possibly true, but don't kid yourself: such a divider chain would cost you
way more than $1500 worth of your time.  And don't forget that you'll have
to build two to test it!

One of the biggest problems would be the effect of the DRO control loop.  I
haven't seen the NIST papers you're mentioning but the best X-band DRO I've
played with has a loop bandwidth of 300-400 kHz.  Within that bandwidth, it
will just scale up the noise of whatever you're using as a reference, so any
attempt to get low VHF phase noise with a DRO and divider chain will just
end up giving you back the noise of your reference, plus any residual
effects.

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).  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.

>   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.

>   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. :-P

-- john, KE5FX