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

Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Sep 11 14:25:18 UTC 2009

Mike Monett wrote:
>   "John Miles" <jmiles at pop.net> wrote:
>   >> 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.
>   That is  a very interesting answer. No wonder Stein  pushes  ease of
>   use so much for the 5120/5125. But they are $40k to $50k  in Canada,
>   so obviously it's time for a new approach.
>   1) Where  would  you find ADCs with enough speed  and  resolution to
>   capture the  noise  signal from the phase detector?
Try using 4 AD9446's or similar pipelined 16 bit ADCs.
>   2) What  do current systems use for a reference oscillator  to reach
>   -170dBc? I'm not talking about the 5120/5125, or the Rohde
>   > 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.
>   I tried  to identify the U1 and U2 ics on the A12 LNA  board  in the
>   11848A. The  best  I  could  come up  with  was  the  part  number -
>   1826-2081. But  there was no cross-reference in any of the  HP lists
>   on the the HP Museum.
>   Anyway, technology has far surpassed what was available back  in the
>   80's when  the 3048 was designed. Wenzel and Rubiola  both published
>   front ends for PN that probably match anything currently in use:
>   http://www.wenzel.com/pdffiles1/pdfs/lowamp.pdf
> <http://www.femto-st.fr/~rubiola/pdf-articles/archives/2005-arxiv-0503012v1-ampli.pdf>
>   In "The Measurement of AM noise of Oscillators", Rubiola states "The
>   measurement systems   described   exhibit   the  world-record lowest
>   background noise."
>   Since AM  noise is generally less than PM noise,  the  amplifiers he
>   describes should  be  pretty close to state of the art.  Table  6 on
>   page 18 shows the noise parameters of some selected amplifiers:
>   http://arxiv.org/PS_cache/physics/pdf/0512/0512082v1.pdf
>   So the  amplifier front end doesn't appear to be the gating  item. I
>   think the biggest problem is to find low noise oscillators  that can
>   be used as a reference. One approach might be to use 8 Wenzel 100MHz
>   ULN's in a cross-correlation analyzer. That gets expensive.
>   >> 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...
>   As above, I'm looking for more than one:)
>   >> 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!
>   I still don't see why it should take so much time to tweak. There is
>   not that much to adjust, and a good network analyzer should  be able
>   to show the response of each section. So once you have  one working,
>   it whould  be  easy to duplicate. And if they were  that  touchy, it
>   would be  difficult  to sell them commercially.  The  slightest bump
>   would knockthem out of spec.
>   But as described below, I have scrapped the whole idea. It turns out
>   the performance may not be much better than a Wenzel.
>   > 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.
>   The idea  was  to  use the 10GHz oscillator  as  a  low  phase noise
>   source, then  divide down to use at lower frequencies. So it  is the
>   reference. One  application  would be to lock it  to  the oscillator
>   under test  to  make PN measurements, so the  loop  would  be pretty
>   slow. But it turns out the whole concept probably won't  give better
>   phase noise, so I scrap the idea.
>   Here's a bunch of links - you don't have to download them  since the
>   last one demolishes the concept. But here they are as a reference.
>   "Ultra-Low-Noise Cavity-Stabilized   Microwave  Reference Oscillator
>   Using An Air-Dielectric Resonator"
>   http://tycho.usno.navy.mil/ptti/ptti2004/paper16.pdf
>   Siemens App Note 002 shows the pcb layout for a 10GHz DRO:
> <http://www.taconic-add.com/pdf/technicalarticles--resonator-oscillator.pdf>
>   The next paper shows the phase noise of a 10.24 GHz  x-band sapphire
>   oscillator divided down to 640 MHz using regenerative  dividers. The
>   plot in Figure 10 on page 5 shows the result is barely 15  dB better
>   than a  Wenzel  at 1 KHz, and it looks like the  Wenzel  pretty much
>   matches the performance past 10 KHz. On the other end, it looks like
>   a Wenzel  10 MHz crystal would match the sapphire  performance below
>   100Hz.
>   "Low Phase Noise Division From X-Band To 640mhz"
>   http://www.psi.com.au/Pages/LibraryPublished/fcs_2002_lnrd_paper.pdf
>   Since a  cavity  stabilized DRO oscillator at 10  GHz  wouldn't come
>   close to the performance of a sapphire, it means the  best practical
>   source is a Wenzel. So I scrap the idea and start looking  at better
>   crystal oscillators as you discuss next.
>   > 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.
One can lock a high power ocxo (for low phase noise floor) to a passive
crystal for good long term stability using the classical Pound method.
NIST did this successfully some decades ago.
>   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.
The Oscilloquartz BVA OCXOs use a relatively conventional Colpitts
crystal oscillator.
However the BVA crystal is expensive.
>   > 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)
Not exactly, the phase noise floor can be somewhat lower than
-174dBc/Hz, it all depends on the input signal level.
NB white noise contributes equal PM and AM power so the PM level is
If the oscillator power is +10dBm (don't try this with an expensive
crystal) and the buffer amp input noise floor is say -172dBm/Hz then the
phase noise floor is around -182dBc/Hz.
This assumes that the oscillator signal is extracted through the crystal.
You will find that Ulrich Rohde claims that this degrades the noise
floor (at least he used to and its perpetuated in his various books).
However his own measurements indicate that his simplistic analysis of
this technique is fatally flawed.
>   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
>   So the trick is to use smaller parts and put them in smaller boxes.
>   Then fill them with Eccosorb:)
>   http://rfdesign.com/mag/0405rfdf1.pdf
>   > john, KE5FX
>   Mike
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