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

John Miles jmiles at pop.net
Sat Sep 12 01:35:16 UTC 2009

>   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?

As Bruce said, fast ADCs with very low aperture jitter like the AD9446 are
the best candidates.  Undersampling with slower ADCs seems to be a dead end.
I have an AD9446 here but haven't had time to do much with it beyond the
usual "turn my PC into an expensive FM radio" trick.  I'm actively
interested in building a digital noise/timing analyzer, but a lot of the
work will, as usual, consist of general software development rather than R&D
based on any particular ADC chip.

>   2) What  do current systems use for a reference oscillator  to reach
>   -170dBc? I'm not talking about the 5120/5125, or the Rohde

Usual suspects include Vectron, Valpey-Fisher, Wenzel, and Pascall.  Crystek
has also been releasing some nice surface-mount oscillators with noise
floors in that region.

Ideally a cross-correlating analyzer would not need multiple high-$$$
references, as it would be capable of self-cancellation to some extent.  The
downside is complexity and measurement time.

>   > 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.

There's nothing exotic in the 11848A's signal path.  It is a simple
quadrature PLL with some switched highpass filters at the mixer output,
which they needed to work with the low-dynamic-range FFT analyzer du jour
(HP 3561A).  The LNA design is again not too unconventional, as described in
the Wenzel app note (also check the febo.com archives for Bruce's thoughts
on the 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.

No, not at all.  I think it was Rick Karlquist who pointed out that the old
HP 608 signal generators, which were free-running vacuum tube oscillators
with tuned RF buffer stages, had broadband floors better than anything else
in HP/Agilent's inventory.  They did that with resonator Qs in the
four-digit range, no crystals or even cavities.

Use of passive crystal filters to shave the noise sidebands from another
source is also a common way to go, although I believe the newer Agilent
E5500 PN test hardware has gone back to SAW-based PLLs rather than
multiplier/filter chains as used in the '80s era.

>   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.

I have no personal experience with crystal oscillator optimization but Bruce
has some notes at http://www.ko4bb.com/~bruce/XTALOSC.html that might be
helpful.  Also it might be worth checking out Chris Bartram's article at
http://uwsdr.berlios.de/A%20New%20LNXO%20Topology.pdf .

>   I thought  the  noise in a 50 ohm resistor set  the  lower  limit to
>   -174dBc.

Where 'c' is 0 dBm into 50 ohms at room temp, yes.  Low-PN sources are
usually characterized at +10 dBm or higher, especially when driving the
mixer on a quadrature test set like the 3048A.

One further suggestion if you're getting into this stuff: check out Enrico
R.'s new book!

-- john, KE5FX

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