[time-nuts] HP 5071A Electron Multiplier of Cesium Beam Tube
xde-l2g3 at myamail.com
Fri Sep 11 10:59:22 UTC 2009
"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?
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:
In "The Measurement of AM noise of Oscillators", Rubiola states "The
measurement systems described exhibit the world-record lowest
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:
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:
> 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
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"
Siemens App Note 002 shows the pcb layout for a 10GHz DRO:
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
"Low Phase Noise Division From X-Band To 640mhz"
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.
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 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
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)
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:
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:
So the trick is to use smaller parts and put them in smaller boxes.
Then fill them with Eccosorb:)
> john, KE5FX
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