[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
-177dBm/Hz.
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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>
Bruce
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