[HN Gopher] 2 GHz Active Probe
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2 GHz Active Probe
Author : hugolundin
Score : 73 points
Date : 2024-06-16 07:36 UTC (15 hours ago)
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| amelius wrote:
| Is there any open source DIY project to relatively cheaply
| measure the analog performance of USB 3.1 (10Gbit/s) signals?
| ComputerGuru wrote:
| What does "analog performance" mean in this context? Are you
| talking about noise, reflections, etc or something else?
| amelius wrote:
| I mean anything that can give more information than just the
| digital bit errors. Like eye diagrams and such. Basically
| something that can rate the signal quality on some scale
| wider than "works? yes/no". Bit error rate is typically too
| low to give a useful indication.
| hatsunearu wrote:
| with the setup you're implying, the eye diagram is all you
| can get, which to be honest you can extract a lot of
| information about transmitter-side parameters like the
| jitter, but you can't evaluate receive performance at all.
|
| receive performance evaluation is done by injecting known
| amounts of jitter and seeing if the receiver can tolerate
| that jitter.
| dbuder wrote:
| No, there was a DIY diode prob with a headline number high
| enough, but I think he said real world performance would be
| about 8ghz, that's only part of the problem anyway. I had a
| look, I can't find the forum post or the podcast he was on
| (amphour). Either rent an appropriate USB analyzer or scope. If
| you just want to verify the speed of your consumer equipment
| look for a software solution to measure throughput (there is
| significant protocol overhead). At the required speed you can't
| even use BNCs.
| rayray wrote:
| Given the required front-end bandwidth, ADC bandwidth, and
| processing power, I don't think so. The closest thing you can
| do is perhaps get your hands on a FPGA board that has >10G
| serdes, and use the internal eye monitoring features to check
| the eye diagram, mask margin, etc.
| labcomputer wrote:
| > Is there any open source DIY project to relatively cheaply
| measure the analog performance of USB 3.1 (10Gbit/s) signals?
|
| For this kind of thing (measuring eye diagrams and such, as you
| mentioned in another comment) you'd need a probe-oscilloscope
| system with (at minimum) 30 GHz at the probe tip. Realistically
| you'd want 50 GHz, and more is better. Recall that square waves
| are composed of odd harmonics of the fundamental (~10 GHz), and
| the more harmonics you capture the more fidelity you'll have to
| the original.
|
| I don't think there are any real-time scopes with this much
| bandwidth, so you will need a (equivalent-time) sampling
| oscilloscope. Keysight might make one, but it probably costs
| "call us" dollars.
|
| [Edit: Actually, it looks like the SD-32 sampling head for the
| Tektronix 11800 series of sampling scopes does claim a
| bandwidth of 50 GHz. That plugin does not have internal
| triggering though, so you would need to add a trigger
| recognizer head and arrange for an external trigger signal.
| Also, don't buy these used on eBay unless the sell offers
| returns because the sampling diodes in the frontend are very
| delicate.]
|
| One additional wrinkle is that super-speed USB uses a
| differential pair with a characteristic impedance of 50 Ohms
| (each side of the pair is 100 Ohms, and terminated with a 100
| Ohm resistor). That means you'll need a differential probe (or
| two probes and an oscilloscope that can do math).
|
| I don't know of any DIY project with those ambitions, but the
| most likely route for something DIY is a passive Z0 probe: You
| want the DUT-probe interface to look like an impedance-matched
| power divider. That calls for a probe impedance of 10x (or even
| 100x) the transmission line, so 500 or 5000 Ohms.
|
| Anyway, I don't know of any DIY project that fits your needs,
| but the terms to search for include "Z0" and "passive".
| amelius wrote:
| I was thinking of skipping the probe entirely, and just
| having a USB receptacle on the board, then have a front-end
| IC that is hopefully available and that is fast enough to do
| the sampling. Along with some circuitry to make the other end
| talk USB3.1.
| KK7NIL wrote:
| The old rule of thumb that you want the bandwidth to be 3 to
| 5x your baud rate is no longer true. It has been found that
| BW as low as 0.7x has very little effect in TDECQ and so the
| newest optical ethernet standards have moved to that for
| standards testing and the electrical standards will likely
| soon follow. This BW is also applied as a 4th order Bessel-
| Thompson instead of an oldschool 1st order gaussian response.
|
| Real-time scopes go up to 110 GHz now! Sampling scopes are
| unfortunately dying out now a days.
| CamperBob2 wrote:
| There is an interesting new 6 GHz sampling scope that could
| potentially be useful for some of this work:
| https://www.eevblog.com/forum/testgear/pocket-
| sized-6-ghz-1-...
|
| Haven't used it myself, but they seem to have a really
| solid customer support ethic, given the way the thread
| developed.
| wrs wrote:
| And re the original topic, that thread has a reference to
| a 2.7GHz active probe for $226!
|
| https://www.lasmux.com/product/single-ended-active-
| probes/
| nyanpasu64 wrote:
| Is the analog Nyquist frequency of the signal 0.5x the
| digital baud (effectively sampling) rate?
| bobmcnamara wrote:
| Not exactly. All other parameters the same, the digital
| baud rate plays into how much bandwidth the signal uses,
| but not directly where it is in the spectrum.
|
| Ex:9600 baud FSK signal centered at 440MHz still needs
| something capable of capturing around 440MHz, but this
| has nothing to do with baud rate. A 500MHz scope would do
| well for acquiring the signal.
|
| Practically radios often divide the problem into two
| parts, first use a mixer to shift the signal down and
| filter out the parts you don't want, then decode the FSK
| which remains at a much lower frequency.
|
| This is the governing limit: https://en.m.wikipedia.org/w
| iki/Shannon%E2%80%93Hartley_theo... and it considers
| things like SNR as well.
| photon_rancher wrote:
| Realtime scopes exist at this bandwidth but are crazy
| expensive, and then they have frequency-stitching artifacts
| all over the fourier plane. (Because they use mixing
| techniques)
|
| Sampling systems are about $20k - 50k without probes in this
| bandwidth.
|
| Keysight DCA-M and picoscope have the best public prices, and
| exfo has an eye analyzer in that price range. There's also
| DCA-X but nobody wants to pay 10k extra for a touchscreen.
|
| Unfortunately tek doesn't make electrical sampling systems
| anymore, just optical.
| KK7NIL wrote:
| Not that I'm aware of.
|
| You could generate an eye diagram with an old sampling scope or
| BERT for probably less than $2k, but those likely wouldn't have
| the proper clock recovery, which means your horizontal would be
| sort of pointless. You'd have to make a separate clock recovery
| module to sit between your USB 3.1 DUT and the instrument.
|
| The other option would be to try and make a real-time ADC with
| enough BW (at Tek we demoed USB 3.1 debugging with just 10 GHz
| on an MSO6B, even though compliance testing requires 12 or 15
| iirc) and memory depth to do the clock recovery in DSP. This
| would be a very significant challenge, but might be possible,
| depending what ADC's are on the market now a days.
| CamperBob2 wrote:
| _The other option would be to try and make a real-time ADC
| with enough BW (at Tek we demoed USB 3.1 debugging with just
| 10 GHz on an MSO6B, even though compliance testing requires
| 12 or 15 iirc) and memory depth to do the clock recovery in
| DSP. This would be a very significant challenge, but might be
| possible, depending what ADC 's are on the market now a
| days._
|
| It strikes me that if you ran it long enough, there is
| probably no reason why a sampling scope couldn't do the same
| thing. It would need even more DSP eggheadery than the
| realtime scope, of course.
| KK7NIL wrote:
| No because modern high speed serial uses clock recovery for
| the timing (implemented as a first or second order PLL in
| the receiver), so you need to implement this in your scope
| too. In a real-time scope this can be done in software but
| in a sampling scope it has to be done in hardware and then
| fed into the trigger.
|
| If you just trigger off the edges in the signal all you'll
| be measuring is the scope jitter.
| CamperBob2 wrote:
| If all you want is an eye diagram, you may not
| necessarily care about clock recovery, though. You'd just
| need a high degree of control over your own sampling
| clock (i.e., better than whatever you're trying to
| measure.)
| KK7NIL wrote:
| For amplitude (the vertical part of the eye), yes. For
| jitter (the horizontal part), which is usually the most
| critical, especially for NRZ protocols like USB 3.1, no,
| you need clock recovery.
| hatsunearu wrote:
| if you want to do this, you generally want a breakout board
| that breaks out the high speed differential signals to 50 ohm
| RF connectors and hook that up to a 50 ohm terminated real time
| scope, and not use a probe.
|
| also for 10gbps per lane, you need at minimum 5ghz bandwidth.
| cushychicken wrote:
| In case folks don't know about the "El Cheapo Special" - you can
| make very respectable 2GHz passive scope probes with 150ps rise
| times out of RG-174 coax cable and 1k resistors.
|
| Clip the cable in half, solder the core to the 1k resistor, and
| the other end of the 1k resistor to the signal you want to probe.
|
| Solder the shield braid to board GND.
|
| 50ohm terminate your scope, set probe amplification to 20x, and
| voila!
|
| Excellent tip recommended both by Paul Horowitz (Art of
| Electronics) and Howard Johnson (High Speed Digital Design).
| Works like a charm!
| 0l wrote:
| The catch however is that this can cause (relatively) high
| resistive loading, so isn't really suitable for all
| applications.
| jjoonathan wrote:
| It really is suitable for most transmission line
| applications, though, because transmission line impedance is
| a fixed target. Top-shelf 20-30GHz probes use this exact
| technique as their "front end" followed by a 50/100 ohm
| amplifier to lock in the noise figure:
|
| https://www.tek.com/en/datasheet/trimode%28tm%29-probe-
| famil...
| cushychicken wrote:
| True - but those are the exceptions, not the rule.
|
| Most modern systems aren't discrete transistor amplifiers
| where an arbitrary 1k to gnd will massively affect bias point
| or circuit operation.
|
| For digital interfaces, or power supplies below ~24V, it's a
| great high bandwidth probe option. Much cheaper than active
| probes too.
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