[labcomputer]

> 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".

[KK7NIL]

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]

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]

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]

Is the analog Nyquist frequency of the signal 0.5x the digital baud (effectively sampling) rate?

[KK7NIL]

There is no Nyquist frequency in analog. The Nyquist frequency only appears when we're going between a continuous (analog) system and a discrete (digital) system.

For a real receiver circuit you'd want to sample at least once a symbol obviously (so RX sample rate equals the baud rate). In practice you'd want more than that so you can better do DSP afterwards to remove channel losses and such. For a test instrument, it's going to depend on what exactly you want to see. Most standards define a required filter shape and BW for testing and you need enough sample rate in your scope to be able to implement this (usually in DSP).

[bobmcnamara]

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/wiki/Shannon%E2%80%93Hartley_theo... and it considers things like SNR as well.

[KK7NIL]

I think the discussion has been all about high speed serial, so it's correct to say that the signal bandwidth starts at DC (or very close to it, there's often AC coupling) due to NRZ or PAM encoding.

[bobmcnamara]

Oh that makes sense!

[photon_rancher]

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]

The only scopes using frequency mixing are the LeCroy scopes over 33 GHz and these have been commercial failures.

All the other scopes on the market use time interleaving (time multiplexing) which has much better characteristics but requires that the ASICs have much higher Ft, which requires a significant capital investment.

Sampling scopes are not actually that much cheaper now a days, especially after you take into account the cost of the clock recovery device that you would need to build a proper eye with a sampling scope (I discussed this further in other parts of this thread).

[amelius]

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.

[labcomputer]

> [...] skipping the probe entirely, and just having a USB receptacle on the board [...] long with some circuitry to make the other end talk USB3.1.

Practically, this probably ends up fairly similar to what I mentioned above, since "making the other end talk USB3" means receiving with the bits it sends and responding in a way it expects.

But, yes, in-situ / solder-down probes can definitely simplify things because it gives you the flexibility to do things like remove the existing termination (assuming the termination isn't in the receiver IC itself) and use the 1x Z0 soldered-in probe as the terminator.

I just probably wouldn't do this part:

> then have a front-end IC that is hopefully available and that is fast enough to do the sampling

This gets complicated fairly quickly. It sounds like you want both real-time sampling (so you can do something with the bits and "make the other end talk USB3") fairly high resolution (so can see something in the eye diagrams). That calls for a digitizer with like >=5 bits and a sample rate of >100 Gsps... Which basically doesn't exist, AFAIK. Oscilloscopes which sample that fast are interleaving multiple digitizers, and this isn't really something you can DIY.

However, if you just want eye diagrams, you can use ET sample techniques (running at a pedestrian ~10-100 ksps), together with something to synchronize the sample to the USB clock, to fill in the entire horizontal width of the eye diagram. There are several catches: First is that it takes a very large number of triggering events to fill in the diagram. Second, the 100 ksps sampler needs a very short capture window (on the order of picoseconds). Third is that you "miss" most bits in the data stream, so you don't capture any real USB data while making the eye diagram.

Fortunately, you can buy a used sampling oscilloscope which does this for you. It captures the samples, relates them to the USB clock, and places them in the correct place on the x-axis. You just need to construct probes to bring the signal into the scope. The 11800 I mentioned is one such example, but there are others, too.