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2 GHz Active Probe

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Re: 2 GHz Active Probe

#21

Post by photon_rancher »

labcomputer wrote: post #7
amelius wrote: post #2

Is there any open source DIY project to relatively cheaply measure the analog performance of USB 3.1 (10Gbit/s) signals?

> 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 fundam…

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.

Re: 2 GHz Active Probe

#22

Post by KK7NIL »

CamperBob2 wrote: post #13
KK7NIL wrote: post #11

Earlier quoted context omitted.

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 w…

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 pro…

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.

Re: 2 GHz Active Probe

#23

Post by CamperBob2 »

KK7NIL wrote: post #22
CamperBob2 wrote: post #13

Earlier quoted context omitted.

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 pro…

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.

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.)

Re: 2 GHz Active Probe

#24

Post by KK7NIL »

CamperBob2 wrote: post #23
KK7NIL wrote: post #22

Earlier quoted context omitted.

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.

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.)

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.

Re: 2 GHz Active Probe

#25

Post by bobmcnamara »

nyanpasu64 wrote: post #15
KK7NIL wrote: post #10

Earlier quoted context omitted.

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. R…

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

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.

Re: 2 GHz Active Probe

#26

Post by KK7NIL »

nyanpasu64 wrote: post #15
KK7NIL wrote: post #10

Earlier quoted context omitted.

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. R…

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

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).

Re: 2 GHz Active Probe

#27

Post by KK7NIL »

bobmcnamara wrote: post #25
nyanpasu64 wrote: post #15

Earlier quoted context omitted.

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

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, fi…

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.

Re: 2 GHz Active Probe

#28

Post by labcomputer »

amelius wrote: post #8
labcomputer wrote: post #7

Earlier quoted context omitted.

> 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 fundam…

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.

> [...] 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.

Re: 2 GHz Active Probe

#29

Post by KK7NIL »

photon_rancher wrote: post #21
labcomputer wrote: post #7

Earlier quoted context omitted.

> 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 fundam…

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 to…

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).

Re: 2 GHz Active Probe

#30

Post by bobmcnamara »

KK7NIL wrote: post #27
bobmcnamara wrote: post #25

Earlier quoted context omitted.

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, fi…

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.

Oh that makes sense!
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