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Comment by KK7NIL | Hacker News Reader

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.

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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.

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/

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.

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!

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).

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