This keeps needing to be pointed out every thread this appears on. Plenty of cables have eMarkers that claim "40Gbps" or whatever speeds, or support 240W PD...but absolutely cannot deliver either of those. And none of these consumer level testers can test either of those claims.
Resistance measurements can cursorily test claims of 240W but resistance changes with temperature which changes with current...so unless your tester puts 5 amps down the wire for 5 minutes and then tests resistance it's not a very good test.
I had a cheap no-name cable which claims 3.1 speeds (and thus needs a marker), though it lacks all differential pairs and borks every usb connection (after I cut it, it was obvious). No clue why it contains a marker chip but doesn't have any conductors. Likely for usb-pd but you do not want to put any power though it anyway as the conductors were cheap copper clad aluminium.
Yeah — "the eMarker said 40Gbps" is basically the same problem as trusting the print on the jacket.
For home use I've ended up treating real capability as an empirical property of the specific cable+port+device combo: does this exact cable drive this monitor at the mode I want, under load, for an hour? If yes, it gets a colored heat-shrink band and never leaves that use case. Everything else is a phone/charge cable until proven otherwise.
You need to generate a precise, calibrated, 40GHz signal, transmit it, then precisely measure the returned signal for phase, amplitude, and waveform integrity -at that speed.
Oscilloscopes and logic analyzers that run at that speed, are damn expensive.
But for bandwidth can't you just have two simple computers at either end that can send a 100GB file and if it arrives in 20 seconds, you know you have 40gbps of bandwidth?
> But for bandwidth can't you just have two simple computers at either end that can send a 100GB file and if it arrives in 20 seconds, you know you have 40gbps of bandwidth?
Because that is not reproducible and introduces all sorts of other random variables to your test.
Expensive equipment is the only answer here.
Its a bit like when you ask an electrician to install network cable (yuk !).
They consider their job done after a continuity test (if they bother to do a continuity test at all).
Meanwhile, if you put a Fluke on it, you will pick up all sorts of subtle faults that the electrician's continuity tester will never pick up, but will cause subtle and hard-to-diagnose problems when you try to push data over the cable.
Its the same thing with all those cheap USB testers.
They might have all sorts of fancy displays, but in the end they are no substitute for the real thing and will end up lulling you into a false sense of security.
If you buy decent cables from reputable companies (CableMatters, LTT, Apple, etc...) you could end up with a drawer full of high quality mystery cables. A cheap tester is probably all you really need to sort out that mess.
You mention a Fluke, which I'm assuming is in relation to an ethernet cable tester (made by Fluke). I've seen/used continuity testers, but not something better (I've been using iperf3) Could you link/include the sku/model number of what you are refering too?
You have to first identify and ensure the two devices and their ports are capable of reliably creating and absorbing that data rate too or you'll never know if it's the cable or the devices that causes the cables to fail. What if your devices can but only intermittently or a software/firmware update messes with it's capabilities?
Dedicated test equipment that can reliably and verifiably test these high speeds is niche and therefor expensive. There's not really a way around that issue.
But do you actually need that at the consumer level?
Make some properly formatted garbage, send it down the wire to the other end of the tester and see if you can read the result. It won't tell you if the cable is marginal, but how often is that actually an issue? Network cables are connected to hardware that retries, masking such errors.
You need to at least look at waveform integrity (which can be approximated, by doing things like looking for harmonics).
For example, you may send in a 40 GHz square wave, and get back a wave full of ringing, or a sine or sawtooth wave. It’s damn difficult to maintain clean square waves, at that frequency. The cable could also double as a microwave antenna.
Since it’s digital information, some waveform alterations are OK, but not too much. You need fairly clear state transitions. Ringing, or waveform distortion, can introduce extra (or fewer) transitions; thus, completely changing the data.
That's not really what I mean. You don't have to diagnose the signal, just the medium, which can tell you what will happen to the real signal. It requires a fast edge (or a very broadband sweep), but nothing much beyond that.
E.g., here's what a bad connector at the far end of an SFF-8654 cable looks like on a TDR: https://i.imgur.com/AiLnSuC.jpeg . This type of plot can be interpreted as impedance versus distance along the line. It could be made with a small handheld instrument, but currently a human is needed to read the tea leaves. While I was making that measurement, I couldn't help thinking that it would be nice if it didn't require either a 50 GHz VNA or equally-expensive training to read.
Equivalent-time sampling gets you out of needing an expensive network analyzer, but it doesn't help with the diagnosis itself. And of course when checking a USB cable you are most likely just after a go/no-go result, not the gory details. That further calls the market economics of such a gadget into question. Still, if somebody offered one at a reasonable price, they'd sell at least one to me.
As long as we don't get false positives. They are worse than false negatives, and is what happens, when you just read the markers from the chip. As we know well, lots of dodgy C.M.O.T. Dibbler-types will deliberately program the chips to emit bogus markers.
Testing in that way will only tell you if the two devices work with the given cable. It won't tell you what sort of margins you have. Not all devices are made equal.
A device isn't even necessarily better if it works with one cable when another doesn't, it might just happen to have a slightly lower impedance on its internal traces that happens to better match an out of spec cable, or any other one of a number of parameters.
If you want to test bandwidth, that's a five- or six-figure bit of gear.