Ok to use BNC breakout wire as oscilloscope input for aux trigger - Page 1
Forum section: electronics / testgear

author: Infraviolet
author_rank: Super Contributor
timestamp: June 07, 2023, 05:01:49 am
content: My scope (Tektronix TBS1052) came with two Tektronix branded probes for the two signal inputs, but I'm planning some projects where I need to observe two waveforms at once relative to a third clocking waveform. I've used oscilloscopes a lot away from home and there are always stacks of spare probes lying around to put a third one on the auxillary input and thereby read the clocking waveform.But spare scope probes cost £30 minimum, £40 minimum for the same sort as I already have, BNC breakout leads (terminating in two clips, banana plugs or dupont leads) are <£4.Is it ok to use one for signals on an auxillary trigger input where you don't need great accuracy? It would only be for probing 5V and 12V circuits, so I assume the attenuation and anything else built in to probes to protect oscilloscopes from spike voltages aren't relevant here? I'm sure it would have nasty capacitances compared to a proper probe, but so long as that would only distort the measured signal on the BNC lead, and not affect (like causing ringing) measurements made on the real probes, is there any reason not to just use a BNC breakout lead in place of a proper probe?Thanks

author: bdunham7
author_rank: Super Contributor
timestamp: June 07, 2023, 05:11:20 am
content: Quote from: Infraviolet on June 07, 2023, 05:01:49 amYou can do it and I have, but you have to understand and accept the consequences. Your BW will be limited and there might even be circuit loading issues as you get near that limit. However, depending on the exact leads and how you have them laid out, it probably isn't any worse than a 1/10X probe set to 1X.

author: tautech
author_rank: Super Contributor
timestamp: June 07, 2023, 05:25:54 am
content: There are limitations of an external trigger input, especially of the voltages it can withstand. RTFM.

author: wasedadoc
author_rank: Super Contributor
timestamp: June 07, 2023, 07:48:37 am
content: You have not mentioned what frequency the clock signal is but those less than £10 probes at Aliexpress are easily good enough for triggers at several tens of MHz.

author: Pilatus
author_rank: Contributor
timestamp: June 07, 2023, 08:07:21 am
content: Not knowing your pertinent circuit details... Cheap probes, as already mentioned above.

author: Ian.M
author_rank: Super Contributor
timestamp: June 07, 2023, 08:29:03 am
content: "Clocking waveform" implies logic. Logic signals, unless slew rate limited, generally have fairly fast edges, and a cheapo clips to BNC cable into a 1 Meg scope input will give you horrible reflections and ringing on those edges, which will make reliable triggering difficult.One workaround if your signal is capable of driving a 1K load, is to terminate the scope end with 50 ohms (either internally if it has that option, or with a BNC T (m-m to f) with a 50 ohm terminator on the other arm of it), and add a 1K resistor in series with the signal, at the clip. That gives you 21:1 attenuation, but with a reasonably flat response and no reflections. Even better is to use a 950 ohm resistor for 20:1 attenuation.Ground clip lead length is still problematic for ringing - shorten it if nessercery!

author: Infraviolet
author_rank: Super Contributor
timestamp: June 07, 2023, 11:50:02 pm
content: I had mainly been looking at BNC breakout cables for this by checking electronic selelr sites and comparing to probes sold on those more official sites, if cheap probes from discount sites are acceptable enough (in terms of no risk of damaging the scope) then one of them might be as suitable an option, hadn't realised they sold that cheaply.

author: PwrElectronics
author_rank: Regular Contributor
timestamp: June 08, 2023, 12:45:44 pm
content: QuoteI've done this many times for cases where I had to semi-permanently have probes in place like when the board is in the thermal chamber or I have numerous higher speed things to monitor at the same time and cannot hold probes with short ground clips in place by hand. One issue can be the 1k load this presents to the circuit and I ran into the issue once where the op-amp driving it could not source the current and started clipping the signal. In those cases, I have used higher resistor values (larger divider ratio) to increase the load impedance.

author: Ian.M
author_rank: Super Contributor
timestamp: June 08, 2023, 01:20:38 pm
content: Yep. 4.93K 0.1% SMD, tombstoned, with the 50R coax center conductor cut short and soldered to its top end, and the braid split in two, pigtailed and soldered to directly adjacent ground pads either side of the signal in question + termination as above, is within 0.5% of 100:1, only a 10K load on the signal and good up to your scope's bandwidth or the limits of the coax, whichever comes first!

author: Infraviolet
author_rank: Super Contributor
timestamp: June 13, 2023, 02:02:32 pm
content: Just found something odd. Got a cheap BNC cable with the intent of making one of those DIY probes (as you describe with the resistor, and similar to Horowitz and Hill's tips too), seems a normal BNC doesn't fit my scope?It will slot on to the connecotr alright, and I can get a signal through, but the distance between the bayonet pin on the oscilloscope's BNC connecotrs seems ever so slightly shorter than the distance the BNC wire connector needs so it an rotate far enough to clip in. Has tektronix started using non typical BNC length sizes on scopes?

author: Ian.M
author_rank: Super Contributor
timestamp: June 13, 2023, 04:36:35 pm
content: Unlikely. More probable the cheap BNC lead's plug doesn't meet specs.

author: Infraviolet
author_rank: Super Contributor
timestamp: June 13, 2023, 09:03:27 pm
content: If I measure the cheap BNC connecotor with calipers from the "bottom" of the bayonet dip to the "face" of the locking ring I get:4.8mm(see attachment for dimension image)I can't make this measurement on my existing probes as they are plastic shielded around the BNC connector, so can anyone please check some BNC connectors they have to hand and see how this length turns out for them. The I'll know if the cheaplead has too long a length here.Thanks

author: Ian.M
author_rank: Super Contributor
timestamp: June 13, 2023, 11:00:21 pm
content: I don't think the measurement you took is necessarily meaningful, as the base of the locking ring doesn't contact any surface of the jack in normal use so its height relative to the contacts may vary. The position of the end holes of the spiral locking grooves relative to the contacts is what matters.Amphenol have mechanical specs for the BNC connector. Seehttps://www.amphenolrf.com/connectors/bnc-connectors.htmlThe Jack drawing at the bottom of the page has the key dimensions you need, giving the distance the locking pins are from the lip of the shell, and the min. clearance between them and the panel. (in fractional inches).Note thatRussian clones of BNCshave slightly different dimensions due to (deliberate) rounding of US customary measurements when converting to metric, so don't mate easily with BNCs.Try mating the offending BNC a few time with a sacrificial BNC jack, and forcing the ring round. The locking ring spring may well collapse a bit allowing it to fit.