Changing oscilloscope input range for signal generator - Page 1
Forum section: electronics / beginners

author: medical-nerd
author_rank: Regular Contributor
timestamp: May 01, 2017, 02:00:58 pm
content: HiyaI've been watching Dave's tutorial on scope probes and understand that x1 setting has a 1M resistor which he mentions is internal to the scope.How then can there be settings less than this? - are there switchable resistances internally - if so what setting should I use if I want to connect the 50ohm output from a signal generator directly to a scope input with a coax patch lead without affecting the waveform displayed ? Do I need to worry about termination impedance ?ThanksSteve

author: rstofer
author_rank: Super Contributor
timestamp: May 01, 2017, 02:29:05 pm
content: It depends on the scope. Some scopes have a selectable 50 Ohm internal impedance (plus the usual 1 MOhm) but many DSOs do not. If you want to terminate the line, use a BNC T fitting at the scope and plug a 50 Ohm terminator in the open port.Whether you need to worry about termination depends on frequency and the higher it is, the more important matching will be. At 100 kHz you don't even need coax. So, it depends...

author: TimFox
author_rank: Super Contributor
timestamp: May 01, 2017, 08:31:56 pm
content: More convenient than a TEE with 50-ohm terminator is a feed-through 50-ohm terminator, with a male and female connector at opposite ends, which takes up less panel space on the CRO input panel. If your signal generator has a calibrated output, it probably assumes voltage into a 50-ohm load. At higher frequencies, an unterminated long coaxial cable will suffer from reflections that will affect the final voltage, or distort a fast-rising waveform (pulse, square wave, etc.).

author: james_s
author_rank: Super Contributor
timestamp: May 01, 2017, 08:48:51 pm
content: X1 setting does not have any resistor, the probe is then effectively just a wire. The 1M termination is within the scope itself, or 50 Ohm can be selected in some as mentioned above.

author: rdl
author_rank: Super Contributor
timestamp: May 01, 2017, 09:13:25 pm
content: In the past, a T-fitting with a 50 ohm terminator was roughly 1/5 or less the cost of a 50 ohm feed-through, but a bit awkward to use. I have never understood why the feed-through type is so expensive. Lately though, I have found what appear to be decent feed-through types on ebay for reasonable prices, under $20, and I did buy a couple. So far, they have not fallen apart, and the connectors fit correctly on my equipment.

author: TimFox
author_rank: Super Contributor
timestamp: May 01, 2017, 10:29:33 pm
content: I have bought many 50-ohm terminators (of both types) through e-Bay. It is a good idea to check every one with a normal ohmmeter--many of mine were way off resistance, or even open circuit. Some were much better than 1%. You can disassemble the bad non-feedthrough units and substitute a different value resistor (75 or 600 ohms) if you need a different termination.

author: james_s
author_rank: Super Contributor
timestamp: May 01, 2017, 10:50:44 pm
content: Some cheap terminators are just a BNC with a 50 ohm resistor across it, there is a Jim Williams app note that mentions the importance of properly designed terminators. Proper terminators from reputable brands are not very expensive.

author: rdl
author_rank: Super Contributor
timestamp: May 01, 2017, 11:56:18 pm
content: Pomona 50 ohm BNC feed through is $45, 50 ohm end cap terminator is $9. Other manufacturers prices I have seen are similar with respect to the difference. From China the price runs 1/2 to 1/4 or even less, but who knows how good. Used name brands are probably a safer choice, but are often not that cheap, maybe 1/2 to 1/3 of new cost. The point actually was that feed through types are significantly more expensive than end cap termination, at least from the same supplier.

author: james_s
author_rank: Super Contributor
timestamp: May 02, 2017, 12:01:08 am
content: I think the terminators I bought were around $2.50 each from Digikey. They're not Pomona but they do seem to be reasonably well made. I've just seen so many fake/counterfeit/junk parts on ebay recently that I'm starting to avoid stuff like that in most cases.

author: tautech
author_rank: Super Contributor
timestamp: May 02, 2017, 12:05:35 am
content: Member whitevamp in the US had some Tek 50feedthroughs available in this thread:https://www.eevblog.com/forum/buysellwanted/fs-us-misc-tek-parts-and-fairchild-voltage-regulator/I bought something else from him, no hassles and it was perfect.

author: medical-nerd
author_rank: Regular Contributor
timestamp: May 03, 2017, 02:30:36 pm
content: HiyaThank you for the replies, I think I will cobble together a terminator (50ohm across BNC) while looking to buy a few feedthrough ones.Now a couple of really stupid questions regarding this.1. Do I consider the design of a terminator the same as that for a dummy load??2. Would a 50ohm 3dB SMA attenuator (with BNC adaptors) act as a feedthrough terminator with the obvious reduction in signal accounted for?CheersSteve

author: tautech
author_rank: Super Contributor
timestamp: May 03, 2017, 06:01:52 pm
content: Yes and yes.If you don't know about these things they don't seem obvious but remember our replies help others too so never be too shy to ask a question even if you think it's stupid.

author: TimFox
author_rank: Super Contributor
timestamp: May 03, 2017, 07:22:33 pm
content: No, a 50-ohm 3dB attenuator is not a good termination, but if you add a mediocre terminator to a good attenuator, the attenuator will improve the termination over just the mediocre terminator. An unterminated 3dB attenuator has an input resistance of approximately 150 ohms.

author: tautech
author_rank: Super Contributor
timestamp: May 03, 2017, 07:26:45 pm
content: Quote from: TimFox on May 03, 2017, 07:22:33 pmYep, but you missed the word "feedthrough".

author: TimFox
author_rank: Super Contributor
timestamp: May 04, 2017, 08:01:30 pm
content: No. Question 2 was if the attenuator plus BNC adapters would act as a feedthrough terminator. Since the shunt resistance of the attenuator is not 50 ohms, it is not a suitable termination.

author: tautech
author_rank: Super Contributor
timestamp: May 04, 2017, 08:49:05 pm
content: Quote from: TimFox on May 04, 2017, 08:01:30 pmHmmmWe're talking about sig gen termination @ 50into a scope without 50inputs, right ?I have Tek 50BNC feedtrough terminators, 1x and 10x for just this use when scopes don't have 50inputs.The OP proposes to use similar in SMA with BNC adaption so that he then has 3dB attenuation and make allowances for such.To quote him:QuoteIt's clear to me that he understands he must make allowances for a 3dB signal reduction, why is it not for you ?There are two impedances to consider, firstly the 50shunt resistance AND the feedthrough resistance/attenuation.With a known accurate sig gen and using his contrived feedthrough his displayed signal should be 3dB down, easy to check.Please educate me to what I overlook.

author: medical-nerd
author_rank: Regular Contributor
timestamp: May 04, 2017, 08:58:56 pm
content: I'm clearly out of my depth here.Can you recommend a text biased towards using test equipment in an amateur homebrew environment.I'm interested in building receivers covering shortwave to UHF.I do have rtl-sdr dongles, but also want to do it the old-fashioned analogue way.I realise this is a waste of time with SDRs available but it is what I would find most fulfilling.I've collected some old test equipment including:HP5335A counter;Systron-Donner 6054B counterHP8656B signal generatorTF2370 spectrum analyserHP8754A network analyserRacal-Dana 9301A rf millivoltmeter'DS1054Z oscilloscope'and the obligatory DIP meterSo I'm definately retro orientated.Cheers

author: TimFox
author_rank: Super Contributor
timestamp: May 04, 2017, 10:51:14 pm
content: Read the thread. The question was what to use for a termination if the oscilloscope does not have a 50-ohm position on the input. We were discussing the use of BNC tee with cap terminator, or a feedthrough terminator, to get the required 50-ohm termination. The attenuatorby itselfwill not make a 50 ohm termination.

author: tautech
author_rank: Super Contributor
timestamp: May 04, 2017, 11:03:00 pm
content: Quote from: TimFox on May 04, 2017, 10:51:14 pmUnless we know and see the attenuator the OP intends to use you may have some point.The Tek's I use specify on the unitboththe impedance and the attenuation: eg. 10x 502W.These work as expected givingbotha 50load and 10x attenuation.

author: timb
author_rank: Super Contributor
timestamp: May 04, 2017, 11:31:05 pm
content: Quote from: tautech on May 04, 2017, 11:03:00 pmI don't think this is correct...I have a bunch of Tek attenuators as well and they do specify the impedance as 50 Ohm, but that doesn't mean they act as terminators.Think about it: A 50 Ohm BNC cable doesn't act as a terminator, right? A 50 Ohm BNC tee adapter doesn't act as a terminator, right? They're just telling you the characteristic impedance it's designed for. You wouldn't want to use a 75 Ohm attenuator, cable or tee adapter on a 50 Ohm system.To me, this is backed up by the fact that all the Tek service manuals I've seen call for the attenuators to be used *with* 50 ohm termination behind them (where needed).

author: tautech
author_rank: Super Contributor
timestamp: May 04, 2017, 11:44:20 pm
content: Quote from: timb on May 04, 2017, 11:31:05 pmIt is if you have the correct one. See table in link:http://www.alliedelec.com/m/d/4131d9983037410f04b31b9863bccd34.pdf

author: TimFox
author_rank: Super Contributor
timestamp: May 05, 2017, 01:12:26 pm
content: Which one do you mean in that table? All the Tek attenuators I used in the past required a 50 ohm termination after the attenuator, as do all the SMA attenuators in the Pasternack catalog.It's not obvious to one unfamiliar with coax attenuators that the input impedance of an unterminated unit is not the characteristic impedance.However, as the attenuation increases, the input impedance does approach the characteristic impedance asymptotically. At 20 dB (10:1), the unterminated input would read 51.010 ohms. Furthermore, the voltage ratio with a high-impedance load is only 5.05:1, not 10:1.The post asked about a 3 dB attenuator, which gives 150.450 ohms.(See the Pasternack website athttps://www.pasternack.com/t-calcualtor-tee-attn.aspx?gclid=CP_T35vr2NMCFZG2wAod_iUBCg)