Using an Oscilloscope - Page 1
Forum section: electronics / projects

author: Erica
author_rank: Regular Contributor
timestamp: May 06, 2015, 10:29:20 am
content: HelloI'm trying to find a high impedance audio signal on a telephone line.Can someone point out how to do this using an oscilloscope?Many thanks.E

author: Fungus
author_rank: Super Contributor
timestamp: May 06, 2015, 10:45:43 am
content: Quote from: Erica on May 06, 2015, 10:29:20 am"Very carefully".Under no circumstances should you connect the 'scope directly to the lines. Magic smoke will escape.It's illegal to do but I'm sure you'll find something if you google images of "telephone wire tap circuit".

author: Erica
author_rank: Regular Contributor
timestamp: May 06, 2015, 10:50:01 am
content: A thread author should have the right to delete any post in their thread.

author: Fungus
author_rank: Super Contributor
timestamp: May 06, 2015, 11:09:59 am
content: Quote from: Erica on May 06, 2015, 10:50:01 ama) Nope.b) Can we assume youdidn'tgo to google images and type "telephone wire tap circuit"? In that case what do you expect from people here?

author: Erica
author_rank: Regular Contributor
timestamp: May 06, 2015, 11:15:39 am
content: I did and I see nothingInstead I googled "oscilloscope telephone line"I found thishttp://www.edaboard.com/thread45343.html

author: Fungus
author_rank: Super Contributor
timestamp: May 06, 2015, 12:29:22 pm
content: Quote from: Erica on May 06, 2015, 11:15:39 amIn that case you're inbigdanger. Google is blocking search results to your computer based on keywords like "wire tap". They're probably watching this forum as well.It may already be too late to do anything except go into hiding.

author: tggzzz
author_rank: Super Contributor
timestamp: May 06, 2015, 12:51:02 pm
content: Quote from: Erica on May 06, 2015, 10:29:20 amYou'll find some good suggestions here:https://www.eevblog.com/forum/projects/telephone-eavesdropping/msg667646/#msg667646Oh, wait a minute, the OP has the same name! Now there's a turnout for the books! (Not)

author: nanofrog
author_rank: Super Contributor
timestamp: May 06, 2015, 02:40:33 pm
content: Why not start with the basics? TheOscilloscope Tutorialpage would be of real use (multiple primers in here that are well worth the effort).I'd recommend starting with the XYZ's of Oscilloscopes, followed by the one on probes.

author: G7PSK
author_rank: Super Contributor
timestamp: May 06, 2015, 02:55:43 pm
content: I made a little device some time ago when trying to look at the ADSL signal,it was just a bit of vero board with a telephone plug and a socket on it and 2 small capacitors to to take the sample with, the isolated the scope from the phone line, I did not get very much on the ADSL signal but the audio showed quite well. I just tried to find the thing to photo but it is now lost in all my other junk if I find it I will post a picture but it's nor really needed as it is so simple.

author: Fungus
author_rank: Super Contributor
timestamp: May 06, 2015, 04:13:34 pm
content: Quote from: G7PSK on May 06, 2015, 02:55:43 pmSo ... a bit like the first image I get when I type "telephone wire tap circuit" into google images.

author: paulie
author_rank: Frequent Contributor
timestamp: May 06, 2015, 04:27:39 pm
content: Can somebody explain what those yellow squares are in everybody's avatar? How come Blueskull gets 4 and me and Fungus only 2? Are we second rate citizens? Or related to number of times you say "shit"?

author: SeanB
author_rank: Super Contributor
timestamp: May 06, 2015, 05:45:29 pm
content: Post counter.....As to the OP, connecting a scope direct to the phone lines, as discovered, means you find out the phone line has -48V on the one leg when on hook, and around -7 to -15V when the phone is off the hook and acting like a roughly 600R load.If you want to see what is on the line with the scope you need to use 2 capacitors ( any value from around 47n to 1uF at a voltage from 160V to 400V) in series with the probe input and the probe earth . that way you only will get the Ac signals on the line, not the standing DC bias voltage.That way when the phone is on the hook, you should see only a small amount of noise on the line, mostly AC mains induced noise from the long line to the exchange. You might also see some what looks like broadband noise from your ADSL connection, or bleedthrough from the other ADSL connections that share the trunk cable to your local exchange and the DSLAM in the street by it.Any signal on it that looks like room noise would be from what is called an infinity bug, which will be picking up the room noise, using a sensitive microphone in the phone base. It typically is done with a module added to the phone, as the mic needs to be very sensitive, which the phone microphone and ringer is not.Typically though the 3 letter agencies do not tap phones, they prefer other methods, like using a separate microphone placed secretly in the room, and by using the thing you all have, a mobile phone, which has a very sensitive microphone already and decent DSP, for use in the hands free mode.

author: Erica
author_rank: Regular Contributor
timestamp: May 06, 2015, 05:53:13 pm
content: Quote from: SeanB on May 06, 2015, 05:45:29 pmJudging from your response you have read my other thread.When I connected the scope to the telephone line , the phone stopped working. A poster mentioned that I need a diffential scope. Is that correct?Can you show me some images of what this capacitor connection should look like. I don't know what a capacitor is or does.I am maybe out of my league.

author: PA0PBZ
author_rank: Super Contributor
timestamp: May 06, 2015, 05:58:21 pm
content: Quote from: Erica on May 06, 2015, 05:53:13 pmIf your scope has more than one channel (it should if I remember the model correctly) you can use 2 channels to probe the signal. Clip the 2 ground leads together but don't connect them to anything else. Use the 2 probe tips to connect to the area of interest and set the scope so that the channels are added and invert one channel. There you have your differential probe.

author: Erica
author_rank: Regular Contributor
timestamp: May 06, 2015, 06:00:09 pm
content: Quote from: PA0PBZ on May 06, 2015, 05:58:21 pmAwesome!

author: Erica
author_rank: Regular Contributor
timestamp: May 06, 2015, 06:01:52 pm
content: Why do I need to connect capacitors to take out the DC signal. Can't I just put the scope into AC coupling mode to get AC only?

author: PA0PBZ
author_rank: Super Contributor
timestamp: May 06, 2015, 06:06:35 pm
content: Quote from: Erica on May 06, 2015, 06:00:09 pmAlso, set the inputs to AC coupling and the input attenuators the same on both channels.Probably an unnecessary comment for most, but you seem to be a beginner with a scope

author: Erica
author_rank: Regular Contributor
timestamp: May 06, 2015, 06:12:29 pm
content: Quote from: PA0PBZ on May 06, 2015, 06:06:35 pmThanks.So this will measure AC only and ignore the DC component?Do you think 100x probes may be better able to detect the weak signal?

author: PA0PBZ
author_rank: Super Contributor
timestamp: May 06, 2015, 06:15:55 pm
content: Quote from: Erica on May 06, 2015, 06:12:29 pmYes, and because the DC component is a lot larger than the AC you are looking for it is impossible to see the AC component with the input dividers set to a level that you can display the DC.QuoteWhy would you attenuate the signal 100x when looking for small signals? That makes no sense.

author: paulie
author_rank: Frequent Contributor
timestamp: May 06, 2015, 06:16:59 pm
content: Quote from: SeanB on May 06, 2015, 05:45:29 pmCan't be total count because Blueskull had very few but lots of squares compared to me and Fungus. Must be some kind of 24 hr or so average? I wonder if there's an explanation somewhere?ps. The more "x"s in a probe the less sensitive. IE 1x better for weak signals than 10x. 100x would be virtually useless for measuring microvolt levels.

author: PA0PBZ
author_rank: Super Contributor
timestamp: May 06, 2015, 06:19:03 pm
content: Quote from: paulie on May 06, 2015, 06:16:59 pmHe's a supporter as you can see, that gets you 4 immediately. I lost one when I stopped supporting via the forum and started using patreon. I don't care

author: paulie
author_rank: Frequent Contributor
timestamp: May 06, 2015, 06:25:13 pm
content: Quote from: PA0PBZ on May 06, 2015, 06:19:03 pmSo a supporter gets more? What about athletic supporter? So not related to post count? Or to both? The mystery deepens.

author: Erica
author_rank: Regular Contributor
timestamp: May 06, 2015, 06:25:45 pm
content: Quote from: paulie on May 06, 2015, 06:16:59 pmNow I'm confused.http://www.radio-electronics.com/info/t_and_m/oscilloscope/oscilloscope-probes.phpThat site says:"To enable better accuracy to be achieved higher levels of impedance are required."When I read what they say about 10 and 100x probes I get the impression that more x's mean better accuracy.

author: PA0PBZ
author_rank: Super Contributor
timestamp: May 06, 2015, 06:26:31 pm
content: Quote from: paulie on May 06, 2015, 06:25:13 pmNon-supporter: Based on post countSupporter: 4Don't make it more difficult than it is...

author: Erica
author_rank: Regular Contributor
timestamp: May 06, 2015, 06:27:38 pm
content: Quote from: blueskull on May 06, 2015, 06:23:36 pmI'm sorry but I have no idea what this all means!

author: PA0PBZ
author_rank: Super Contributor
timestamp: May 06, 2015, 06:27:56 pm
content: Quote from: Erica on May 06, 2015, 06:25:45 pmYes, because the load on the circuit is smaller. Totally irrelevant for what you are trying to do.

author: PA0PBZ
author_rank: Super Contributor
timestamp: May 06, 2015, 06:28:57 pm
content: Quote from: Erica on May 06, 2015, 06:27:38 pmHe's basically saying the same I did, but using more grown-up words.

author: Erica
author_rank: Regular Contributor
timestamp: May 06, 2015, 06:30:42 pm
content: Quote from: PA0PBZ on May 06, 2015, 06:27:56 pmOk. Let me give it a shot.I hope I can find the audio signal.

author: PA0PBZ
author_rank: Super Contributor
timestamp: May 06, 2015, 06:38:52 pm
content: Quote from: blueskull on May 06, 2015, 06:25:39 pmI can still access the supporters lounge by the way, but I also get an email from Patreon when there's a new video to watch.(to stay off topic a bit more...)

author: PA0PBZ
author_rank: Super Contributor
timestamp: May 06, 2015, 06:49:15 pm
content: Quote from: blueskull on May 06, 2015, 06:45:01 pmYes, I remember that now. Anyway, no need to pay extra to get the 4thsquare back.

author: lincoln
author_rank: Regular Contributor
timestamp: May 07, 2015, 04:01:28 am
content: Quote from: Erica on May 06, 2015, 10:50:01 amKind of a rough first day on the internet?

author: tautech
author_rank: Super Contributor
timestamp: May 07, 2015, 04:18:52 am
content: Quote from: Erica on May 06, 2015, 06:25:45 pmWrong.1, 10, 20, 50, 100x etc denotes the DIVISION of the waveform amplitude by the probe.Correctly expressed it is 10:1, 100:1 and so on.For general usage best sensitivity is gained with a 1:1 probe.Probe and scope impedance must also match for correct waveform reproduction.As explained by others the lines carry a voltage offset with any signal present riding on the offset.The simplest method to scope this will be to use AC coupling and proceed as blueskull has outlined.

author: Erica
author_rank: Regular Contributor
timestamp: May 07, 2015, 06:50:25 am
content: Quote from: tautech on May 07, 2015, 04:18:52 amMaybe I don't understand this. When I read the wiki page on impedance matchinghttp://en.m.wikipedia.org/wiki/Impedance_matchingI get the impression that minimum reflection is obtained when the impedances of the source and load are the same.In this case I think the phone line is the source and the load is the probe. Therefore the higher the impedance on the line, the higher the impedance on the probe should be.

author: kjs
author_rank: Regular Contributor
timestamp: May 07, 2015, 06:53:34 am
content: but your phone line is already terminated by the phone itself. In theory you shouldn't connect anything but the impedance of even a 1x probe is that high that in parallel to the 600R of the phone you won't recognize it. However, if you divide the available voltage by 10 or even worse 100 your signal gets really small and all you see on your scope is the noise it generates on its own.

author: Erica
author_rank: Regular Contributor
timestamp: May 07, 2015, 07:20:13 am
content: Quote from: blueskull on May 07, 2015, 07:09:16 amReally? I'm just about to try this with 1x.You're certain the oscilloscope will be damaged?I'm connecting the phone to the line and the scope to the line. I want to dial my phone and see the AC on the scope.Is this then a bad idea?

author: tautech
author_rank: Super Contributor
timestamp: May 07, 2015, 07:23:59 am
content: Quote from: blueskull on May 07, 2015, 07:09:16 amNot if the scope input is AC coupled.

author: tautech
author_rank: Super Contributor
timestamp: May 07, 2015, 08:24:54 am
content: Quote from: blueskull on May 07, 2015, 07:37:28 amSo scope will display a 25 Hz ringtone, yes?What scope are you talking about with ONLY 30 Vpk inputs?

author: kjs
author_rank: Regular Contributor
timestamp: May 07, 2015, 08:40:45 am
content: Quote from: tautech on May 07, 2015, 08:24:54 amI would really like to know that too. Mine has no problem when I touch 240VAC with the probe.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: May 07, 2015, 11:56:17 am
content: Quote from: tautech on May 07, 2015, 08:24:54 amJeez, there's 600V caps on the inputs of my 1A4.

author: G7PSK
author_rank: Super Contributor
timestamp: May 07, 2015, 12:08:18 pm
content: Sounds like it's either a soundcard scope or a usb one.But even with a proper scope it is a good idea to put a cap in the ground line of the probe when looking at things like phone lines so as not to short the phone line to ground,both lines are live on the phone.

author: Erica
author_rank: Regular Contributor
timestamp: May 07, 2015, 01:34:19 pm
content: Quote from: lpc32 on May 07, 2015, 01:30:09 pmI have this already.What do you intend to do with it?

author: lpc32
author_rank: Frequent Contributor
timestamp: May 07, 2015, 02:22:09 pm
content: Quote from: Erica on May 07, 2015, 01:34:19 pmJust skimmed. Ignore.

author: tautech
author_rank: Super Contributor
timestamp: May 07, 2015, 08:49:59 pm
content: Quote from: Alex Eisenhut on May 07, 2015, 11:56:17 amExactly.Quote from: blueskull on May 07, 2015, 02:15:10 pmGood on you for being aware of the limitations of your equipment.But as you are probably aware "stand alone" scopes have either 300 or 400 Vpk BNC inputs.Add a 10:1 or 100:1 probe and the waveforms that can be examined can be quite high voltages,depending on the voltage and frequency rating of the probe.

author: Fungus
author_rank: Super Contributor
timestamp: May 08, 2015, 10:37:06 am
content: Quote from: Alex Eisenhut on May 07, 2015, 11:56:17 amSo? You don't know anything at all about the grounding of the phone line relative to the mains socket your 'scope is plugged into.Better stick a multimeter on it first to make sure it's not more than 600V difference.