Using scope without probes? - Page 1
Forum section: electronics / testgear

author: K3mHtH
author_rank: Regular Contributor
timestamp: June 14, 2014, 05:11:24 pm
content: Hey guys, this might be a silly question but I figure I'd ask first.Can I run an audio signal directly into the BNC connector of an oscilloscope? I'm asking because I don't know what's inside of the probe, if there's some necessary isolation or something happening in there. I have a few DIY pieces of test gear that I use on the bench regularly that output audio, around 1-3VPpp and I'd like to add BNC connectors to them so I can attach them to my scope easier and more permanently.Is it literally as easy as sticking a BNC jack in one of these things and then running a BNC-BNC cable between the gear and scope?

author: edavid
author_rank: Super Contributor
timestamp: June 14, 2014, 05:13:11 pm
content: Yes, it's that easy.

author: KJDS
author_rank: Super Contributor
timestamp: June 14, 2014, 05:34:23 pm
content: Provided that the audio is at a sensible level then it can be connected directly. Probably not so good an idea to directly connect a big power amp to a scope

author: KL27x
author_rank: Super Contributor
timestamp: June 14, 2014, 08:14:32 pm
content: You can connect a big power amp to a scope. Nothing is gonna happen to the scope, as long as your amp isn't supplying 100V P-P.A power amp supplies a varying voltage. It can't force that voltage through the scope. The amp is supposed to be connected to speakers with an impedance of 4 or 8 ohms or so... that's what allows the amps to run freely. The scope input impedance is on the order of a Mohm. So there's absolutely nothing the amp can do to the scope.Voltage will kill a scope, not amperage. The probe resistance attenuates voltage. To measure high voltage, you need to add DC resistance. The 9Mohms on the 10x setting of the scope probe, for measuring up to 600V, gives a voltage attenuation of 10x. A straight input should theoretically be good for voltage approaching 60V, then, by comparison.When you remove the probe from the equation, what you need to start worrying about is what is the scope doing to the signal. There will be more loading of the signal.90% of the time I use my scope with no probes. Unless I have problems with undue loading of the signal, it's just a straight piece of 50 ohm coaxial probe cable and 50ohm terminator connected to my signal. I have measured the resistance of the scope probes I cut off, and they were in the 100 ohm range on the 1x settings. Some of my other scope probes measure closer to 200 ohms.I have also used a BNC connector and loose wires connected. No coaxial impedance there, at all. But scope probes are so cheap, it's easier to just buy cheap 50-100Mhz scope probes and cut them off. Then you get the color coding rings, too.

author: K3mHtH
author_rank: Regular Contributor
timestamp: June 15, 2014, 02:42:46 am
content: Quote from: KL27x on June 14, 2014, 08:14:32 pmSo should I include a 100 ohm resistor before the BNC connector then? It would be nice if the measurements were accurate as well.

author: c4757p
author_rank: Super Contributor
timestamp: June 15, 2014, 02:51:06 am
content: No need to. The oscilloscope inputs have 1 megohm impedance, so adding 100 ohms in series will change your voltage by just 0.01%.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: June 15, 2014, 03:26:40 am
content: Quote from: K3mHtH on June 14, 2014, 05:11:24 pmWhy not get a few of thesehttp://www.ebay.ca/itm/10x-RCA-Female-Jack-to-BNC-Male-F-M-Plug-Coax-Adapter-Connector-for-Camera-CCTV-/310656285459?pt=US_Surveillance_Cables_Adapters_Connectors&hash=item48548ebb13-or-http://www.ebay.ca/itm/Lot-of-10-ATLONA-Gold-Plated-RCA-Female-to-BNC-Male-Adapter-Connector-for-CCTV-/221079769320?pt=US_Audio_Cables_Adapters&hash=item33796190e8then you can just run plain old RCA cables.Anyways I assume you meant analog audio, single ended...

author: MarkL
author_rank: Unknown
timestamp: June 15, 2014, 05:48:59 pm
content: 1-3V audio levels are fine for direct input into a scope.What occurs to me is that if you're retrofitting BNC connectors onto existing audio equipment, just be aware that the outer shell of the BNC will now be connected to earth ground via the scope. You haven't said anything about the audio equipment and perhaps this is not a problem. But if the signal you're tapping is not floating or you're connecting to something like an amplifier with a bridged output, it may not be expecting one side to be grounded. You might also introduce hum into your audio via the ground path through the scope.There's no added isolation benefit even if you use a scope probe since one side will still be grounded. (With the exception of differential scope probes, which keep both sides floating).

author: German_EE
author_rank: Super Contributor
timestamp: June 15, 2014, 06:12:37 pm
content: Yeah, make sure that the signal you're measuring doesn't have any funky DC bias on it. As a young man I once connected my scope across the speaker terminals of my parents HiFi not knowing that it was a bridged output with each terminal +60V above chassis. The bang was rather loud.The secrets are a) learn from your mistakes and b) learn from the mistakes of others .

author: Dave Turner
author_rank: Frequent Contributor
timestamp: June 17, 2014, 05:32:19 am
content: Reinforces the point of eyes & brain first then multimeter and only then scope.

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: June 17, 2014, 05:50:09 am
content: Still for all practical it is probably make sense to connect output from amplifier to 16 or 8 ( or whatever the correct resistance ) ohm power resistor and then measure signal across that resistor. Looking at how output signal looks like when it is going into hi-Z load will be misleading.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: June 17, 2014, 05:47:50 pm
content: Quote from: Alexei.Polkhanov on June 17, 2014, 05:50:09 amI agree, especially with a tube amp, but even "only" 5W of audio power is going to be a mighty hot resistor. Get the appropriate size!Also use caution in case of the bias voltages mentioned earlier. Especially if it says "Martin Logan" on the speakers...

author: macboy
author_rank: Super Contributor
timestamp: June 17, 2014, 06:38:30 pm
content: Quote from: K3mHtH on June 15, 2014, 02:42:46 amThe reason for the apparent series resistance isn't that there is literally a series resistor, but that the probe cable itself has a very high resistance per length: 39 Ohm per foot is one figure I dug up! This is due to an extremely thin center conductor. This very fine wire has a very small surface area, and that means very low capacitance (to the shield). Normal RG-58 coax has about 25 pF/ft, and it would be very hard to make a high impedance, high bandwidth probe with cable like that. Remember that capacitance is a parasitic load that increases with frequency, so the small series resistance is a good compromise to get the extremely low capacitance.Probes look simple, but they are one of the most highly engineered parts of the oscilloscope.

author: alexwhittemore
author_rank: Frequent Contributor
timestamp: June 17, 2014, 08:43:10 pm
content: To answer the original question as directly as possible, "Can I just run audio straight into my scope without a probe?" - the answer is yes, but lots of things are simpler and better if you just use the probe.Signal integrity (you don't care): When dealing with high frequency systems, impedance matching is important. The scope has either a 1 MOhm input impedance or a 50 Ohm (cheaper scopes aren't selectable, and are just 1Mohm). Typical coax systems are 50, 75, or 100 ohms. When you mix impedances, the junctions can cause things like reflections and standing waves, such that what you measure in one spot (the scope itself) isn't the same as what is elsewhere in the system on the same electrical net. For example, you are measuring a 50-ohm source through 50-ohm coax into a 50-ohm load. Let's say you put a BNC tee in the middle somewhere and just run another bit of coax into your scope. If it's terminated properly, you won't see any reflections or standing waves or anything like that: what you measure at the scope is really what's happening on the line. If you have it in 1Mohm impedance mode, this isn't true: what the scope measures and displays may well be different from some arbitrary point elsewhere on that transmission line, like at the source or load (what the scope sees isn't what the load sees, perhaps). A properly engineered probe actually has losses in it to prevent this from being a problem when you're measuring real systems. Basically, what it does is present a 1Mohm or 10Mohm impedance AT THE EXACT POINT of the probe tip, so if you're measuring a transmission line you won't inadvertently add a big bit of line that can mess up the measurement by adding reflections. But at audio frequencies this isn't really a big concern.Safety: The probe is most useful in 10x mode and people usually leave it there. In 10x, it has an impedance of 10Mohm, which is to say it's basically 10-times safer from a voltage standpoint. With a 10x probe you could expect to measure 1000v without anything blowing up, whereas if you run that into the scope it will CERTAINLY damage it. Not that doing so is SAFE by any means.The important part here is that if you don't know exactly what you're measuring, but expect it should be safe for a scope, start the measurement with a 10x probe.Only switch to 1x when you verify that it's safe and desirable (by the way, you'll reduce your measurement bandwidth by 10x in most cases, though for audio no big deal). Someone above mentioned "it's not like you're measuring 100v peak to peak," but the thing with audio isyou actually might be.If you're measuring the output side of an amplifier, the voltages can actually be kind of high. Take my car subwoofer, 1200w at 4 ohms. Power=V^2/R, and if you solve for V, it's 70 in this example. That's RMS power, so RMS voltage. 70V rms on a pure sine wave is actually 100V.The other point about safety, as someone mentioned, is that scope ground is earth ground. I measured a system once that had a negative rail which was supposed to be floating. In fact, it was held at 12V above earth by an internal short, and this was a supply capable of sourcing 100A. When I hooked up the ground lead to my scope, it literally caught fire (it turns out, scope chassis can handle a lot of abuse). The proper way to make sure this isn't a problem is to take a DC measurement of the 'negative' to earth with a multimeter. But if you're being cautious, you can make a poor man's differential probe by using channel A for positive, channel B for negative, short the grounds together, and use math to calculate B-A.

author: KL27x
author_rank: Super Contributor
timestamp: June 17, 2014, 11:16:36 pm
content: QuoteOn all my probes, there appears to be some resistor inside the probe, itself, even at 1x setting. I had in mind to possibly recycle my cutoff scope probes as general instrument/DMM probes. They had on the order of 100 ohms of resistance in and of themselves on the 1x setting. I believe there's also a small capacitor inside between signal and ground.I've googled to figure out what is in there and why, and I'm just more confused. I found one schematic showing that there is supposed to be 1Mohm resistance in the probe on the 1x setting! All my probes are on the order of 50-200 ohm on the 1x setting (9Mohm on the 10x), if you substract ~100R for the cable and termination.

author: w2aew
author_rank: Super Contributor
timestamp: June 18, 2014, 12:24:47 am
content: Quote from: KL27x on June 17, 2014, 11:16:36 pmThe coax of 1x and 1x probes is *intentionally* lossy, mainly to de-Q the probing system. Dave did a really good video on this:

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: June 18, 2014, 05:23:02 am
content: Quote from: KL27x on June 17, 2014, 11:16:36 pmaries.ucsd.edu/najmabadi/CLASS/COMMON/ABC-probe.pdf