Scope Measurements are all double above what they should be - Page 1
Forum section: electronics / beginners

author: properharmony
author_rank: Newbie
timestamp: August 19, 2020, 05:44:12 pm
content: Ok so this is a pretty early on newbie question so please be kind.I watched Alan's tutorial on "why are my oscilloscope measurements wrong?" and that shed a little bit of light on this, but I'm still kinda stumped on another riddle within it.So recently after fixing a Tek 2467B I inherited, I found the input coupling option of DC 50 ohms. I thought, "sweet. plug signal generator in there...Vp-p should be correct." Tested and works just fine; makes sense with matched impedance. Checked against DMM * sqrt(2) gave me the same reasonably accurate number in going from RMS back up to peak.However, if I use a passive probe on basically any AC source (computer's audio output,signal generator, bench stereo with 1k test tone, etc), the Vp-p is doubled what it should be. 1x and 10x just does exactly those operations perfectly but the number itself remains double what it should be with a passive probe. I checked this on my Rigol 1054 and...embarrassingly...I've never noticed this before. I simply assumed what I was seeing was in fact what I should be seeing on that scope. But nope. It does this same thing too; peak to peak is 2x what it should be. Is this me just not knowing what I'm doing (likely)? What's happening here? Anyway thanks for any help clarifying. It hasn't been exactly easy to figure out where to look to figure this out and I'm kinda new to this subject.

author: Benta
author_rank: Super Contributor
timestamp: August 19, 2020, 07:02:32 pm
content: If you're using 50 ohm 'scope inputs, your source must also be 50 ohms. Then it'll be correct.Using a non-matched source is looking for trouble.Use the normal 1 Mohm input with suitable probes instead. 50 ohms is for specialists.

author: tggzzz
author_rank: Super Contributor
timestamp: August 19, 2020, 07:34:30 pm
content: A signal source will have a defined output impedance, Ro. For RF sources Rois 50ohms or 75ohms, for audio it may be 600ohms. The output voltage is defined when driving a a load Rl, where Rl=Ro.Rland Roform a potential divider which halve the source's internal(Thevenin) voltage. If Rlis replaced by 1Mohm, then the full interna! voltage appears at the output.So, to get the stated voltage, always use the correct load resistance.

author: SilverSolder
author_rank: Super Contributor
timestamp: August 19, 2020, 08:10:14 pm
content: Quote from: properharmony on August 19, 2020, 05:44:12 pmJust to completely understand your question let's narrow it down: for example, if you probe your bench stereo's speaker output with the scope, the Vp-p is twice what it should be? (How do you know?)

author: magic
author_rank: Super Contributor
timestamp: August 19, 2020, 08:33:58 pm
content: Just for the record, with a sine waveVpp = 2·sqrt(2)·VrmsRight?

author: Electro Fan
author_rank: Super Contributor
timestamp: August 19, 2020, 08:38:24 pm
content: To simplify, if the source (such as a generator) is set to HiZ1M ohms, set the 2467B to 1M ohms; if the source is 50 ohms, set the 2467B to 50 ohms (easy to do with the button corresponding to the channel). (btw, congrats on the 2467B!)With the 1054 you are always going to be on 1M ohms unless you get and use a "feed-thru terminator". Probably good to have a couple of these on hand (or maybe 4 if you use 4 channels at the same time).https://www.tequipment.net/RigolADAPTOR-50-OHM.html?v=0https://www.ebay.com/itm/Tektronix-011-0049-01-50-ohm-2-watt-BNC-feed-thru-Termination-/254654717704?epid=1600623857&hash=item3b4a9a9f08%3Ag%3AhBUAAOSwwiVfD14d&nma=true&si=a3cPlcDQpr9YJEHgzXAXBBCDk6M%253D&orig_cvip=true&nordt=true&rt=nc&_trksid=p2047675.l2557Edit made to address tggzzz's good point below.

author: tggzzz
author_rank: Super Contributor
timestamp: August 19, 2020, 09:00:34 pm
content: Quote from: Electro Fan on August 19, 2020, 08:38:24 pmGood luck finding a generator with an output resistance of "1M ohms" - for most purposes that could only be a low valuecurrentsourceI think it would be useful to review the theory of Thevenin voltage sources and voltage dividers, and apply that to this situation.

author: Benta
author_rank: Super Contributor
timestamp: August 19, 2020, 09:18:51 pm
content: OK, let's go completely back to basics:An oscilloscope measures voltages, right?And in that case it's best if it has infinite input resistance and zero input reactance, to minimize influences on the circuit it's measuring.This is why 'scopes normally have an input resistance of 1 Mohm, and with a x10 probe 10 Mohm.The 50 ohm input of a 'scope is not intended for normal voltage measurements. It's intended for matched power measurements in specific situations.The model for 50 ohm measurements is:An ideal voltage output signal generator (=0 ohm output impedance).A 50 ohm series resistor between output and 'scope input.This is a 50 ohm signal generator.Now the 50 ohm series resistor and the 50 ohm 'scope input impedance form a voltage divider. Due to this, the 'scope will only display the correct output voltage from the generator when multiplying the signal by 2. If your signal generator has 0 output impedance, the voltage divider is gone.This may be useful in certain RF measurements, but youreallyneed to know what you're doing. Any impedance mismatch will lead to wrong results.My recommendation is: stick with the 1 Mohm mode, this always works.

author: tggzzz
author_rank: Super Contributor
timestamp: August 19, 2020, 09:38:17 pm
content: Yes, but the voltage measured will be twice what the generator is set to generate. That's the OP's question - why?

author: Benta
author_rank: Super Contributor
timestamp: August 19, 2020, 09:44:46 pm
content: Quote from: tggzzz on August 19, 2020, 09:38:17 pmNo. The voltageshownis twice the generator voltage due to the missing 50 ohm source impedance.

author: tggzzz
author_rank: Super Contributor
timestamp: August 19, 2020, 10:03:42 pm
content: Quote from: Benta on August 19, 2020, 09:44:46 pmFor that to make sense, you are going to have to explain the difference "shown" and "measured".

author: bdunham7
author_rank: Super Contributor
timestamp: August 19, 2020, 10:07:19 pm
content: If you observe the same thing on your Rigol, I'm sure you are just making a math error. The discussion of impedance matching is a little off--you usually do not want to 'match' impedance when doing measurements, especially when you are well below the transmission line domain (your cables are shorter than 1/10 of the wavelength of your signal). The reason it works out with signal generators is that, depending on the specific device and settings, the generator isexpectinga 50R load and is displaying the output amplitude that would result. If the signal generator has an output impedance of 50R and is expecting a 50R load but is connected to a 1M or 10M meter or scope, the voltage will, in fact, be double.You mentioned audio amplifiers, which are designed to have a very, very low source impedance and yet should be measured with an actual speaker-like load and a high-impedance DMM or scope. So, if you would, set up your audio amp to put out 1Vrms @ 1kHz as measured by a decent DMM, then show us what you have on the scope. What you should have is 2.818 Vp-p or so.

author: Benta
author_rank: Super Contributor
timestamp: August 19, 2020, 10:20:01 pm
content: Quote from: tggzzz on August 19, 2020, 10:03:42 pmWe're into semantics. Play with your 'scope, and you'll see what I mean.

author: Electro Fan
author_rank: Super Contributor
timestamp: August 19, 2020, 10:32:00 pm
content: From Siglent 2042X manual (in case this helps connect theory and practice):4. Channel Parameters AreaDisplays the load and output settings of the currently selected channel.Load ----Value of the output load, as selected by the user.Press Utility → Output → Load, then use the softkeys, number keys or knob tochange the parameter value; or continue pressing the corresponding outputkey for two second to switch between High Impedance and 50Ω.High Impedance: display HiZ.Load: display impedance value (the default is 50Ω and the range is 50Ω to100kΩ).Note: This setting does not actually change the instrument‘s output impedanceof 50Ω but rather is used to maintain amplitude accuracy into different loadvalues

author: tggzzz
author_rank: Super Contributor
timestamp: August 19, 2020, 10:43:18 pm
content: Quote from: Benta on August 19, 2020, 10:20:01 pmIt is more than semantics; I've been intimately familiar with this for 45 years, so I do know exactly what is happening and why!Your statement "The voltageshownis twice the generator voltage" is false.The generator is modelled as a Thevenin source of a voltage Vsin series with a resistor Rs. If there is no load then the output voltage is Vs.The control on the generator indicates a voltage Vg(i.e. not Vs) when the generator is connected to a load Rl= Rs. When correctly loaded like that, the output voltage is Vs/2 = Vgdue to the potential divider action of Rsand Rl.

author: properharmony
author_rank: Newbie
timestamp: August 19, 2020, 10:54:52 pm
content: Quote from: bdunham7 on August 19, 2020, 10:07:19 pmAh bingo! What you had for that the value is essentially what I got (2.87Vpp on the Tek). Best I could do to bump the volume there was 0.989Vrms on my Fluke DMM. Which I now see that the value you said, and essentially what I saw is 2*sqrt(2).

author: magic
author_rank: Super Contributor
timestamp: August 20, 2020, 07:52:00 am
content: Quote from: tggzzz on August 19, 2020, 10:03:42 pmTo measure something you need to get the measurement setup right; otherwise you are just showing a snake on the screen

author: tggzzz
author_rank: Super Contributor
timestamp: August 20, 2020, 08:14:33 am
content: Quote from: magic on August 20, 2020, 07:52:00 amPrecisely - and that implies understanding the instruments and their settings.Electrofan's post indicate that some generators have settings that allow you to change what's shown on its screen. Those settings appear to be buried in a menuing system (ugh) and designed to avoid confusion by people that aren't thinking what is happening - but confuse experienced people. There can be similar problems on scopes with buried settings for *1/*10 probes, to the extent I often prefer to fix the setting as *1 and do the mental arithmetic myself.The OP is to be commended forthinkingandchecking, which is not as common as I would like!

author: David Hess
author_rank: Super Contributor
timestamp: August 21, 2020, 07:38:48 pm
content: A source with 50 ohm impedance, like a signal generator, will generate twice the output into an unterminated or high input impedance load like a 1 megohm oscilloscope input, at least at low frequencies.

author: ejeffrey
author_rank: Super Contributor
timestamp: August 21, 2020, 07:53:08 pm
content: Almost every reasonably modern low frequency signal (like your typical DC-10 MHz) generator works the following way.The output always has a 50 ohm source impedance regardless of any instrument settings. That means that the open circuit voltage is half the voltage you get driving a 50 ohm load.The signal generator will usually have a switch/button/setting for high-Z vs 50 ohm. This is essentially for display purposes only. The output is always 50 ohm source impedance. If you select high-Z, the front panel will display the open circuit voltage. If you select 50 ohm, the front panel will display half of the open circuit voltage, the expected voltage that can be delivered to a 50 ohm load. The output stays the same.Oscilloscopes measure the voltage at the input. The termination setting actually adds or removes a 50 ohm resistor from the input. Therefore, if you connect a 50 ohm source like a signal generator and enable the 50 ohm input, the voltage will be half the open circuit voltage. If you change to 1 M input, the voltage will be approximately equal to the open circuit voltage (but you will have reflections, so be careful). In either case the oscilloscope is displaying exactly what it measures -- the difference is due to a real change in the circuit. An oscilloscope will not try to "correct" for that 50 ohm load or display the expected open circuit voltage.If you use a standard 10x probe, the scope should be set to high-Z 1 Mohm input. The probe plus scope input will have a 10x voltage divider. This means the measured voltage will be 1/10 the voltage seen at the probe tip. However, the scope can display 10x higher than the measured votlage to give you a direct reading of the voltage at the probe tip. Some oscilloscopes with some 10x probes will automatically detect the 10x ratio and change the display automatically. Others combinations may require that to be set in a menu.