Fun with digital scope - Page 1
Forum section: electronics / testgear

author: bob91343
author_rank: Super Contributor
timestamp: June 16, 2021, 04:50:20 pm
content: I am lucky to have some nice gear. I was trying to learn to use my Tektronix 2440 digital scope. I thought it would be interesting to create an AM signal by multiplying an audio signal with an rf signal.So I put 1 kHz into one channel and 5 MHz into the second vertical channel and selected MULT. What I got on the screen was confusing, to say the least. So I pressed the AUTO button and let the scope decide for itself how to display the result.It was laughable. Such nonsense I would never be able to create myself but the scope essentially went mad and showed three traces that made no sense to me.Has anyone tried this? Can it be made to work? Or is multiplying not what I want to do?

author: ataradov
author_rank: Super Contributor
timestamp: June 16, 2021, 05:13:57 pm
content: It would help of you show some captures or pictures.But what I suspect is that is it hard to observe 1 kHz and 5 MHz on the same time scale, so if you set your time scale to be such that you can see 1 kHz envelope, then 5 MHz may be aliased and not display correctly.

author: xmo
author_rank: Regular Contributor
timestamp: June 16, 2021, 05:24:20 pm
content: In a digital oscilloscope, multiply is a mathematical operation where the trace data values are multiplied point by point and the result displayed.It's quite useful when you want to see a power result where one channel input represents voltage and the other represents current.Keep experimenting. It would be interesting to set up a modulated waveform with a dual channel waveform synthesizer and see how the digital scope shows the modulated envelope.

author: Grandchuck
author_rank: Super Contributor
timestamp: June 16, 2021, 06:29:22 pm
content: See attached:Direct coax connection to the DS2202 from SDG 2122X, both outputs sine, 1 kHz @ 2Vpp and 5MHz @ 3 Vpp. Math AxB

author: Grandchuck
author_rank: Super Contributor
timestamp: June 16, 2021, 06:59:52 pm
content: Some fiddling for a nicer display of the double side band result and the two generator signals.

author: xmo
author_rank: Regular Contributor
timestamp: June 16, 2021, 07:16:32 pm
content: Bob. Did you try trace addition? That might be interesting too.When you have had enough fun playing with your scope, mosey back over to ARF and tell Paul what's wrong with his RCA128.

author: radiolistener
author_rank: Super Contributor
timestamp: June 16, 2021, 07:20:04 pm
content: Quote from: bob91343 on June 16, 2021, 04:50:20 pmThis is not AM. This is DSB, you're forgot to add carrier to the result of multiplication.AM modulation:y(t) = cos(2πfct) + μ cos(2πfmt) ∙ cos(2πfct)= cos(2πfct) + 0.5μ cos[2π(fc− fm)t] + 0.5μ cos[2π(fc+ fm)t]DSB modulation:y(t) = cos(2πfmt) ∙ cos(2πfct)= 0.5 cos[2π(fc− fm)t] + 0.5 cos[2π(fc+ fm)t]whereμ = modulation index [0..1]fc= carrier frequencyfm= message frequencyIn order to get AM with 1000 kHz carrier, 1 kHz message and modulation index 100%, you're needs to sum 3 signals:1) 1000 kHz2) half amplitude 999 kHz3) half amplitude 1001 kHzAnother way is to multiply carrier with message and then add carrier.Here is how AM 1000 kHz / 1 kHz signal looks on a digital oscilloscope (Siglent SDS1102X):

author: radiolistener
author_rank: Super Contributor
timestamp: June 16, 2021, 07:57:47 pm
content: And here is DSB modulation of 1000 kHz carrier with 1 kHz message obtained with MATH multiply function on digital oscilloscope (the same SDS1102X) from two sources 1000 kHz and 1 kHz taken from PSG9080 signal generator:As you can see, DSB signal has a little different shape than AM.

author: bob91343
author_rank: Super Contributor
timestamp: June 17, 2021, 06:01:17 am
content: Good show, guys! Those are the waveforms I was seeking. I still don't know how to obtain them.When I use an amplitude modulated signal generator, I get the expected display on my analog scope. Getting it on a digital scope is an entirely different matter, and I don't know how to do it.

author: rf-loop
author_rank: Super Contributor
timestamp: June 17, 2021, 08:32:46 am
content: Quote from: bob91343 on June 17, 2021, 06:01:17 amYoy told you have Tek 2440.This is problem number 1.Why?Because its record length is 1024 samples aka 1k acquisition memory length.These modern digital scopes have least thousand fold amount.Take example.Lets define imagined example. Scope have 1000 points memory, and display have 10 horizontal div and this memory length is equal with this display width. Now if your sample memory length is 1000 samples and your horizontal speed (aka timebase) is set so that this whole memory length take example 5ms (5 cycle 1kHz modulating signal).In this case your timebase is set for 500us/div.Now I ask what is sampling speed. Yes Tek 2440 max sampling speed is 500MSa/s (without talking about how they did it with using CCD).5ms and 1000 samples. Result is 200kSa/s. With this "Nyquist limit" is 100kHz.Now if your modulated carrier is example 1MHz it is ten times higher than Nyquist limit.With your scope you can try set carrier freq example to 50kHz and modulate it it using 1kHz.This you can look with your scope so that it start somehow look as conventional AM shape.Just this short memory was big problem in old digital scopes.(I have old times owned and used just also this Tek 2440 model. But it was so long time ago I do not remember anymore all details... somehow I think it have 50% memory lenght on display and 25% overlap in both sides... but now this my memory may be messed with what ever other old times digitals...Think about it...You need keep sampling frequency, in theory, least 2 (in practice usual modern DPO with Sinc interpolation, 2.5x) times higher than frequency components in your signal under test.If need look normal AM signals this 2440 is nearly or fully hopeless. Even when it is still nice well made scope for some kind of use. Tens of years ago even with it and 2430/A we did many "miracles".

author: bob91343
author_rank: Super Contributor
timestamp: June 17, 2021, 06:08:42 pm
content: Thank you rf-loop; you have explained it nicely. Next opportunity I will try modulating 5 kHz with 1 kHz and see what happens.

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: June 17, 2021, 06:30:59 pm
content: I got this on my 2440. Multiply 1KHz with 1MHz. Needs to be viewed with ch 1 & 2 off and acquisition mode in envelope.And AM modulation

author: bdunham7
author_rank: Super Contributor
timestamp: June 17, 2021, 07:31:37 pm
content: Quote from: Ground_Loop on June 17, 2021, 06:30:59 pmWhat happens when you try to look at the carrier, say at 500ns/div?

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: June 17, 2021, 07:54:28 pm
content: I get this in envelope modeAnd in normal mode just the carrier

author: bdunham7
author_rank: Super Contributor
timestamp: June 17, 2021, 08:00:07 pm
content: Quote from: bob91343 on June 17, 2021, 06:01:17 amI thinkGroundloophas that figured out. The special 'envelope' acquisition mode, which is a modified peak-detect, will work at the modulation frequency. I'm not sure if or how you can display the typical AM modulation signal at the carrier frequency.To generate an AM-modulated signal by multiplication, you have to add a DC bias equal to the amplitude of the carrier. If you look at the post fromradiolistenerhe refers to needing the 'add the carrier back in', this amounts to the same thing. An AM signal at 100% modulation goes from 0% to 200% of the base carrier amplitude--it never goes negative. Negative would flip the phase of the carrier 180 degrees.

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: June 17, 2021, 08:15:28 pm
content: With a 0.5 VDC positive offset on the modulating frequency I did get a good AM trace by multiplying 1 and 2.Thanks BD7 I learned something today.

author: bob91343
author_rank: Super Contributor
timestamp: June 17, 2021, 09:49:34 pm
content: Wow this is great! Now I have to try it. But I thought I already did this.More details please. I put 1 kHz into Ch1 and 1 MHz into Ch2. I shut off display of both channels. Go into MULT. Okay so far?And where do I fall apart due to insufficient memory depth?

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: June 17, 2021, 11:46:09 pm
content: You need to use the envelope mode in the acquisition menu. See attachedYou'll also need to add a DC offset of about a half volt to the 1KHz signal.Trigger off the 1KHz signal.

author: bob91343
author_rank: Super Contributor
timestamp: June 18, 2021, 02:58:17 am
content: Do I need an actual offset to the signal, or just move the trace position?I got an interesting display but it's not right yet.

author: bdunham7
author_rank: Super Contributor
timestamp: June 18, 2021, 04:27:11 am
content: Quote from: bob91343 on June 18, 2021, 02:58:17 amYou need the actual offset, moving the trace does not affect the multiply operation and you can't have the modulation signal going negative if you want to simulate an AM signal.

author: rf-loop
author_rank: Super Contributor
timestamp: June 18, 2021, 05:58:46 am
content: Quote from: Ground_Loop on June 17, 2021, 06:30:59 pmNaturally this can do. Lot of old digital scopes have peak detect / envelope mode implemented way or other, just because it is mandatory due to extremely acquisition memory what drops samplerate down with slow timebases.(And Tek2440 have some special nice things, in its time it was amazing tool for work. Also its internal working principle is (was in that time) brilliant)But carrier loosed depending its freq and mr Nyquist and Shannon. If stop and try look details - game over. Depending different old digital scope models how zoom/2nd timebase works if you need zoom in to carrier, no way due to undersampling - aliasing - again - game over. And "envelope" mode or "peak detect" mode do not help here anything. But, yes AM mod envelope figure can see. But if look inside there is no carrier. Also this kind of solution with old dso's are many times in big troubles with trigger if modulating frequency change fast, depending available trigger features.Today many modern DPO can easy go down to example 10ms/div and still whole HF band carrier freq range do not alias.Also modern scopes trigger engines are far better than these old ones - in most cases. Example for trig rock solid to fast and large scale variable AM modulating frequency and modulating depth.But yes, this all meet this thread topic, "Fun with digital scope" and there is possible to watch wide range of amazing phenomena and amazing patterns on the oscilloscope screen - when this is wanted.

author: bdunham7
author_rank: Super Contributor
timestamp: June 18, 2021, 03:36:12 pm
content: Quote from: rf-loop on June 18, 2021, 05:58:46 amI don't see how Nyquist limits apply here. In envelope mode, the peak detect circuits effectively eliminate the carrier, so there's no aliasing even when the sweep rate is slow.Ground_Loopclearly shows the 1kHz envelope without any ill effects. And if the sample rate was an issue under different circumstances, the scope has the ETS (REPET) mode that should eliminate any aliasing well past the bandwidth limit of the scope.Here is my ancient Tek 2221 (20MSa/s, 60MHz) displaying 1MHz modulated to 80% by 1kHz, and then at 20MHz (obviously "over Nyquist") by 1kHz. The 20MHz ones were a bit touchy to set up as you need just the right acquisition mode and setup, and the displays look bad at first and take form over time. The secret weapon is that on these hybrid scopes, you can set them up in analog and then go digital.

author: rf-loop
author_rank: Super Contributor
timestamp: June 18, 2021, 04:20:10 pm
content: Quote from: bdunham7 on June 18, 2021, 03:36:12 pmBut, perhaps my poor language or what ever... there was now misunderstood.Sampling speed is under Nyquist. But due to envelope mode carrier is loosed, of course and no aliasing visible on screen. But as I told you can not look carriage (of course),but with more memory and enough sampling speed you can. With runtime zoom or stopping scope and zoom in. All is there.But yep, fun images can do.

author: bob91343
author_rank: Super Contributor
timestamp: June 18, 2021, 06:25:05 pm
content: bdunham7 I don't quite get it. Is the modulating signal the one that requires offset, or is it the carrier? I am thinking the carrier, because the modulation can affect the carrier both increase and decrease.Accomplishing this requires adding dc to the carrier signal without causing problems in the signal generator. I guess a coupling capacitor and a battery or power supply.

author: bdunham7
author_rank: Super Contributor
timestamp: June 18, 2021, 06:55:14 pm
content: Quote from: bob91343 on June 18, 2021, 06:25:05 pmThe modulating signal. The carrier will swing from positive to negative and you want it to continue doing so. However, if you multiply it by a negative number, you invert its polarity or flip its phase 180 degree, however you like to think of it. So, you never want the modulation signal to be negative, thus the need for the DC bias. I'm not sure the source for your modulating signal--on a cheapo modern DDS/AWG you can just set it directly.

author: bob91343
author_rank: Super Contributor
timestamp: June 19, 2021, 12:58:37 am
content: I am using a simple audio oscillator. I do have some old function generators so maybe I'll break one out and see about offset.

author: bob91343
author_rank: Super Contributor
timestamp: June 19, 2021, 02:19:09 am
content: That seemed to work. I now can see the classic amplitude modulated wave.However, I am unsure of just what I am doing. As I vary the dc offset I see the wave move up and down until clipping. If I vary the ac component the wave reaches 100% modulation and continues beyond to something I can't really understand. It folds over and rather than flatten at the axis, it goes beneath and remains sinusoidal.

author: David Hess
author_rank: Super Contributor
timestamp: June 21, 2021, 01:22:16 am
content: More amusing is that some of these old DSOs, like the mentioned 2221, can synchronously demodulate AM directly from RF to recover the original audio by using their external sample-clock input.

author: bob91343
author_rank: Super Contributor
timestamp: June 30, 2021, 05:39:24 pm
content: I still feel that the analog scope's days are not yet over. Much simpler to use for troubleshooting and casual measurements. For demanding applications, the digital is probably superior.

author: Fungus
author_rank: Super Contributor
timestamp: June 30, 2021, 08:12:15 pm
content: Quote from: bob91343 on June 30, 2021, 05:39:24 pmI don't think anybody denies they're beautiful and awesome, but... very limited in what they can do.And they take up a lot of space.

author: bob91343
author_rank: Super Contributor
timestamp: July 01, 2021, 12:43:23 am
content: My digital and analog scopes are about the same size.

author: tautech
author_rank: Super Contributor
timestamp: July 01, 2021, 01:12:06 am
content: Quote from: bob91343 on July 01, 2021, 12:43:23 amQuote from: bob91343 on June 16, 2021, 04:50:20 pmI hate to inform you a 2240 would be considered as a dinosaur today.A modern and compact DSO has far more functionality.

author: David Hess
author_rank: Super Contributor
timestamp: July 01, 2021, 02:43:05 am
content: Quote from: tautech on July 01, 2021, 01:12:06 amBut not delayed sweep, so most modern DSOs cannot for instance measure the timing difference between adjacent pulse per second pulses at their best resolution. Doing so would require upwards of a 1 Gigasample record length. An old DSO only requires a 32 bit counter. Modern high end DSOs get around this limitation with segmented memory.I would agree that analog and older digital storage oscilloscopes are long obsolete, but that does not make every "modern" DSO better for every application. Some modern instruments are pretty horrid.Quote from: bob91343 on July 01, 2021, 12:43:23 amSince my most used DSO is a Tektronix 2232, my most used digital and analog oscilloscopes are exactly the same size.

author: bob91343
author_rank: Super Contributor
timestamp: July 01, 2021, 03:15:11 am
content: My old HP 54542A has a larger front panel but it's not too deep. That is a very old digital scope that I like a lot.