question about square pulse on the oscilloscope - Page 1
Forum section: electronics / beginners

author: Galaxy
author_rank: Newbie
timestamp: February 15, 2024, 07:11:40 am
content: hello everyone on the forumPlease, anyone who knows palmistry well, tell me if the signal I see is correct.one photo is the output of the generator and the other is the output of the amplifierThanks for your answer

author: Jwillis
author_rank: Super Contributor
timestamp: February 15, 2024, 07:37:43 am
content: Your getting over shoot from the amplifier. If you zoom in on the spikes you'll probably see some ringing due capacitive discharge. Could be directly from the amp or parasitic.I'm not a 100% sure.

author: selcuk
author_rank: Frequent Contributor
timestamp: February 15, 2024, 08:17:35 am
content: Are you using the same probe for both measurements? If not, make sure the probe is adjusted to show square waves properly. There should be a trimmer on the probe for this purpose.

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: February 15, 2024, 09:54:26 am
content: Hi there,it's just 1kHz, but put a terminator (50 Ohm) at the input of the (audio ?) amplifier.You'll have to double the output-level of the generator with termination.There was no load on the output, right?Try it with some 8 Ohm and show us the results.

author: CaptDon
author_rank: Super Contributor
timestamp: February 15, 2024, 02:09:30 pm
content: W.T.F.? You are generating sine waves and getting square wave output from the amplifier??? Is this what you want? An audio amplifier is a real poor choice for making square waves!! What is your target exactly?? As for the overshoot, wiring, probe ground lead length, improperly compensated probe. The amount of overshoot I see is probably meaningless for your experiments at 1KHz. What does the box with all the knobs do? Just turn sine waves into different shapes of sort of squarewaves? You could draw your own wave shapes saved as .csv files and load them into a cheap Arbitrary Function Generator and 'avoid the middle man'.

author: wasedadoc
author_rank: Super Contributor
timestamp: February 15, 2024, 02:13:46 pm
content: Quote from: CaptDon on February 15, 2024, 02:09:30 pmLook at the first photo more carefully. Don't you see "CH1=SQUR" in the top left corner of the generator's screen and the cable plugged into CH1 BNC socket?

author: donlisms
author_rank: Frequent Contributor
timestamp: February 15, 2024, 05:10:26 pm
content: Remember that a square wave has as close to infinite speed as possible in the transitions between the high and low. Once you get something moving that fast, it's hard to stop it. What you see is a little disturbance in the extremely high frequency range, far far above audio. So... meh.For audio, it would be much more meaningful to use sine waves kept in the 20kHz range; any other 20kHz waveform will require higher frequencies in order to produce the necessary harmonics, and you're asking the amp for superpowers. Or... maybe that's what you're trying to find out?

author: CaptDon
author_rank: Super Contributor
timestamp: February 15, 2024, 06:11:48 pm
content: Ahh, O.K., I do see the SQ on the sig generator. The sine shape was what first caught my eye. Most audio amplifiers don't do well with square waves. The feedback circuits are to slow for proper overshoot / undershoot control and certainly any amplifier with magnetics in the audio chain is out of the question. The old Peavey DECA 700 / 724 class D switching amplifiers were clocked at 500KHz and had very good sharp transient response. I think they could do sinewave out to about 50KHz so squarewave out to about 5KHz 'should' look pretty good. I still wonder what the box with all of the unmarked knobs does??

author: Galaxy
author_rank: Newbie
timestamp: February 15, 2024, 11:32:38 pm
content: Τhank you very much for the answers.I have finished an amplifier with mosfet IRFP9240 and IRFP240 and I want to see what it does on the oscilloscope.Τhe black box in the photo is a preamplifier.I am using probe of the oscilloscope and it is set correctly.The load test 4ohm.Τhe Sine wave looks good until 11khz.What I want is to learn good oscilloscope and to fix the circuit I have as best as possibleI am concerned about the spikes I saw in the square pulseSorry my english is not very goodI first check the signal of the generator on the oscilloscope and then on the amplifier. I understand that the square is a difficult wave but I put the same wave on other amplifiers built and I see it right.NOTE. Τhe amplifier works very well in the ear. the offset is less than 10mv, the quiescent current is 50ma with a continuous 45-0-45 volt power supply.two mosfets on each sidethere is no noise in the speakers.

author: CaptDon
author_rank: Super Contributor
timestamp: February 16, 2024, 03:11:58 am
content: The Peavey DECA 528 Class D amplifier uses the IRFP240 and IRFP9240 however due to the P-Channel device having a higher RDS they paralleled a pair of the P-Channel devices on one rail and a single N-Channel on the opposite rail. I was baffled when I first looked at the schematic but it becomes clear when you look at the RDS specs. I don't know what the clock speed of the DECA 528 is but I think it is lower than the 500KHz used in the older DECA series.

author: tggzzz
author_rank: Super Contributor
timestamp: February 16, 2024, 11:34:07 am
content: Quote from: Galaxy on February 15, 2024, 11:32:38 pmWell done on providing photos and describing your situation.Too often people looking at audio amplifiers only think about the frequency domain. With amplifiers some behaviour is best described in the time domain.The classic example is trying to describe and measure "transient intermodulation distortion" (TID), since the numbers are a function of frequency and amplitude. It is much better and easier to think in terms of "slew rate".So, have a look at theedgeof the response to a square wave input. The period/frequency is not particularly important.Characterise the "overshoot" visible in the traces. If it looks like half of a sine wave, what would be the period of the sine wave (and hence frequency)? Alternatively, if your scope supports it, show a single cycle on the display and get the scope to transform it into the frequency domain (i.e. do an FFT!).Make sure the load and amplitude are representative of a real system.

author: Galaxy
author_rank: Newbie
timestamp: February 16, 2024, 06:43:53 pm
content: we've been doing the hobby for many years, but I've had an oscilloscope since last year.I will read the manual again if I can FFTI didn't understand exactly but I will tryit's also the translation, you see

author: tggzzz
author_rank: Super Contributor
timestamp: February 16, 2024, 09:25:46 pm
content: Quote from: Galaxy on February 16, 2024, 06:43:53 pmSounds like you would benefit from doing some research on the relationship between the time domain and the frequency domain. The Fourier Transform decomposes the time domain waveform into the equivalent frequency and phase components. The inverse Frequency Transform takes the frequency and phase components and creates the equivalent time domain waveform. The FFT and IFFT are merely computationally efficient transforms.There are many subtelties, but if you understand "band limiting" and the Gibbs phenomenon (or ears!), That will be a good start.