strange sine waves - Page 1
Forum section: electronics / beginners

author: ysccc
author_rank: Contributor
timestamp: October 09, 2025, 02:27:59 am
content: Hi, few days ago I measured the output of my circuit and it seems well. But today the waves become strange. I try to solve it but I don't know how to do. Please help me

author: golden_labels
author_rank: Super Contributor
timestamp: October 09, 2025, 02:39:49 am
content: It’s hard to tell, why a circuit misbehaves, without knowing the circuit.It looks like you’re picking up some external noise, that is an order of magnitude larger than the intended output. There seems to be a clear 1 MHz tone with unstable phase, but also some other components.

author: ysccc
author_rank: Contributor
timestamp: October 09, 2025, 03:37:11 am
content: Thanks for your reply. Here is my sch. I don't understand why. I didn't do anything to the circuit for couple days. TUT

author: TimFox
author_rank: Super Contributor
timestamp: October 09, 2025, 03:43:07 am
content: I don't see any power bypass capacitors in your drawing.In a Spice simulation, those power lines are perfect (zero impedance), but real circuits have impedances in the wiring.

author: Bud
author_rank: Super Contributor
timestamp: October 09, 2025, 04:30:49 am
content: Your two screenshots were made with different timebase. First one is 1ms/ but the second one 500ns/. You have to compare using a same timebase, then go from there.

author: ysccc
author_rank: Contributor
timestamp: October 09, 2025, 06:27:31 am
content: Thanks for your reply. I added bypass capacitors in real circuit.

author: ysccc
author_rank: Contributor
timestamp: October 09, 2025, 07:16:49 am
content: Thanks for your reply. I change timebase into 1ms/. It looks like unknow noise.

author: golden_labels
author_rank: Super Contributor
timestamp: October 09, 2025, 07:34:39 am
content: Where did you add the capacitors? What capacitors?Where do you measure? Your scope shows two traces, while your circuit has only one output. The “noise” waveforms also seem identical, while the good ones were not, which is even more suspicious.The schematic you posted doesn’t contain any circuit, that would generate the good waveforms. It’s a differential pair, which in this diagram already receives 1 kHz waves from ideal sources.How do you expect anybody to help you, if you’re not telling us anything? And, when you tell something, it appears to be some unrelated element.

author: MariuszD
author_rank: Regular Contributor
timestamp: October 09, 2025, 12:27:07 pm
content: On a random oscilloscope setting, something like this looks like noise.These are usually interference from switching power supplies. Setting the time base appropriately will allow you to find periodic waveforms.Knowing the characteristic features of the interference source, it can be easily identified by bringing a shorted oscilloscope probe close to power supplies, other devices and cables nearby.P.S. In school, I was taught that "noise" (pl. szumy) is term used only for random fluctuations that occur within electronic components. For external disturbances, we had a different term. It's a shame that in English literature, there isn't such a distinction. Now, even in Poland, few people remember this distinction because people learn from translated books. Whenever I see topics like this, I want to shout, "That's not noise!"

author: Bud
author_rank: Super Contributor
timestamp: October 09, 2025, 12:38:57 pm
content: The scope is on 500mV/, it is not scope's own noise, it is either his circuit stray oscillate due to instability or something wrong with the signal source.

author: ysccc
author_rank: Contributor
timestamp: October 10, 2025, 02:24:54 am
content: Sorry for my careless.This is an ultralow noise circuit. The first stage is built by LSK389C, the output is a pair of differential signal, tested in Vd1 and Vd2. Pic1 shows the simulation and measurement of Vd1 and Vd2(the green wave and yellow wave). The second stage is built by LT1028. The blue wave is the output of LT1028, at that time LT1028 is faculty so there is no output. Few days ago my measurement finally matched the simulation. Pic2 shows the simulation and measurement of LT1028(the blue wave).I added bypass capacitors 0.1uF to the V+ and V- of LT1028.Actually this is not my first time to see the strange waves, basically I adjust the ground terminal of the oscilloscope probe to connect to the reference ground of the board and the normal waves appear. Yesterday I tried to fixed the problem(shows in pic3) by old way but it didn't work. This circuit is put in a mental box because it is easily affected by electromagnetic interference. When I touched somewhere outside the box the blue wave appeared, but the wave have many glitches.The problem is so confusing and I don't know what I can do.

author: ysccc
author_rank: Contributor
timestamp: October 10, 2025, 02:31:15 am
content: Thanks for your reply. I learn a new way to figure out interference source. I use Li-ion battery for power supply.Before this I seldom build circuit. Obviously I should learn more about noise.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: October 10, 2025, 03:31:39 am
content: probably oscillation , add cap on back feeding , see what happens

author: Bud
author_rank: Super Contributor
timestamp: October 10, 2025, 04:30:27 am
content: It maybe the way you built your circuit that is causing this. If you can post a photo we may advise.

author: ysccc
author_rank: Contributor
timestamp: October 10, 2025, 07:52:16 am
content: Here is my circuit. I don't know why the wave become close to normal, except the green wave. The green wave comes from Vd1 and it should be about 60mV, 1kHz(Input differential signal 2mV, 1kHz. The magnification of first stage is about 30). The oscilloscope shows that the green wave is always 32mV, frequency changes from tens of kHz to over one MHz. It looks like some kind of high frequency noise(I already open BW limit).Besides, the blue wave from the output of LT1028 is a little different from calculation. The magnification of second stage is 100. So totally is about 3000. Input 2mV so the output should be close to 6V. But actually is close to 5V. Is this reasonable?

author: GigaJoe
author_rank: Frequent Contributor
timestamp: October 10, 2025, 03:22:10 pm
content: still ... thinking of oscillation ... R10 I would add capacitor shunt of few hundred pF ...your input and output more less OK on PCB, then it crossed as naked cables on connectors , close positive input to output with such amplification you have an an oscillation.ideal shielded cable for both input and output to connectors.don't see any coupling capacitors for opamp ±15 V , at least 0.1 to 1 uF on both as close as possible to pins.same for input fets i would add 20-50 ohn 100uf RC filter for ±15 V ; but even better drop to ±10 V and more clean feed using 10v zener + RC filter + transistor follower

author: ysccc
author_rank: Contributor
timestamp: October 11, 2025, 12:42:32 am
content: Thanks for your reply. I will try it.

author: Bud
author_rank: Super Contributor
timestamp: October 11, 2025, 01:48:30 am
content: Your board's ground does not seem to be connected to the box. You do not have ground wires from the board to the front panel connectors. By looking at the photo your signal generator is only connected with the center wire to each input and no ground because there is no ground connections to the board. You must fix that first.

author: ysccc
author_rank: Contributor
timestamp: October 13, 2025, 07:20:33 am
content: Thank you sir, I never taught to do so. I will connect the board's ground to the box. Besides, the signal generator needs to connect to board's ground like oscilloscope, signal to signal, ground to ground, am I right?

author: Terry Bites
author_rank: Super Contributor
timestamp: October 13, 2025, 10:46:59 am
content: I dont think its a good idea to tie the npn bases directly to ground. Maybe use a resistor bypassed with a cap?

author: Bud
author_rank: Super Contributor
timestamp: October 13, 2025, 06:00:23 pm
content: You can start with connecting ground wires to each front panel connector. Better yet, remove the single wires that you currently have, and use shielded wires, for each input and output. Your circuit has high gain and the input signals are only 1mV, so it makes sense to use shielded wires.