Noise with sinusoidal signal with a tension divider - Page 1
Forum section: electronics / beginners

author: SC
author_rank: Newbie
timestamp: June 04, 2025, 08:57:56 pm
content: Hi,I'm using a FY6900 to generate an signal (sinusoidal wave) with a frequency of 1 kHz and Vpp of 20 mV. The signal with this small amplitude is very noisy (I know, FY6900 is a cheap choise), so as the signal at 2 Vpp seems ok I tried to scale using a voltage divider. I though, all be scaled, also noise on original signal, so results will be ok if the current is small. But the signal scaled with the voltage divider is very noisy. Why is this happening?The oscilloscope is a Rigol DHO804.Thank

author: RoGeorge
author_rank: Super Contributor
timestamp: June 04, 2025, 09:17:44 pm
content: The noise is not from the generator, most probably those spikes are induced from the surroundings.The signal is small, and high impedance inputs are very sensitive to induced noise (for example noise from LED lights SMPS, or from other powered devices nearby).Make sure you use a shielded cable, and use the short spring for the probe tip (not the crocodile + parrot probe tip). You may also try to set the probe switch on 1x instead of 10x.

author: Benta
author_rank: Super Contributor
timestamp: June 04, 2025, 09:18:39 pm
content: What values do your voltage divider resistors have?

author: SC
author_rank: Newbie
timestamp: June 04, 2025, 09:27:57 pm
content: Thanks for your feedback.So, it may be due to the electronic equipment nearby, I will try.The strange thing is that it is not the breadboard (I tried without connecting the resistors to the breadboard and the result is the same). I have a LED lamp near and a PC monitor, they may be the responsible. The probe is already at 1X.Thanks

author: SC
author_rank: Newbie
timestamp: June 04, 2025, 09:29:10 pm
content: I tried 47 and 4700 ohm.After I tried 10 and 1000 ohm, nothing changed.

author: RoGeorge
author_rank: Super Contributor
timestamp: June 04, 2025, 10:02:36 pm
content: Put a picture with the physical setup.Use the spring GND adapter (like the ones seen at minute 5:26 herehttps://youtu.be/sPSJnj7gVJA?t=326,and NOT the crocodile with a the long wire. So, not the crocodile + parrot as seen on the probe at the beginning of the video. At small signal, the GND alligator of the probe acts as an antenna, capturing induced parasites from the surroundings. Ignore the title of that video, probe capacitance is not a problem in your case, that video is only to show why the spring tip adapter for the probe. The spring comes as an accessory, look for it in the ziplock bag in which the probes were packed.Alternatively, you can DIY a small adapter for the probe tip, like this one:Also, make sure the probe GND goes to the generator's GND.

author: radiolistener
author_rank: Super Contributor
timestamp: June 05, 2025, 02:03:47 am
content: Quote from: SC on June 04, 2025, 08:57:56 pmThe noise leaks through parasitic paths, such as ground currents, capacitive and inductive coupling, etc. You can't simply remove it with a voltage divider - eliminating noise is not an easy task.This is why it's much easier to replace the device with one that doesn't produce noise, rather than trying to fight a noisy one, which is sometimes nearly impossible.Just imagine someone placing a loud gasoline generator right next to you - the noise is overwhelming. Now try to block out that noise - not so easy, right? It's the same with electronic noise - once a strong noise source is present, mitigating its effects becomes a complex and often impractical challenge. It's usually far more effective to eliminate the noisy component entirely and replace it with a quieter or low-noise alternative.

author: SC
author_rank: Newbie
timestamp: June 05, 2025, 09:12:22 pm
content: Hi,here it is the setup.I found the mass spring, so still have to understand how to mount it.I downloaded the PVP3150 userguide, but there is no info on how to mount. I will try on internet or youtube.Thank

author: RoGeorge
author_rank: Super Contributor
timestamp: June 05, 2025, 09:39:54 pm
content: Pull out the GND wire and the plastic hook. This will reveal the metal pin tip of the probe, and a metal ring/tube like, right at the naked probe tip.Click to enlargeFit the spring onto the metal ring right at the tip, as seen in the video with timestamp I've given you before. Stop the video and zoom in to observe how the spring sits at the probe tip.Once you do this, set the generator on 20mVpp without any cable. Gently touch the oscilloscope probe tip to the middle of the (empty) BNC connector right at the generator front panel, while the spring foot you just attached is touching the GND of the BNC connector at the generator side.!!! DO NOT PUSH the probe tip inside the middle connector !!!, only gentle touch it to make contact with the middle of the generator's BNC connector. You should see a clean 20mVpp sinusoid on the oscilloscope screen.If you still see those noise spikes, your oscilloscope is defective and needs service/replacement.Note 1: if you set the probe on 1x, then you should check that the oscilloscope input channel is also set to 1x (not 10x), otherwise a 2mVpp signal will be wrongly display as 20mVppNote 2: if your generator allows selection for High Z vs 50 ohm output, set the generator for high Z, otherwise it will generate double of the Vpp you are settingNote 3: make time to read the user manual of your instruments (oscilloscope + generator), and exercise with the instrument settings while reading from the user manuals

author: SC
author_rank: Newbie
timestamp: June 05, 2025, 10:05:37 pm
content: Hi,I mounted the spring and the level of noise is slightly decreased with the spring (download1 is with crocodile, download2 is with spring).At the end, I tested your suggestion to connect the spring directly to the generator BNC and the signal is very clear (RigolDS3). So likely the oscilloscope is fine :-)The spring however is almost impossible to keep fixed in the setup, expecially if you use more probes.I'm practicing with the book "Analog Electronic Circuits Laboratory Manual" and in one of these exercises it was needed to inject a 20 mV sinusoidal signal...is where all started!Thank you for your help, I will try to figure out how to reduce this noise.

author: RoGeorge
author_rank: Super Contributor
timestamp: June 05, 2025, 10:34:00 pm
content: Quote from: SC on June 05, 2025, 10:05:37 pmYes, that touching of the generator's BNC directly with the oscilloscope probe was only to check if the oscilloscope is OK, and NEVER to be used in practice.Glad to hear your instruments are OK. Good call to exercise following a book.Now, that you know both the DSO (DSO=the oscilloscope) and the AWG (AWG=the generator) are OK, you should next attach the cable to the AWG, then clip the cable's alligators directly to the DSO GND spring, and to the DSO probe tip. !!! Gentle when doing this, so to not bend/damage the DSO probe tip.You should see a clean 20mVpp from the AWG cable's crocodiles.If not, maybe there is something (some other device) very noisy nearby. Most often the LED lights produce sharp spike that interfere with small signals. Turn off the lights, one by one, until the spike noise disappear. If not, start unplugging other devices you may have, PC/laptop, phone chargers, etc, until you identify which device produces the spikes.After you identify where from are coming those spike noises, you can move practicing with the breadboard and additional components.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 07, 2025, 01:13:06 am
content: One trick is to connect the "long" GND clip of the scope to the tip of the scope. This way the loop forms a pickup for magnetic fields, and you can wave your scope probe around to see where the most noise comes from.Also, switching gadgets off one by one is an easy way to narrow it down. At the moment the noise disappears, you have turned off the equipment that generated it. It can be anything, from your soldering iron, keyboard or monitor to desk lighting. Anything that runs on electricity is suspect.

author: Aldo22
author_rank: Super Contributor
timestamp: June 07, 2025, 08:50:52 am
content: I probably won't solve your problem, but here's a tip anyway, because it's simply much more practical and less sensitive to noise than your voltage divider.With my even cheaper FY3220s, 20mVpp looks really bad (Screenshot 1). A lot of noise and also a visible offset.I simply put a20dB attenuatorin between and I see a nice signal (Screenshot 2). You then have to set the SG Vpp 10x higher (200mV will be 20mV).So you don't have to fiddle around with a tinkered and perhaps not very accurate divider every time.

author: radiolistener
author_rank: Super Contributor
timestamp: June 07, 2025, 01:55:57 pm
content: Note that high impedance inputs such as 1MΩ are highly susceptible to EMI, as only a small amount of energy is required to induce high-amplitude signals. Consequently, even low-power interference can result in significant noise.To minimize noise and EMI interference, use a low-impedance oscilloscope input with a direct coaxial cable connection. Standard oscilloscope probes are not suitable for this purpose.If your oscilloscope supports it, just enable the 50 Ω input mode to reduce noise and reflections. However, be careful - this mode typically has a maximum input voltage rating of 5 Vrms. For higher signal amplitudes, a suitable high-power attenuator must be used to prevent damage.If your oscilloscope does not support a 50 Ω input mode, you can use an external 50 Ω feed-through terminator connected directly to the oscilloscope input.However, for improved sensitivity and lower noise, a dedicated RF front-end with a low-pass filter, low noise opamp and a high-speed ADC is recommended instead of using an oscilloscope.What Aldo22 suggested above relies on the same low-impedance effect, but it is implemented incorrectly. In his setup, the attenuator acts as a feed-through 50 Ω terminator, but with signal attenuation. There are two main issues in his approach:First, the 50 Ω termination must be placed at the receiver end - that is, at the oscilloscope input, not at the signal generator side, as Aldo22 did.Second, using an attenuator for this purpose is impractical: the signal is already weak (20 mV), and attenuating it by 20 dB (a tenfold voltage reduction) lowers it to just 2 mV, making measurements more difficult and more susceptible to noise.To avoid unnecessary signal loss, a proper 50 Ω feed-through terminator should be used instead of an attenuator. This device is typically a BNC-to-BNC or SMA-to-SMA adapter with a non-inductive 50 Ω resistor between the signal and ground lines inside.

author: Aldo22
author_rank: Super Contributor
timestamp: June 07, 2025, 02:39:26 pm
content: Quote from: radiolistener on June 07, 2025, 01:55:57 pmThat's not what I do.I increase the voltage in the signal generator tenfold so that interference coming from the signal generator (Which is very visible at low voltages. PSU?) is attenuated and other effects of the lowest range (e.g. offset) are attenuated.Screenshot 2 proves that it is helpful.Both screenshots show at the end 20mVpp, not 2mVpp.

author: radiolistener
author_rank: Super Contributor
timestamp: June 07, 2025, 02:56:03 pm
content: Increasing the output voltage of the signal generator to compensate for attenuator losses is also not a good idea. Driving the generator at high amplitude can lead to nonlinear distortion introduced by its output stage, especially if it's pushed near its limits.Moreover, this approach is fundamentally flawed because increasing the output amplitude also amplifies the generator's own internal noise by the same factor - by 10x times in your example, making the signal more prone to noise-related issues.In contrast, using a proper feed-through 50 Ω terminator eliminates the need to boost the generator output. This allows you to maintain a lower, cleaner signal level without introducing additional distortion or amplifying the generator's noise floor.

author: Aldo22
author_rank: Super Contributor
timestamp: June 07, 2025, 03:05:31 pm
content: @radiolistener: No. Compare screenshot 1 and screenshot 2 above.The aim is to iron out the weaknesses of the signal generator in the lowest voltage range.In this case, we are talking about 200mVpp -> 20mVpp and not the upper limits of the SG (20Vpp).In the latter case, the attenuator would make no sense.