Lowpass filter for scope - Page 1
Forum section: electronics / beginners

author: salomonander
author_rank: Regular Contributor
timestamp: October 06, 2025, 10:08:28 pm
content: Hello,I work with audio devices that cover the 20hz-20khz range and i want to build myself a little box that acts as a lowpass filter for my oscilloscope. Im wondering if i could get away with a passive rc network (say 8th order), or if i should use something like a max295 by analog devices. I would prefer the passive version as i could build it into a tiny bnc feedthrough box. But in case it would introduce too much noise or mess with the impedance, i guess id have to go active. I use an rtb2002 scope with 1M input impedance.

author: mtwieg
author_rank: Frequent Contributor
timestamp: October 06, 2025, 10:51:51 pm
content: If the probe is a standard 10Mohm 10:1 passive probe then the short answer is no. Those kind of probes are designed to work specifically with an oscilloscope's input, and adding anything in between (even an extra length of coax cable) will severely degrade its performance.However, if your scope probe has adjustable compensation, you might be able to get what you want by intentionally reducing (or even removing) the compensation capacitor (the one in parallel with the probe's internal 9meg resistor). This will cause the probe's resistance and the scope's input capacitance to form an RC lowpass filter. However its stopband attenuation probably won't be very good, maybe 20dB or so.

author: salomonander
author_rank: Regular Contributor
timestamp: October 06, 2025, 11:12:09 pm
content: Thanks,I also have a differential probe if that changes things.

author: mtwieg
author_rank: Frequent Contributor
timestamp: October 06, 2025, 11:31:46 pm
content: Quote from: salomonander on October 06, 2025, 11:12:09 pmDepends on the output impedance of the active probe. What's the model number?

author: mawyatt
author_rank: Super Contributor
timestamp: October 06, 2025, 11:38:21 pm
content: Don't know rtb2002 but if it has ERES this can be utilized as an effective LPF. Also, some of the modern entry level DSOs have built-in selectable/configurable filters.Best

author: salomonander
author_rank: Regular Contributor
timestamp: October 06, 2025, 11:51:46 pm
content: Its an older hameg hz109. Here is a link to the manual:https://scdn.rohde-schwarz.com/ur/pws/dl_downloads/dl_common_library/dl_manuals/dl_user_manual/HZ109_Manual_en.pdfBut yes, 50ohms output. Actually im not certain if the probe works correctly with the scope though, as the square wave coming from the scope looks a little off in the initial rise. Im a scope noob though. Still, the probes rise time is slower than the scope. Here is what i just posted in the units dedicated thread:Can anyone comment on this waveform? I bought an older type of differential probe as i cannot afford the new version.Im wondering if it works properly as the square wave looks a little undershot. But im pretty new to scopes and would appreciate someone more experienced to chime in. I did get a lot of overshoot in 10:1 mode, but limiting the bandwidth to 20Mhz took care of that. Makes sense i guess, as the probe only has a bandwidth of 30Mhz. Which is plenty for what i do. With the limit engaged the waveform now looks the same in 1:1 and 10:1. Still, there is this slight undershoot and no trimmer in the probe to adjust. I could still return it at this point. But it was only 100$ while the dedicated probe for the rtb is 2000$. Way out of reach for me.

author: BennoG
author_rank: Frequent Contributor
timestamp: October 07, 2025, 05:54:30 am
content: That is a nice scope, that should have low pass filtering build in the menu somewhere.Benno

author: jonpaul
author_rank: Super Contributor
timestamp: October 07, 2025, 06:09:44 am
content: in 1/2 century of audio work, never needs this.Just use good probe practice, set BW limi in scope for HF noise.j

author: mtwieg
author_rank: Frequent Contributor
timestamp: October 07, 2025, 03:57:45 pm
content: Quote from: salomonander on October 06, 2025, 11:51:46 pmThen you can definitely implement a simple RC filter on the output. For example, just adding 150nF between output ang GND should set a cutoff of around 20kHz (assuming the scope input impedance is 1Mohm).QuoteHard to say without knowing more about the test setup. My guess would be that the probe has some internal trim caps that need adjustment (similar to compensation on a passive probe).Quote from: mawyatt on October 06, 2025, 11:38:21 pmQuote from: BennoG on October 07, 2025, 05:54:30 amYes, built-in filtering should be used when possible. Most scopes don't give much flexibility though, unlikely they will give a cutoff below 1MHz....

author: salomonander
author_rank: Regular Contributor
timestamp: October 07, 2025, 04:24:41 pm
content: Thanks guys, the only low pass filter option the scope has is 20Mhz. Nothing below that.

author: mawyatt
author_rank: Super Contributor
timestamp: October 07, 2025, 04:46:13 pm
content: Quote from: mtwieg on October 07, 2025, 03:57:45 pmThe entry level SDS824X HD allows very low corner frequencies utilizing adaptive sampling, here's an example of a 10Hz (#34), 1Hz (#35) LPF, and recovering a Sinc function with a 10Hz LPF, you can even go below 1Hz if required.Best

author: David Hess
author_rank: Super Contributor
timestamp: October 07, 2025, 06:20:55 pm
content: The old school solution which applies to any oscilloscope is to use a Tektronix AM502 which has high frequency cutoffs in the 1-3 series from 1 MHz down to 100 Hz. I sometimes use mine to make spot noise measurements.https://w140.com/tekwiki/wiki/AM502I think Preamble, bought by LeCroy, made a more modern version at one time.

author: mawyatt
author_rank: Super Contributor
timestamp: October 07, 2025, 07:01:50 pm
content: Here's what the SDS824X HD LPF looks like with a 10mv input and 1KHz corner. It's a Sampled Data FIR LPFilter so you can scale the frequency axis as required (adjusting the sample rate) and it will look the same relative-wise.Best

author: radiolistener
author_rank: Super Contributor
timestamp: October 08, 2025, 01:00:04 am
content: I’m afraid an analog low-pass filter might not be ideal for this purpose. A passive RC network — even a high-order one — will inevitably introduce both amplitude and phase distortion within the audio band. The roll-off of analog filters is relatively gentle, so to avoid noticeable impact on the signal, the cutoff frequency usually needs to be set well above the highest frequency of interest.Most oscilloscopes already include a built-in LPF 20 MHz option, which is the standard way to suppress high-frequency noise without affecting audio-range signals.

author: Bud
author_rank: Super Contributor
timestamp: October 08, 2025, 01:14:45 am
content: What are you trying to achieve by adding an external LPF? This won't reduce trace noise because the scope's bandwidth noise remains the same. If what you want is to filter out harmonics/spurs before the signal hits the scope's input then it is a different matter.

author: MrAl
author_rank: Super Contributor
timestamp: October 08, 2025, 06:04:02 am
content: Quote from: salomonander on October 06, 2025, 10:08:28 pmHi,If you only have to go up to 20kHz it would be easy to use a LP filter with just a single R and C.You do want to stay above the 20kHz by a significant factor though, and it depends what you are going to measure also.The RC LP filter could go on the input to the probe not inline. You can't go too high on the R value but you should be able to get to at least 100k. Check with and without the filter to see what effect it has.

author: David Hess
author_rank: Super Contributor
timestamp: October 08, 2025, 07:04:11 am
content: Quote from: radiolistener on October 08, 2025, 01:00:04 amPhase distortion is not audible, and an amplitude distortion of 3dB is barely discernible at best. (1) Many workable audio filters have greater than 2nd order roll-off without adverse effects, but a Butterworth filter shape with maximum amplitude flatness is typically the sharpest filter used except in anti-aliasing applications. Crossovers are a good example where more than 2-poles can be beneficial, and for reducing intermodulation distortion no less!An interesting test would be to insert a variable cutoff Tektronix AM502 amplifier into an audio chain and listen to the effect as its high frequency cutoff is reduced. While not specified, the AM502 appears to use a single-pole high frequency response, which agrees with testing that I did many years ago. Down to 30 kHz, I doubt any difference could be heard and it does reduce displayed noise. This should work even better with an oscilloscope's internal bandwidth filtering, but it is difficult to know what is going with a modern DSO unless the details are revealed in its specifications.I would consider a 4th order or even higher Bessel filter at maybe 200 kHz to be completely acceptable for oscilloscope use, and a lower cutoff frequency could be used if amplitude distortion at 20 kHz is acceptable, but this is more about preserving transient response than good audio response.(1) Check out the response of good cassette or reel-to-reel tape systems. They have horrible phase and poor amplitude response which is practically inaudible.

author: radiolistener
author_rank: Super Contributor
timestamp: October 08, 2025, 07:12:09 am
content: Quote from: David Hess on October 08, 2025, 07:04:11 amYes, that’s true for listening applications — a few dB of amplitude ripple or some phase shift are hardly noticeable to the ear. But in this case, he’s not trying to hear the signal, he wants to see it on the oscilloscope. Such distortions can become quite visible on the waveform display.

author: tggzzz
author_rank: Super Contributor
timestamp: October 08, 2025, 07:48:02 am
content: Quote from: salomonander on October 06, 2025, 10:08:28 pmSo you think that would solve your problem. But whatisyour problem?There may be preferable ways of achieving whatever you are doing, or of avoiding the problem.