Equipment to measure RC filter attenuation - Page 1
Forum section: electronics / testgear

author: slugrustle
author_rank: Frequent Contributor
timestamp: September 13, 2022, 05:32:44 pm
content: I often use 1-stage and 2-stage RC filters for various purposes, such asFilter AC noise from voltage divider intended to measure DCTurn PWM signal into adjustable analog DC signalFilter power supply to sensitive circuits with low current draw (opamps, references, etc.)I like to measure the filters to make sure they are operating close to expectations. I typically measure rising and falling input voltage step response (sig gen / knife switch input, scope on input and output) and attenuation. Attenuation measurement is giving me trouble.Usually, I use a signal generator with an injection transformer and a lab supply so I can see if DC bias affects the filter response (I sometimes use X7R capacitors in filters—sue me). I typically use 10x probes on filter input and output to avoid loading the output too much, AC couple the probes, enable 20MHz bandwidth limit, and turn on my scope's averaging statistics. The trouble is that I often run into the noise floor of the scope at the frequency of interest, even with a large input signal to the filter. The other trouble is a non-negligible DC offset on the 5mV/div range when AC coupled and the lack of an AC RMS measurement on my scope (Siglent SDS2504X+) so I use pk-pk measurement, but this is artificially inflated by noise.At this moment, I'm trying two Rigol DM3068 DMMs measuring AC voltage on input and output. Frequency limitation is 300kHz, and I'm having trouble with the low voltage AC measurement taking a long time to settle on the trend chart.What's a good setup to measure filter attenuation? I don't really care about phase. Some ideas come to mind:Frequency response analyzer (Omicron Labs Bode 100, Venable 6305, etc.)Picoscope 4262 in a sig gen + scope setupSpectrum analyzer for input / output amplitude measurement instead of scope, still use sig gen + transformer + DC supply as stimulus. Can this handle DC offset? I'm guessing it is always or can be AC coupled.Vector network analyzer, although they tend to have high minimum frequencies (9kHz, etc.). Can this handle DC offset?Any tips on which option above is best? Are there other or better ways to do this?

author: tautech
author_rank: Super Contributor
timestamp: September 14, 2022, 07:33:15 am
content: Quote from: slugrustle on September 13, 2022, 05:32:44 pmTo 50 MHz, your scope Siglent SDS2504X+ using the inbuilt AWG and the Bode plot function.You can push higher with a standalone AWG but the limit is 120 MHz.Some guidance here in Siglents App note where they use a SDS1104X-E that hasn't 50 Ohm inputs therefore external feedthrough terminations are used.https://siglentna.com/application-note/bode-plot-filter-oscilloscope-generator/There might also be some help and guidance in the following threads:https://www.eevblog.com/forum/testgear/bode-plots-useful/Here rf-loop explores the limits of the Bode plot feature in 4ch X-E's:https://www.eevblog.com/forum/testgear/siglent-sds1x04x-e-bodeplot-ii-(sfra)-features-and-testing-(coming)/Simple 2 stage RC pass filter linked in one of the threads above.Tables, markers and results = OFF to reduce clutter.

author: tszaboo
author_rank: Super Contributor
timestamp: September 14, 2022, 08:01:53 am
content: If you have a programmable signal generator/AWG and one or two multimeters, that will give you a versatile setup to measure this. Just automate the setup with a script, and you are fine all the way to your DMMs AC range, and drink a coffee while it works. Other setup I could use in the lab is a Keysight 3104T, which has the generator, and Bode plot function up to 20MHz. This is easier to setup, one instrument, and it totally goes to the frequencies where the cable already matters.

author: slugrustle
author_rank: Frequent Contributor
timestamp: September 14, 2022, 07:43:27 pm
content: To state it more succinctly, the specific problem I'm running into is the noise floor / minimum measurable voltage on both my Siglent SDS2504X+ scope and my Rigol DM3068 DMMs, even when using a 20Vpk-pk sine wave as the stimulus. I hit this amplitude measurement limit often before getting to the frequency of interest, where the filter is often attenuating by 80dB or so. The frequency of interest is often below 1MHz.Is there a good method that would allow measurement of filter attenuation down to say 100dB while allowing DC offset in the stimulus? Perhaps something in the list below or other options...Frequency response analyzer (Omicron Labs Bode 100, Venable 6305, etc.)Picoscope 4262 in a sig gen + scope setupSpectrum analyzer for input / output amplitude measurement instead of scope, still use sig gen + transformer + DC supply as stimulus. Can this measure in dBVrms units? Can this handle DC offset? I'm guessing it is always or can be AC coupled.Vector network analyzer, although they tend to have high minimum frequencies (9kHz, etc.). Can this handle DC offset?

author: tautech
author_rank: Super Contributor
timestamp: September 14, 2022, 08:02:25 pm
content: Quote from: slugrustle on September 14, 2022, 07:43:27 pmAnd that stimulus source is from ?

author: tszaboo
author_rank: Super Contributor
timestamp: September 14, 2022, 09:50:14 pm
content: I'm, not surprised, 80dB is a lot. Scopes have 8 bit ADCs so very low dynamic range when it comes to these sort of measurements. You could try setting the AWG into white noise mode and measure the filter with your scope's FFT mode, based on your windows and your setting, you might be able to lower the noise floor. Higher dynamic range is usually done by instruments called dynamic signal analyzers. But I only used one once, not sure how to set it up. DMM is useful because it can measure 10V and 100mV without too much error, but that's also going to have trouble above 80dB dynamic range.How about specialized audio gear?

author: tautech
author_rank: Super Contributor
timestamp: September 14, 2022, 09:56:35 pm
content: Quote from: tszaboo on September 14, 2022, 09:50:14 pmYet comfortably within the capability of the Siglent Bode plot feature.Proof is here:https://www.eevblog.com/forum/testgear/siglent-sds1x04x-e-bodeplot-ii-(sfra)-features-and-testing-(coming)/

author: slugrustle
author_rank: Frequent Contributor
timestamp: September 14, 2022, 11:19:10 pm
content: 20Vpk-pk sine wave source from SDG1032X signal generator. Usually through 1:1 injection transformer (Picotest J2100A) so I can also provide DC offset.Maybe the Siglent Bode plot thing can do 80dB with ideal input settings. I was using 10x scope probes. Even with AC coupling and 20MHz bandwidth limit, I was getting near the noise floor of the scope on the filter output signal.That ties into a solution that might actually work though: A low noise amplifier with high input impedance that can work at 10x gain into a 50Ω input on my scope. This way the signal is larger and can work with a scope input that has lower noise.

author: tautech
author_rank: Super Contributor
timestamp: September 14, 2022, 11:47:58 pm
content: You can do it all in 50 Ohms but you are limited by the 50 Ohm input max rating of 5V in SDS2000X Plus scopes.Instead of using probes get some/at least 2 of these:Then with a BNC Tee from the stimulus connect the croc clips to your DUT and the other output of the Tee to the reference input selected in the Bode plot feature. Then output of the filter/DUT with croc clips to the channel chosen for the Bode plot result.Set the stimulus output to 50 Ohm and apply the corresponding settings and appropriate amplitude limits/range in the Bode plot menus and just let her rip ! The scope takes charge of the AWG to control and performs the sweep.Yes, it's almost that darn simple.

author: slugrustle
author_rank: Frequent Contributor
timestamp: September 15, 2022, 12:03:18 am
content: So far, I've been using a 10x probe on the filter output to avoid loading the filter output too much (10MΩ vs. 1MΩ), hence the idea of using an amplifier. For the stimulus signal, I do have some nice 1x probes that I could have used, but I didn't have noise problems with the stimulus signal...An ADA4637-1 opamp is looking good as an amplifier for this sort of application, although I'll either need to keep the scope input at 1MΩ or put a buffer after that opamp to drive a 50Ω scope input. If I do this, I'll start with the 1MΩ input impedance and see how it goes. Unfortunately, the SDS2000X+ series datasheet has very little information on noise levels.

author: tautech
author_rank: Super Contributor
timestamp: September 15, 2022, 12:31:15 am
content: Then set everything up at HiZ then and you'll have no voltage limits and not load the filter.Lemme connect the bandpass RC filter I have here to Bode plot (No SDS2000X Plus available) but it will be with SDS1104X-E or SDS6204A and sweep it at low levels, something I've not done before but only a 10x attenuator to get 'right down there' could be an issue but we'll suck it and see how it goes.Get some rest as it might be a few hours before I can get onto this.

author: slugrustle
author_rank: Frequent Contributor
timestamp: September 15, 2022, 12:46:25 am
content: What do you mean by HiZ? I only see 1MΩ and 50Ω input impedance options on the scope. One of the filters I was testing had 110kΩ impedance at DC, hence preferring the 10MΩ load of a 10x probe vs. the 1MΩ scope input impedance.

author: tautech
author_rank: Super Contributor
timestamp: September 15, 2022, 12:54:06 am
content: Quote from: slugrustle on September 15, 2022, 12:46:25 amhttps://www.maximintegrated.com/en/glossary/definitions.mvp/term/Hi-Z/gpk/1175

author: slugrustle
author_rank: Frequent Contributor
timestamp: September 15, 2022, 01:23:27 am
content: Right, that is a standard definition of the term. Can you explain what you mean in the context of this thread?

author: tautech
author_rank: Super Contributor
timestamp: September 15, 2022, 01:48:07 am
content: Quote from: slugrustle on September 15, 2022, 01:23:27 amThat I'll be using HiZ scope inputs and AWG output, ie. 1MIf we refer to your SDG2000X specs, the output can be either HiZ or 50 Ohms and when we set up both the scope and AWG input/output settings to match the measurements/settings on each will match.As such we need set the Bode plot settings correctly for this to hapen so to get properly accurate measurements as a result.A strong advocate of SDS2000X Plus Bode plot, member mawyatt has joined the thread and I hope he can offer some further tips based on his extensive experience with this feature.

author: mawyatt
author_rank: Super Contributor
timestamp: September 15, 2022, 02:52:05 am
content: Agree with tautech about using the Hi Z modes, use directly with AWG BNC cable shown to DUT and 10X/1X scope probe for DSO CH1 (Input) and set to 1X scope probe for CH2 (Output). You want to place the highest possible signal on the DUT input assuming the filter input can handle the larger voltage swing (caution if polarized caps are present, then you'll need to add DC bias), this produces the highest DR within the Bode Plot routine since the input and outputs are dynamically scaled during the sweeps which produces a DR well beyond what an 8 bit ADC can provide. The built-in AWG with the SDS2000X+ has lower output capability and you would be better served with a separate AWG which can supply a higher voltage stimulus for the input.What are the details of the LP filters you wish to measure?We were/are impressed with the Bode capability built-in the SDS2000X+. The "Frequency Selective Measurement" method works very well to "pull out" the signal from the noise environment, and the dynamic scaling also works well to enhance the overall DR. This is not a typical hobby level feature you might expect on a lower end DSO, but in the hands of a capable user, a very powerful and useful feature and our only criticism is it's a little slow and forgets the IP address of AWG when using LAN (fixed with latest firmware update)We found this Bode feature so useful we built up a couple Isolation Transformers for use with Closed Loop Bode Measurements that the OP might find worthwhile.https://www.eevblog.com/forum/testgear/diy-transformer-for-use-with-bode-plots/Anyway, hope this helps.Best,

author: tautech
author_rank: Super Contributor
timestamp: September 15, 2022, 03:19:22 am
content: Thanks very much Mike and FYI slugrustle has the SDG1032X standalone AWG with similar capability to your SDG2000X and IIRC you have a SDG6000X too ?And I still need find time to get onto this............

author: rf-loop
author_rank: Super Contributor
timestamp: September 15, 2022, 08:25:37 am
content: Quote from: tszaboo on September 14, 2022, 09:50:14 pmSiglent FRA have continuous AGC and also receiver channels are frequency selective.FRA itself whole dynamic range is over 130dB independent of if 8 bit ADC as in SDS2kX+ or 12bit ADC in SDS2kXHD and even roughly same with SDS1kX-E.But there is no single simple truth. Now of course it need define what we mean with dynamic range. In bottom level we need define what noise level we accept - what level measurement uncertainty and so on, so on.Siglent FRA (also known as "BodePlot") have continuous AGC (Automatic channel Gain Control) what can step full level range in every freq step(1). And more, also receiver channels are frequency selective (center is moving during sweep).(1)50ohm 500uV/div to 1V/div and 1M 500uV/div to 10V/div.FRA itself whole dynamic range is (nearly(2)) independent of if 8 bit ADC as in SDS2kX+ or 12bit ADC in SDS2kXHD and even roughly the same also with SDS1kX-E.(2)If AGC is on, as is default. User can turn it also off (more fast sweep but reduced dynamic range)But there is no single simple truth - as nowhere. Now of course it needs to define what we mean by dynamic range. In bottom level we need to define what noise level we accept - what level measurement uncertainty and so on, so on.One limit is also maximum level what depends about generator and other things..If we think in some freq sweep DUT top level in some frequency point is 0dBm. Distance to FRA bottom noise level is over 100dB.If DUT max out level is 10dBm then distance to FRA noise level is over 110dB... And so on.If use oscilloscope internal 50ohm it set limit to maximum level. It isa then around 22dBm (because 1V/div max)If use external 50ohm load and inputs 1M. Max is now 42dBm. Now FRA noise level is over 140dB down from this level.One common way to tell oscilloscope FRA dynamic rangeis tell dynamic range from 0dBm level to down. (example Keysight use this, but without definition about bottom level quality etc)Naturally if user turn FRA AGC off then dynamic range is reduced.If do not want talk dBm levels everyone can calc what are dBV or what ever other units.Here is one tiny example about FRA dynamic range down from 0dBm level. (SDS2504X HD)C4 is just only for display FRA bottom noise level. (cascaded 2 cheap IF filter for more stop band attenuation)Here other example, old quick test with SDS2kX Plus. And in this case I have extremely heavy problems to step this attenuator.(0dBm signal go to attenuator as DUT in and attenuator out is as DUT out.)Most of sweeps go to garbage due to atten step problems and finally I get this... and after then I throw this mad stepping attenuator "out from window". Steps levels was ok but stepping mostly fails ... arrghhh.., )C3 trace is just for display FRA bottom noise level.As can see -90dBm have some random noise and -100dBm more. It is up to user who can define what he accept. Still visually it is quite clear where is average.But if there is narrow dip...and need measure its bottom level. After every sweep level is different, how accurate user need measure, again up to user needs.In next image: Vertical top 40dBm to bottom noise level is Siglent FRA dynamic range if we define it using noise level.Note: Base noise level depends freq. (here limited to 60MHz, you can extrapolate with eyes how it continue to 120)If want think dBV then top is 27dBV and 100dBm line is -113dBV(As can see sweet pot is around 400kHz just nice for 450-500kHz IF filters)This image, below, is also with SDS2kXHD but Xplus is roughly same in this matter.Sad we do not have trace average function for reduce FRA displayed random noise.---------------For OPI believe Siglent FRA (SDS2kXPlus) and SDG1000X can do this job quite easy. Specially if you can drive circuit using 20Vpp and filter out in passband have not significant attenuation. (What level is not possible using SDS internal generator).With 10x probe (DUT out) still available dynamic range is quite lot. But then, this BodePlot is also very slow specially with low frequencies.SDS2kPlus FFT can also try.

author: mawyatt
author_rank: Super Contributor
timestamp: September 15, 2022, 03:21:11 pm
content: Quote from: tautech on September 15, 2022, 03:19:22 amYes we have both , and put both to good use often. The waveform quality/performance is outstanding for the $, so one can overlook the UI quirksFor the OP, since the DR limiting factor with the Bode Mode as shown by rfloop is the DSO Output Channel noise floor. One wants to drive the DUT input with the largest practical input signal as we mentioned and use no attenuation on the DSO Output Channel, which gets the largest signal to the DSO Channel for the best Output sensing SNR.Also the OP mentioned measuring filter responses with a quality DMM, this is certainly practical at lower frequencies but does not convey any phase information. Many filter applications the phase response is very important (signal processing for example), so here the DSO Bode Mode or a full fledged Network Analyzer is in order.For analysis of power line/supply filters such as "T" Notch, LRC Low Pass and RC Low Pass the modern Bode Capable DSO is an excellent instrument of choice IMO. One can "see" the effects of loading, DC bias in somewhat real time (OK the SDS is a little slow) without having to resort to a PC and plotting.....this is why we always prefer a stand alone DSO rather than a PC enabled scope (we have the excellent Picosope 4262 but rarely used).Best,

author: Someone
author_rank: Super Contributor
timestamp: September 15, 2022, 10:43:21 pm
content: Quote from: rf-loop on September 15, 2022, 08:25:37 amIt is very complex with no single truth as you say. Were your example plots using a 20 or 40dBm injection signal? the channels didn't seem to be high enough scale for that. Other effects come to play with increasing the stimulus signal, crosstalk of the scope channels, and leakage/crosstalk in the DUT. So it is not always adding more stimulus amplitude = more dynamic range.It would be good to show an actual configuration achieving more than 120dB dynamic range, rather than just moving the reference value and promising itmightcould achieve it.

author: David Hess
author_rank: Super Contributor
timestamp: September 16, 2022, 03:52:20 am
content: Take advantage of your DSO's external trigger to synchronously demodulate the signal of interest. Attach the external trigger or another channel to the signal generator's output and use that as the trigger source. Now when averaging is used on the DSO, everything except the signal from the function generator, including noise, is removed.

author: rf-loop
author_rank: Super Contributor
timestamp: September 16, 2022, 08:22:26 am
content: Quote from: Someone on September 15, 2022, 10:43:21 pmQuoteIn these examples naturally not because my test signal to DUT input was 0dBm for look things down from 0dBm (previously told why).QuoteNow we need separate things what we are talking. I am talking more about FFT performance. DUT leakage/crosstalk is what it is, I do not handle here DUT characteristics. Let's keep these separate.Crosstalk inside FRA apparatus aka example inside Siglent some oscilloscope is of course part of FRA performance - characteristics and need take care for avoid traps. Every engineer can think how to arrange test setup and how to reduce these kinds of things and/or think how much these affect result. For high dynamic range needs, one possibility is reduce reference signal level so that it is not as high as DUT in level, just for reduce crosstalk effect.In Siglent FRA all 4 channels (Reference channel aka "DUT in" all 3 DUT out channels) and have independent and continuous automatic gain control during sweep (every freq step check level) and this AGC range is 500uV/Div to 10V/div (1x level). (except when internal 50ohm then it is reduced to max 1V/div (1x levels))Naturally example 40dBm level need external 50ohm load (if we are talking about 50 ohm systems)In many cases FRA reference channel input level can attenuate for avoid cross talkaffect inside oscilloscope between channels.But in multichannel FRA there can be crosstalk between channels inside oscilloscope what may be more challenging.One tiny example about crosstalk.This is SDS2kXHD. (naturally whole crosstalk is much more things than just this.)Channels 2 and 3 do not have signal and inputs also have metal cap (not shorting model) for isolate possible external noises.These channels display just FRA noise floor in this setup.Channel 4 is DUT out. (Reference channel (C1) level is lot less than DUT out max peak)Now if look CH2 and CH3 there can see crosstalk from Dut out Ch4 when DUT out peaks. If look Ch3 and Ch2 peaks distance to CH4 peak (~84090Hz) it is well over 125dB.Naturally cross talk is highly dependent about frequency.So, user need (know his equiment and) think this specially when do multi channel FRA or if need very high dynamic range so that reference channel cross talk to DUT out channel does not spoil the result.In this image sweep step is 1Hz and span 500Hz. (For narrow things 1Hz is like 12" wrench on watchmaker table.)Let's hope Siglent give some day more steps/more resolution. For example, 10kHz span resolution is only 20Hz.User need have enough knowledge and experience for use these things and very importantly know his instruments performance and features.QuoteYes. Indeed, of course.I think this place/thread is not the best possible place to go deeper and talk about only the performance and features of Siglent FRA, and of course that includes grains and chaff. Good and bad sides - as in everything that man does. It also depends a lot on the individual needs of the user - and the pocketbook - which can affect what inconveniences are tolerated.

author: Someone
author_rank: Super Contributor
timestamp: September 16, 2022, 09:06:17 am
content: Quote from: rf-loop on September 16, 2022, 08:22:26 amThank you for additional examples, I think crosstalk and feedthrough is more realistically measured with the probe/cable connected with the DUT but to/through a short. Higher end equipment is usually using differential measurement to get such large dynamic ranges. I do not think there is real demonstration of 120dB with such easy setup/equipment.

author: mawyatt
author_rank: Super Contributor
timestamp: September 16, 2022, 05:40:58 pm
content: Out of curiosity we did some Bode Plots of RC and an Active (2N3904) LPF using the Bode feature within the SDS2000X+ and SDG2042X AWG.1) Screen capture #77 is with 100 ohm series R with 10uF Mylar Shunt C.2) #78 is with 10uF electrolytic and using Voltage Offset Feature to keep cap from "seeing" a negative voltage a low frequencies.3) #79 is with 10uF Polypropylene (Two 5uF in parallel).4) #80 is with 1uF Mylar.5) #81 is with 1uF Polypropylene.6) #82 is an "active" type LPF with 2N3904 acting to lower output impedance as "seen" by load, shunt C from Base is 0.47uF Mylar, R from Collector to Base is 1K and load is 1K on Emitter, Input is to Collector.7) #83 is same as 6 but with an additional 0.1uF Mylar shunt C across load.Note, used the Bode Function Voltage Offset feature to Bias the Active Filter and supply load current (~5ma), so no need for additional equipment. Very handy feature indeed!!Best,

author: nctnico
author_rank: Super Contributor
timestamp: September 16, 2022, 05:54:19 pm
content: Quote from: slugrustle on September 15, 2022, 12:46:25 amA 10M Ohm probe isn't going to do much good either. These have enough capacitive load to seriously affect your readings by adding an extra pole to your filter; you need a different probing solution to accurately measure filters like that at frequencies over 100kHz. Think about incorporating an amplifier in your circuit that buffers the signal into a 50 Ohm output and feed that into an oscilloscope or network analyser.BTW low frequency network analysers do exist. Like the Anritsu MS4630. Such equipment may also be called audio or dynamic signal analysers (but check the max. frequency; some only go to 100kHz).