# Problem measuring notch filter **URL:** \ **Category:** QA401\ **Created:** [June 8, 2021, 3:51pm UTC](https://forum.quantasylum.com/t/problem-measuring-notch-filter/363 "2021-06-08T15:51:41Z")\ **Posts on this page:** 16\ **Page:** 1
**Author:** ![ceulrich](https://avatars.discourse-cdn.com/v4/letter/c/aca169/32.png) [@ceulrich](https://forum.quantasylum.com/u/ceulrich)\ **Post date:** [June 8, 2021, 3:51pm UTC](https://forum.quantasylum.com/t/problem-measuring-notch-filter/363/1 "2021-06-08T15:51:41Z")
I am new to this forum, so if this post blows up, please bear with me. I am working with a 1 KHz passive notch filter. The design is based on the Hall Network as described in a paper by Kuhn ([https://www.kennethkuhn.com/electronics/design\_and\_applications\_of\_the\_hall\_network.pdf](https://www.kennethkuhn.com/electronics/design_and_applications_of_the_hall_network.pdf)) I am using the same component values as Kuhn, except I am using fixed resistors rather than potentiometers for fine tuning. I am getting some anomalous results using the FreqResponseChirp Test Plugin on the QA401 running V 1.920. I read the Notch Filters blog post, but did not find anything that pointed me toward a solution. See the attached screen shot. As a rough check, I collected some single point measurements using a function generator and DMM (HP33120A and HP34401A, respectively). The insertion loss at 50 Hz was -3.97 dBV rather than -10 to -11 dBV, and the shape of the notch was much more symmetrical, i.e., at 50 KHz the output was -1.16 dBV. Any advice would be greatly appreciated. Thanks, ceulrich ![image](https://canada1.discourse-cdn.com/flex032/uploads/quantasylum/original/1X/837313c57b46f2cb51c38f14b3170b9a15a1267c.jpeg) ---
**Author:** ![matt](https://avatars.discourse-cdn.com/v4/letter/m/a6a055/32.png) [@matt](https://forum.quantasylum.com/u/matt)\ **Post date:** [June 8, 2021, 8:53pm UTC](https://forum.quantasylum.com/t/problem-measuring-notch-filter/363/2 "2021-06-08T20:53:53Z")
Hi @ceulrich, how does the notch filter look if you use a series of discrete sines? ![image](https://canada1.discourse-cdn.com/flex032/uploads/quantasylum/original/1X/2894ac4aca5e5ae5b21e821c2336d3bc14a58b35.png) ---
**Author:** ![ceulrich](https://avatars.discourse-cdn.com/v4/letter/c/aca169/32.png) [@ceulrich](https://forum.quantasylum.com/u/ceulrich)\ **Post date:** [June 9, 2021, 4:15pm UTC](https://forum.quantasylum.com/t/problem-measuring-notch-filter/363/3 "2021-06-09T16:15:31Z")
Hello Matt, Thanks for the response. See the attached. Cheers, ceulrich ![20 Hz to 50 KHz - 0 dBV Input - Log 5 per Decade - 10 Hz per Step - Final 06-09-21](https://canada1.discourse-cdn.com/flex032/uploads/quantasylum/original/1X/56b458d0cda8b28bf914f86f51ef938ed720642c.jpeg) ---
**Author:** ![matt](https://avatars.discourse-cdn.com/v4/letter/m/a6a055/32.png) [@matt](https://forum.quantasylum.com/u/matt)\ **Post date:** [June 9, 2021, 6:42pm UTC](https://forum.quantasylum.com/t/problem-measuring-notch-filter/363/4 "2021-06-09T18:42:53Z")
Hi @ceulrich, since your sweep using a chirp and also using tones looks the same, I think it’s believable. If you still have low confidence, then you can do a sweep in loopback mode to convince yourself the QA402 response is flat. There could also be some issues related to input and output impedance. The output impedance of the QA402 is 100 ohms, and the input impedance is 100k. What are the values you are using in your notch? Do you have a schematic? ---
**Author:** ![ceulrich](https://avatars.discourse-cdn.com/v4/letter/c/aca169/32.png) [@ceulrich](https://forum.quantasylum.com/u/ceulrich)\ **Post date:** [June 10, 2021, 6:30pm UTC](https://forum.quantasylum.com/t/problem-measuring-notch-filter/363/5 "2021-06-10T18:30:41Z")
Hello Matt, The schematic is attached. Yes, I did not think about the input impedance of the QA401. With this passive design, the 100K is a bit low to accurately characterize the shape of the left-hand side of the notch. However, at this stage I am only interested in defining the insertion loss for frequencies between 2K and10K, and it looks like the QA401 will be able to do this. Thanks for the guidance. Cheers, ceulrich ![Passive 1 KHz Notch Filter - Fixed Resistors 1](https://canada1.discourse-cdn.com/flex032/uploads/quantasylum/original/1X/11630c34a7907b9d4f413d6ec7d9cf65284a86e7.jpeg) ---
**Author:** ![FdW](https://avatars.discourse-cdn.com/v4/letter/f/a587f6/32.png) [@FdW](https://forum.quantasylum.com/u/FdW)\ **Post date:** [June 10, 2021, 7:37pm UTC](https://forum.quantasylum.com/t/problem-measuring-notch-filter/363/6 "2021-06-10T19:37:25Z")
Maybe I’m thinking to simple, but why not do a simulation ![Notch](https://canada1.discourse-cdn.com/flex032/uploads/quantasylum/original/1X/1610735b44a5e79c2690f9dbbc8353afafecc5c4.png) ---
**Author:** ![matt](https://avatars.discourse-cdn.com/v4/letter/m/a6a055/32.png) [@matt](https://forum.quantasylum.com/u/matt)\ **Post date:** [June 10, 2021, 9:51pm UTC](https://forum.quantasylum.com/t/problem-measuring-notch-filter/363/7 "2021-06-10T21:51:38Z")
Hi @FdW, Is your conclusion that the spice with the 100K load appears as you are seeing? It looks that way to me. ---
**Author:** ![FdW](https://avatars.discourse-cdn.com/v4/letter/f/a587f6/32.png) [@FdW](https://forum.quantasylum.com/u/FdW)\ **Post date:** [June 11, 2021, 9:32am UTC](https://forum.quantasylum.com/t/problem-measuring-notch-filter/363/8 "2021-06-11T09:32:54Z")
Hi Matt, I do not completely understand the question, but what I will say is. I think that a simple simulation would have given a real impression of what to expect in the first place and would have removed to surprise of seeing something that could have been expected. The other ting is, I do have a vested interest in the QA402 and other devices, so when the schematic was published I just simulated the notch-filter to do a reality/virtuosity check and then posted my result to share. Any way, I would have thought that publishing a picture that matched reality (as was measured) with theory(as simulated) would ease any doubts that may exist on the work done and the equipment used. For me it was just a good result that I was happy to show. ---
**Author:** ![FdW](https://avatars.discourse-cdn.com/v4/letter/f/a587f6/32.png) [@FdW](https://forum.quantasylum.com/u/FdW)\ **Post date:** [June 11, 2021, 9:36am UTC](https://forum.quantasylum.com/t/problem-measuring-notch-filter/363/9 "2021-06-11T09:36:01Z")
And I still do not understand your question. In other words, my conclusion is that the measurement seems to indicate that reality matches prediction and that is a good thing I guess. ---
**Author:** ![ceulrich](https://avatars.discourse-cdn.com/v4/letter/c/aca169/32.png) [@ceulrich](https://forum.quantasylum.com/u/ceulrich)\ **Post date:** [June 11, 2021, 4:49pm UTC](https://forum.quantasylum.com/t/problem-measuring-notch-filter/363/10 "2021-06-11T16:49:50Z")
Hello FdW, Hello Matt, FdW, Thanks for doing the simulation, it does clearly show where my “problem” lies. Unfortunately I do not have simulation software. When I got the funny looking notch, I just assumed I had a setting wrong somewhere, and didn’t consider a simple impedance mismatch. I agree with you, the QA401 – and now the QA402 – are great devices, particularly considering the prices, and the support. However, like so many things, it is only as good as the operator. Cheers, ceulrich ---
**Author:** ![DanClark](https://avatars.discourse-cdn.com/v4/letter/d/d26b3c/32.png) [@DanClark](https://forum.quantasylum.com/u/DanClark)\ **Post date:** [June 15, 2021, 12:07am UTC](https://forum.quantasylum.com/t/problem-measuring-notch-filter/363/11 "2021-06-15T00:07:58Z")
Thought you might find this listing of free simulation packages to be of interest… > **[List of free electronics circuit simulators](https://en.wikipedia.org/wiki/List_of_free_electronics_circuit_simulators)** > > List of free analog and digital electronic circuit simulators available for several computing platforms. > Some of the following simulators don't include a graphical visual interface, so a separate program must be chosen to provide that feature, such as Qucs-S, Oregano, or a PCB suite that supports external simulators, such as KiCad or gEDA. ---
**Author:** ![FdW](https://avatars.discourse-cdn.com/v4/letter/f/a587f6/32.png) [@FdW](https://forum.quantasylum.com/u/FdW)\ **Post date:** [June 15, 2021, 7:59am UTC](https://forum.quantasylum.com/t/problem-measuring-notch-filter/363/12 "2021-06-15T07:59:25Z")
Nice list The one used for the simulation above is [LTspice - Wikipedia](https://en.wikipedia.org/wiki/LTspice) User support at [Installing and using LTspice IV (now including LTXVII). From beginner to advanced. - diyAudio](https://www.diyaudio.com/forums/software-tools/260627-installing-using-ltspice-iv-including-ltxvii-beginner-advanced.html) ---
**Author:** ![ceulrich](https://avatars.discourse-cdn.com/v4/letter/c/aca169/32.png) [@ceulrich](https://forum.quantasylum.com/u/ceulrich)\ **Post date:** [June 15, 2021, 2:01pm UTC](https://forum.quantasylum.com/t/problem-measuring-notch-filter/363/13 "2021-06-15T14:01:00Z")
Hello DanClark & FdW, DanClark, thanks for the info. I did manage to learn KiCad, so there is a chance I might also be able to handle a simulation program. I work mostly on a Mac, but I do keep a Windows 7 laptop around just for must have programs like REW and QA. Is there a simulation program you would recommend for a novice? My large screen desk machine runs OS 10.13. Cheers, ceulrich ---
**Author:** ![DanClark](https://avatars.discourse-cdn.com/v4/letter/d/d26b3c/32.png) [@DanClark](https://forum.quantasylum.com/u/DanClark)\ **Post date:** [June 15, 2021, 5:04pm UTC](https://forum.quantasylum.com/t/problem-measuring-notch-filter/363/14 "2021-06-15T17:04:43Z")
I’d try LTSpice first since it supports OS X and also has a visual editor. Unless you’re doing digital circuits that’s probably your easiest option as it looks like you’re more OS X oriented (as am I)… ---
**Author:** ![FdW](https://avatars.discourse-cdn.com/v4/letter/f/a587f6/32.png) [@FdW](https://forum.quantasylum.com/u/FdW)\ **Post date:** [June 15, 2021, 5:43pm UTC](https://forum.quantasylum.com/t/problem-measuring-notch-filter/363/15 "2021-06-15T17:43:11Z")
Seems like a good idea. as you see LTSpice in my choice also. Here is the above notch filter, save all following as a ‘notch.asc’ file Version 4 SHEET 1 880 680 WIRE -48 144 -64 144 WIRE -16 144 -48 144 WIRE 16 144 -16 144 WIRE 336 144 96 144 WIRE 432 144 416 144 WIRE 464 144 432 144 WIRE 480 144 464 144 WIRE -16 224 -16 144 WIRE 16 224 -16 224 WIRE 112 224 80 224 WIRE 176 224 112 224 WIRE 320 224 240 224 WIRE 352 224 320 224 WIRE 432 224 432 144 WIRE 432 224 416 224 WIRE 112 240 112 224 WIRE 320 240 320 224 WIRE 464 288 464 144 WIRE 480 288 464 288 WIRE 480 304 480 288 WIRE 480 304 464 304 WIRE -48 320 -48 144 WIRE 320 336 320 320 WIRE 320 336 208 336 WIRE 112 352 112 320 WIRE 208 352 208 336 WIRE 320 352 320 336 WIRE 464 352 464 304 WIRE -48 448 -48 400 WIRE 112 448 112 432 WIRE 112 448 -48 448 WIRE 208 448 208 432 WIRE 208 448 112 448 WIRE 320 448 320 432 WIRE 320 448 208 448 WIRE 464 448 464 432 WIRE 464 448 320 448 WIRE 208 464 208 448 FLAG 208 464 0 FLAG 480 144 out FLAG -64 144 in SYMBOL res 112 128 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R1 SYMATTR Value 233.7k SYMBOL res 432 128 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R2 SYMATTR Value 4.99k SYMBOL cap 80 208 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C1 SYMATTR Value 6.8n SYMBOL cap 240 208 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C2 SYMATTR Value 6.8n SYMBOL cap 416 208 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C3 SYMATTR Value 6.8n SYMBOL res 96 224 R0 SYMATTR InstName R3 SYMATTR Value 27k SYMBOL res 304 224 R0 SYMATTR InstName R4 SYMATTR Value 2.2k SYMBOL res 96 336 R0 SYMATTR InstName R5 SYMATTR Value 6.4k SYMBOL res 304 336 R0 SYMATTR InstName R6 SYMATTR Value 3.24k SYMBOL res 192 336 R0 SYMATTR InstName R7 SYMATTR Value 238.7k SYMBOL voltage -48 304 R0 WINDOW 123 24 118 Left 2 WINDOW 39 0 0 Left 0 SYMATTR Value2 AC 1 SYMATTR InstName V1 SYMATTR Value SINE(0 1 1k) SYMBOL res 448 336 R0 SYMATTR InstName R8 SYMATTR Value 100k TEXT -80 480 Left 2 !.ac dec 1k 1 1Meg ---
**Author:** ![YamahaCA](https://avatars.discourse-cdn.com/v4/letter/y/57b2e6/32.png) [@YamahaCA](https://forum.quantasylum.com/u/YamahaCA)\ **Post date:** [January 28, 2025, 5:31pm UTC](https://forum.quantasylum.com/t/problem-measuring-notch-filter/363/16 "2025-01-28T17:31:23Z")
To pick your knowledge, how can I test the log plot on a potentiometer? I imagine a chart defining dbv in vs dbv out, with the ability to rotate the shaft to full range and then plot.