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) 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.
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?
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.
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.
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.
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.
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)…