Interdigital capacitor in Bandpass filter - Page 1
Forum section: electronics / rf-microwave

author: dmowziz
author_rank: Contributor
timestamp: September 20, 2023, 09:18:58 pm
content: Hi. The first image is bandpass filter.After running EM simulation, the red response is like with a 'normal' capacitor (But it has insertion loss).The blue response is with an interdigital capacitor which has a value of 0.168pF and q of 162Using a normal capacitor with a low q does not give the response as with the interdigital.I have done EM simulation ( on a layout)Thanks.

author: dmowziz
author_rank: Contributor
timestamp: September 22, 2023, 01:39:14 pm
content: Please any idea will be great.Or any input from someone that has used these kind of capacitorsThanksss

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 22, 2023, 02:35:22 pm
content: Beats me. I see a schematic, and a response that clearly doesn't match the schematic (if it's showing what you intended, anyway). Your answer lies somewhere inbetween.For example: I have no idea what the schematic elements consist of. There are TL elements, implying a port based analysis. Are there common-mode aspects to these components? That is, can the components actually be modeled as one-ports as you intend?A typical issue with narrow bandpass filters, is trying to make a resonator as a one-port (e.g. a conventional ladder topology with alternating shunt L||C and series L+C), when it clearly cannot be implemented as such in the real world (the shunt resonators' Zo must be extremely low, and the series, extremely high; nonphysical values easily manifest e.g. few nH or fractional pF). The solution is to transform it into a chain of coupled resonators, designed at nominal Zo, with matching networks to set the desired (system) Q and thus response. Coupling most often is series caps (which begets a shallow HF slope) or mutual inductance (symmetrical response).Or, embrace the strays and use them to advantage. On this network,https://www.seventransistorlabs.com/Images/FilterBreadboarded.jpghttps://www.seventransistorlabs.com/Images/FilterResponse1G.png(signal source is oscillator on the left, represented by V2 on the schematic; SOT-23s from left to right are: varactor, oscillator, Q1, Q2)it started as a... 3rd order bandpass I think, with the observation that Q1 needs bias so an inductor belongs here (L1), and maximum gain is obtained from minimizing Q1-C load capacitance (about 1pF in the MMBTH10 itself, plus fractions more on the PCB), hence a parallel resonant tank is available to start off with. A series resonant tank goes inbetween, and another parallel resonant tank loads Q2-B. Well, the stray capacitance (to GND) wasn't going to work out for that -- the inductor was reasonable (wirewound ceramic-core chip inductors have good L and Q at this frequency) but the C6-C2-C5 motif is approximately what the middle of that resonator looked like. Which means there's an impedance matching network in the middle of the thing, and... tweaking other values around, gets something closer, but, I don't recall what exactly didn't work out with it but I ended up changing it anyway.In any case, the final design changed to a 5th order lowpass, with 2nd order highpass to make a reasonable bandpass response. The LF asymptote isn't so important (like I said, oscillator; VCO), but harmonic filtering is desired, and stray capacitance can indeed be used for implementing zeroes, making it a customized part-elliptical sort of filter. This gives a much sharper cutoff, and reasonably low stopband,Incidentally, note that R13 is part of the filter. Q2 has significant Miller effect, meaning input impedance depends on gain. I forget what the figure was; something like 5pF || 500Ω maybe, for the impedance looking into Q2-B. Being able to tweak it by R13 makes C3 a lot easier to tune.Oh, anyway, the result ended up something like, about 1kΩ impedance looking into the filter from Q1-C, so there's a bit of impedance-matching gain between Q1 and Q2 (versus a fixed-Z filter). Nice little things like that with customized networks.Tim

author: dmowziz
author_rank: Contributor
timestamp: September 22, 2023, 07:48:34 pm
content: Quote from: T3sl4co1l on September 22, 2023, 02:35:22 pmHi Tim, thanks for the response.The TL are just microstrip lines, c16 is an interdigital capacitor.Anyways, I think I've seen my mistakes.Please can you post the oscillator schematics?On the board, there's a 330 ohms but not in the schematicsis the FilterResponse1G the result from a spectrum analyzer or simulation?and is it just the response of the filter part or the whole schematic? What was V2's powerI'm thinking a 1nF at 100MHz has already started behaving like an inductor?and why didn't you model L5?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 23, 2023, 01:01:11 am
content: Oscillator is just a common-collector Colpitts with dual-diode varactor, and variable bias to both (hence the wires); 330R is osc supply.That's the simulated response of course. I took a rough real measurement by varying the oscillator; which wasn't well calibrated (frequency not proportional to sweep voltage), but was within the margin expected from simulation so I'm fine with it.A 1nF 0805 might be 3nH ESL, which is resonant right about in this band, meaning it has its lowest impedance (best bypass action) here, and in any case that impedance is orders of magnitude less than connected nodes (ohms vs. kohm) so it is effective bypass.What about L5? Surely you would be most concerned about L2 and L3, being involved in the transfer function. I don't recall if I tried them with detailed models (Coilcraft publishes reasonably accurate SPICE models for much of their product line), but this is also well in the middle of their inductive range (seehttps://www.coilcraft.com/getmedia/0d2d6445-1567-49d3-b969-9190d57aa771/1206cs.pdf), so I have no concerns about them.Indeed L5 being used as RFC into 50 ohms, implies it could be as low as 84nH, or have stray capacitance as large as 24pF.Tim