[Mostly Solved] LC Bandpass Filter Resonates, but with 30-40 dB Insertion Loss - Page 1
Forum section: electronics / projects

author: niconiconi
author_rank: Frequent Contributor
timestamp: October 30, 2019, 01:01:32 pm
content: I'm trying to build a Butterworth LC bandpass filter to extract a 28.8 MHz signal for experiments, so I synthesized a 25-30 MHz Butterworth band-pass filter in Qucs and tweaked the component values to match the parts I have. It works fine in simulation. Unfortunately, when I built the circuit, it resonates strongly at ~28.8 MHz, and I can measure a 50 ohm impedance even if nothing is connected to the output. But I measured an insertion loss around 30-40 dB .The simulated circuit, simulation result, actual circuit, PCB layout, and VNA measuments are attached. I omitted C107, L5, L6 - the last stage of the LC circuit (C4, L4 in simulation) and used some solder to fill the gaps. Dummy load R1 is not installed (not needed for two-port measurement), L7 is not installed, JP1 is closed. Capacitors are the old-fashioned ceramic disc capacitors, inductors are random 0603 inductors, probably multilayer construction.I tweaked the capacitor and inductor values in the simulation to check the effects of extra parasitic elements, but an additional 20 pF or 20 nH cannot cause 40 dB of attenuation.One category of possibility is cold solder-joints, badly etched board, or thin traces, but I've tested on a through-hole prototype board yesterday and seen similar attenuation, it would be extremely unfortunate to have the same cold solder joints in two different constructions. Another possibility is that the inductor has bad RF performance, but it's below 30 MHz and I don't think it matters at this frequency. It's also possible that the inductors are mismarked, but I hope it is not the case here...Where should I start looking at?

author: niconiconi
author_rank: Frequent Contributor
timestamp: October 30, 2019, 01:05:55 pm
content: PCB schematics and PCB layout reupload.

author: JohnPen
author_rank: Regular Contributor
timestamp: October 30, 2019, 03:25:21 pm
content: I did a quick check using the AADE filter simulator which confirms your simulation results. The first stage resonates nearer to 28 Mhz the second stage is nearer to 27Mhz. The series element also resonates around 28Mhz. As you do have a resonance around 28.5 Mhz a guess is that your second stage in the physical circuit is defective in some way. Are the 2 x 2200 disc ceramic you are using the decoupling type if so they might cause unwelcome resonances. If you can replace them with an alternative single capacitor. Otherwise a faulty inductor in the second stage would seem likely. Hope this helps.John

author: niconiconi
author_rank: Frequent Contributor
timestamp: October 30, 2019, 04:07:36 pm
content: I'm still troubleshooting it. I used the L_test footprint on the board and measured the inductors using NanoVNA.Quote68 nH and 27 nH looks good. 1 uH and 2.2 uH looks good at low frequency, the extremely bad high-frequency performance I measured here is possibly untrue, when the impedance is too far from 50R, VNA gives all types of bogus results. But it's still a bit suspicious, especially the 2.2 uH one at 20 MHz. Well, at least I know that no inductors are mismarked.Keep debugging, next steps:1. Remove the second stage, test the filter with L1 and C1 only.2. Order some known-good RF inductors and capacitors.

author: niconiconi
author_rank: Frequent Contributor
timestamp: October 30, 2019, 04:13:03 pm
content: Quote from: JohnPen on October 30, 2019, 03:25:21 pmThe 2 x 2200 pF capacitors are the cheapest no-name brand, you know, those small 3mm disc with hard to read numbers staying in my junkbox for years, I think they are the decoupling type.Thanks for your verification. I think it makes sense now, the most likely explanation is that both the inductors and capacitors are defective in some ways.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 30, 2019, 08:33:39 pm
content: Use higher Q inductors? The first resonator has an impedance of 11.9 ohms but you measured 1.1 ohms ESR at 20MHz, so it has a maximum Q near 10. The last resonator is even worse (~8).Buy known C0G type capacitors. That or film (PP, PPS or other high Q type). Nothing else is suitable.Note that parasitic capacitance to ground, on the series branch, causes an impedance transformation effect. This can be harnessed by intentionally transforming one side or the other to a slightly different impedance, so the end result comes out correct.Note also that you have significant ESL in the second resonator, probably around 10nH. This acts in series with the inductor, lowering the resonant frequency. It also contributes an impedance transformation effect (a tapped inductor) which you will again have to model and compensate for.Tim

author: niconiconi
author_rank: Frequent Contributor
timestamp: October 30, 2019, 10:37:35 pm
content: Quote from: T3sl4co1l on October 30, 2019, 08:33:39 pmWell, I just wanted to use the filter to do one job: extract one harmonic frequency from a signal and kill other harmonic frequencies, the low-Q here actually makes the filter tuning-freeThe measurements were uncalibrated, the connector and traces added some ESR that hasn't been zeroed out, I measured them just for a reality check.Thanks for the suggestion. I have been searching for NP0 capacitors that don't 7-15 business days to ship for an hour since I posted the original thread.

author: niconiconi
author_rank: Frequent Contributor
timestamp: October 30, 2019, 10:40:28 pm
content: Quote from: niconiconi on October 30, 2019, 04:07:36 pmDone. With only the first-stage installed, return loss is -11.28 dB, insertion loss is 2.8 dB, it's reasonable for my purpose.QuoteNeed to do some shopping now.

author: OwO
author_rank: Super Contributor
timestamp: October 31, 2019, 02:07:26 am
content: 0603 inductors have low Q, usually around 10-20 at your frequency. That 3.3uH inductor is likely what's causing problems. 3.3uH at 28MHz is very large and ESR is proportional to inductance (when keeping Q and frequency constant). Try a physical inductor model (there are probably approximate models given Q @ frequency).

author: niconiconi
author_rank: Frequent Contributor
timestamp: November 02, 2019, 09:46:25 am
content: Quote from: OwO on October 31, 2019, 02:07:26 amWhile waiting for shipping, I replaced the 3.3 uH inductor with a ferrite inductor using a high-frequency core found in my partbox, and it immediately improved the insertion loss from 40 dB to 20 dB.I also tried another bandpass filter (wider bandwidth) that only requires 1 uH inductance, and tried the 0603 SMD inductor again, and the best insertion loss I got is 12 dB.Your are absolutely correct, all the components I used are low-Q and lossy, and the series inductor in the middle is the prime culprit. Replacing the "ideal inductor" with "inductor with Q" (Q = 20) in the simulator can immediately reproduce a similar result as seen in the experiment.

author: virtualparticles
author_rank: Regular Contributor
timestamp: December 21, 2019, 04:39:36 pm
content: In my humble opinion, you should make your own inductors. Use the formula L (uH) = (R2*N2)/(9*R + 10*L) where R is the coil radius in inches, L is the length of the coil in inches and N is the number of turns. This is surprising accurate and you get Qs on the order of a hundred without too much trouble. You can also spread or squeeze the turns together to "tune" the filter for best performance.Have fun!