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| [Mostly Solved] LC Bandpass Filter Resonates, but with 30-40 dB Insertion Loss |
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| niconiconi:
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 . |O |O 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? |
| niconiconi:
PCB schematics and PCB layout reupload. |
| JohnPen:
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 |
| niconiconi:
I'm still troubleshooting it. I used the L_test footprint on the board and measured the inductors using NanoVNA. --- Quote ---68 nH inductor: 1.10 R + 78.0 nH @ 20 MHz 2.73 R + 76.0 nH @ 100 MHz 27 nH inductor: 0.62 R + 36.6 nH @ 20 MHz 1.30 R + 35.3 nH @ 100 MHz 1 uH inductor: 7 R + 1.44 uH @ 20 MHz 939 R + 27 pF @ 100 MHz 2.2 uH inductor: 0.2 R + 2 uH @ 50 kHz 101R + 784nH @ 20 MHz 287R + 85nH @ 100 MHz --- End quote --- 68 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. |
| niconiconi:
--- Quote from: JohnPen on October 30, 2019, 03:25:21 pm ---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 --- End quote --- The 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. |
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