Why a filter can have low loss but high mismatch - Page 1
Forum section: electronics / rf-microwave

author: jujun
author_rank: Regular Contributor
timestamp: July 18, 2019, 09:07:52 pm
content: Hello,I just built a simple LPF filter for 15Mhz, to be used on 10Mhz and 14Mhz.First I measured the losses at the frequency of interest : 14Mhz, and the losses were quite small about 0,6dB.Unfortunately, then I measured the matching of each port and it was not good at all (I did in both ways).Obviously the filter is a little to low in frequency. I don't understand how a filter can look fine with a low loss band pass, but have a very bad matching. Can you point me to the right direction ?thank you for your help

author: radiolistener
author_rank: Super Contributor
timestamp: July 18, 2019, 10:45:25 pm
content: it seems that your filter is not well tuned. The wave at the top of frequency response is also indicate impedance mismatch. Try to tune it by impedance chart. Impedance chart is much more sensitive for tuning, so you can tune it much more better by monitoring impedance graph. You can check how impedance graph should looks like in the simulator.And don't forget to add dummy load on the other side of filter for impedance measurement.I tuned 7-th order filter in that way (by impedance graph). But it's not easy taskIt's real headache to tune 7-th order filter with 3 coils.When you tune one coil, at some time you're realize that you're needs to tune another coil and vice versa and then you're realize that you're needs to tune third coil... And it happens in a loopIt will be nice if someone can share instruction on how to tune filters in easy way.Because my experience was looks like random tuning and see what happens in a loop

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: July 19, 2019, 08:20:44 am
content: The reason is quite simple really: Your matching at low frequencies is actually kinda good. Imagine you have a return loss of -10 dB. That means that about 1/10th of power is transmitted back. In other words, a lossless filter with a RL of -10 dB will have an output power of 9/10ths the input power. That is only a loss of 0.46 dB!RL of -15 dB means about 96% of the input signal will continue to output. That is a loss of only 0.13 dB. Up to about 14 MHz you almost never dip above -15 dB RL, so the losses due mismatch will never dip above 0.15 dB up to 14 MHz.I quickly made this plot in matlab to show. x-axis is RL, y-axis is IL due to reflections, both in dB.

author: radiolistener
author_rank: Super Contributor
timestamp: July 19, 2019, 09:04:39 am
content: can you show schematic of your filter. It will be more easy to simulate it and compare your impedance chart with ideal one than talking about parameters of abstract filter with unknown schematic.For example, here is 15 MHz Chebyshev LPF 7-th order:And it's simulation assuming inductor Q = 100 at 100 MHz and resonant frequency 100 MHz. It includes deviation due to ±5% components:

author: jujun
author_rank: Regular Contributor
timestamp: July 19, 2019, 09:20:19 am
content: Thank you all for your answsersHere is the filter shematic. I will try to get the smith chart with Qucs.Your L/C values are very close to mines.

author: radiolistener
author_rank: Super Contributor
timestamp: July 19, 2019, 09:28:11 am
content: Quote from: jujun on July 19, 2019, 09:20:19 amHere is simulation for your schema version. I assume that L and C has Q = 100 at 100 MHz for simulation.If you can measure Q of your inductors and capacitors it will allows more precise simulation.

author: jujun
author_rank: Regular Contributor
timestamp: July 19, 2019, 02:35:17 pm
content: @TheUnnamedNewbie : you are right, it was obvious ! Thank youWhat are the ticks to tune it ? I can move the spacing of the windings, but it dosent seems to change that much.I will need to unsolder the inductors, and meter them.Maybe I did an obvious error in the layout, but I guess that at 14Mhz is not very important.

author: radiolistener
author_rank: Super Contributor
timestamp: July 20, 2019, 01:33:36 am
content: Quote from: jujun on July 19, 2019, 02:35:17 pmI used a small move of coils winding. I'm tuned it for 145 MHz, it's much more sensitive. In your case it may be possible even unsolder it and tune with LC meter

author: jujun
author_rank: Regular Contributor
timestamp: July 20, 2019, 09:43:54 am
content: I just read again all your answer, and I still don't understand how a filter with a 0,5dB loss in band pass, can have an RL of -4dB (It's an SWR of 5:1 ! 44.4% of power reflected !) at the same frequency.I guess it's what happened when a filter is detuned: the RL curve don't follow the TL curves. Is it right ? For example on the last cavity filter I built for 2.4Ghz, it was perfectly aligned on the first try.

author: ejeffrey
author_rank: Super Contributor
timestamp: July 20, 2019, 08:01:30 pm
content: Quote from: jujun on July 20, 2019, 09:43:54 amIt can't. The only way you can get that is a calibration error or a measurement error. For instance if you calibrate out too much cable loss or the wrong source impedance you could get results like that.

author: jujun
author_rank: Regular Contributor
timestamp: July 20, 2019, 10:16:07 pm
content: Quote from: ejeffrey on July 20, 2019, 08:01:30 pmI did a new calibration with new cables, I also checked it and 50ohm/short/open are in the right place of the Smith chart.I still have the same result. Maybe the 44% signal reflection is not a 44% loss to the signal, because it bounces forth and back a high number of times and the only losses are from the cable, but many times.(I used a low loss cable at 14Mhz : a very short RG402 cable.)Any ideas ? I will try to use another VNA.

author: radiolistener
author_rank: Super Contributor
timestamp: July 20, 2019, 11:06:52 pm
content: if you're talking aboutthis measurement, it looks great. I would not try to tune it better, it seems that it is already has very good performance.PS: One time I spend a lot of time to tune 144 MHz BPF, I got good result, but I wanted to tune it better, so I desoldered it and started to tune it from scratch. I spend 3x more time, but cannot tune it as good as for the first time. So, if it's already works good, don't touch, best the enemy of the good

author: ejeffrey
author_rank: Super Contributor
timestamp: July 21, 2019, 03:14:27 am
content: Quote from: jujun on July 20, 2019, 10:16:07 pmThat can only happen if the source is mismatched. Otherwise the reflection would be absorbed at the source. That should be handled by your calibration.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 21, 2019, 03:27:13 am
content: If you think about it, a filter that has a good SWR in the rejection band must have a high loss in that band, since the energy has to go somewhere. To an extent, a filter that has low loss in the pass band must have a good SWR in that band, but some applications are sensitive to even a little reflected power that will barely show up as loss of power at the output.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: July 21, 2019, 05:23:12 am
content: Quote from: NiHaoMike on July 21, 2019, 03:27:13 amOr worse still - it could be a good radiator in that band, spewing all that RF all over the air!

author: radiolistener
author_rank: Super Contributor
timestamp: July 21, 2019, 11:36:23 am
content: There is known relation between SWR, bandwidth and Q factor:BW = ( SWR - 1) / (Q * sqrt(SWR))Where BW = 0..1 (in order to get exact bandwidth in Hz you're needs to multiply it with Fc - center frequency)As you can see, if you change one parameter, it will leads to change another...This equation has two solutions for SWR, but if we assume that Q > 0 and BW > 0, then there is just one solution:SWR = (Q*Q * BW*BW + sqrt( Q*Q * BW*BW * (Q*Q * BW*BW + 4) ) + 2) / 2Also we can find SWR value for 3-dB level bandwidth (with replacement Q = 1 / BW):SWR = 3 / 2 + sqrt(5) / 2 = 2.618We can use it to measure filter bandwidth with SWR meter, by SWR level = 2.618.Or by RL level = 6.9897 dB.

author: eb4fbz
author_rank: Regular Contributor
timestamp: July 22, 2019, 03:19:57 pm
content: Inductors are the least ideal components in the real world. Those inductors you built doesn't behave as you expect at 15MHz. Your LCR meter probably measure the reactance at a much lower frequency.If they use an air core, i find this calculator pretty accurate:https://hamwaves.com/inductance/en/index.htmlIf they use ferrite core, things are even harder.

author: ejeffrey
author_rank: Super Contributor
timestamp: July 22, 2019, 04:34:59 pm
content: Quote from: jujun on July 20, 2019, 10:16:07 pmOh, one more thing. Looking at your curve, the point where you see the apparently inconsistent insertion/return loss is right at the edge of your filter so the curves, especially RL are very steep. This suggests that the effect could be an artifact of your VNA sweep rate and its power detector response. Try slowing down your sweep speed and zooming in on the transition to sample more finely and see if that changes the result.