Mechanical filter measurement with nanoVNA - Page 1
Forum section: electronics / rf-microwave

author: ardf
author_rank: Contributor
timestamp: August 10, 2020, 02:09:46 pm
content: Hello,I have a 455 kHz mechanical filter with an unknown characteristic impedance. Can I use nanoVNA to find out it's impedance? As far as I understand the other end of the filter should be terminated properly before taking the measurements.

author: cdev
author_rank: Super Contributor
timestamp: August 10, 2020, 02:35:43 pm
content: I would expect so since its well within the range of frequencies it supports?Did you try it?

author: ardf
author_rank: Contributor
timestamp: August 10, 2020, 03:11:10 pm
content: I did not tried to match it because I don't know the ballpark of the initial in/out impedance. However I've taken the unmatched through measurement and it showed an awful 19 dB of the pass band attention.

author: cdev
author_rank: Super Contributor
timestamp: August 11, 2020, 12:45:27 am
content: If you have two toroids of the appropriate kind you can look into the filter in any one of a number of ways to determine the characteristic impedance and then wind two identical transformers with the approriate turns ratio. Use that as the entry and exit. You can calibrate it also as you would normally except if you want to make single ended measurements (here I am not sure) you could perhaps use instead of a 50 ohm load use a noninductive resistor of the approriate load value for load. I'm sure others could add to this.You may already have the parts in the form of some off the shelf toroids from switching power supplies (there are toroids in burned out LED bulbs) They are probably a decebt value for 455 KHz too.(Just guessing)That seems like a good starting point.You can use your VNA in TDR mode to look into practically anything that acts as a transmission line to see its impedance.

author: bob91343
author_rank: Super Contributor
timestamp: August 11, 2020, 04:32:24 am
content: You might try an old trick. Measure the input impedance with the output open and then again with the output shorted. Take the geometric mean of the two readings.Alternatively, excite the input and load the output. With a dual trace 'scope, measure the input and output of the filter simultaneously. Vary the load until the output drops 6 dB while the input is held constant.Another way is to see how much the input loads the source, with a variable source resistance.Or look up the part number and see what the manufacturer claims.

author: ardf
author_rank: Contributor
timestamp: August 11, 2020, 10:02:44 am
content: Thanks for the replies.NanoVNA had hard times measuring such a high impedance when doing open/short measurements of the filter output. Especially when you short the output at first it shows negative resistance of c.a. -15 kOhms which then gradually decaying to 0 Ohms then positively increasing to the kilo ohms values during the period of 5 to 10 seconds.What I've tried:1)1:9 transformer followed by 1:1 transformer at the filter input/output2) a simple active probe at the filter output.I found that there is no substantial difference in the pass band ripple compared to the direct VNA attachment to the filter ports.

author: cdev
author_rank: Super Contributor
timestamp: August 11, 2020, 12:17:38 pm
content: Try:NanoVNA Port 0 > transformer > filter > transformer > Nano VNA port 1

author: bd139
author_rank: Super Contributor
timestamp: August 11, 2020, 12:55:37 pm
content: ^^^ this.The overall response depends on the impedance presented to the filter. Same is true for crystals. Old filters seem to have around 1.5k impedance so you'd need to transform 50 ohms up to that, then 1.5k back down to 50 ohms.

author: cdev
author_rank: Super Contributor
timestamp: August 11, 2020, 01:31:16 pm
content: I'm in the process of trying to make some broadband fixtures to measure a varety of components.Small is good here, so I am trying to use a commercial part. (A Mini-Circuits 1:1 transformer)What I really need is some way of interfacing to a wide variety of potential DUTs thats physically short and not a huge impedance mismatch and not lossy.It seems like I may be stuck using stripboard. Its cheap and short and versatile and not that bad as far as I can tell so far,under a few hundred MHz.Should be great for making test fixtures for HF, though.

author: bd139
author_rank: Super Contributor
timestamp: August 11, 2020, 01:40:20 pm
content: I tend to make them on demand and then lose them

author: cdev
author_rank: Super Contributor
timestamp: August 11, 2020, 02:00:22 pm
content: I know what you mean.Quote from: bd139 on August 11, 2020, 01:40:20 pm

author: ardf
author_rank: Contributor
timestamp: August 11, 2020, 02:47:23 pm
content: "Try:NanoVNA Port 0 > transformer > filter > transformer > Nano VNA port 1"That's exactly what I did."The overall response depends on the impedance presented to the filter."Apparently the response of the _mechanical_ filter is somewhat immune to the load mismatch on its ports.

author: cdev
author_rank: Super Contributor
timestamp: August 11, 2020, 03:31:33 pm
content: Quote from: ardf on August 11, 2020, 02:47:23 pmBut the 20 db of loss or whatever it is you saw within the pass band might be reduced substantially if you have an appropriate impedance transformation matching it to your input and output.

author: paul002
author_rank: Regular Contributor
timestamp: August 14, 2020, 07:56:16 pm
content: You can Check the data sheet for lt455 filterhttps://www.datasheetarchive.com/pdf/download.php?id=3a010e1a84f32bdea523c2106e8193efc706d3&type=M&term=LT455G

author: wd5jfr
author_rank: Regular Contributor
timestamp: August 16, 2020, 10:02:15 pm
content: If it has a part number or you know the make & model of the radio you might try to find schematic and it'll give you an idea.Hank

author: ardf
author_rank: Contributor
timestamp: August 17, 2020, 06:52:59 pm
content: I think this thread can be closed. I found that it is problematic to measure kiloohms impedance with nanoVNA. Second, it is not necessary to measure the impedance of mechanical filter because impedance mismatch does not reflect on its pass band characteristic. Third, I inserted the filter into the radio and found that it has less attenuation compared to the built in filter. The noise spectrum FFT analysis of the pass band showed less ripple dB than it was shown by the nanoVNA, but it can be due to coarse dB scale setting at the VNA.To conclude: mechanical filter does not require matching for characterisation. If in doubt --- use high impedance probe on the filter output.