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Author Topic: Measuring Kits and Parts band pass filter input impedance.  (Read 2366 times)
KH2BR
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Posts: 239




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« on: October 29, 2017, 11:07:38 PM »

I have a band pass filter from Kits and parts.com that is built for 40 meters. I attempted to check its input impedance. I put a 50 ohm load on the output and used a Mini60 antenna analyzer which displays impedance.
It measured 6 ohms. Is this a suitable way to measure impedance of circuits like this?
It tunes and peaks ok but there sure is a lot of loss in it. It was built according to their instructions.
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K5LXP
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Posts: 5340


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« Reply #1 on: October 30, 2017, 03:26:47 AM »

I have a band pass filter from Kits and parts.com

Looks like they sell toroid kits and you build from a choice of published designs.  Which one did you build?

Quote
It measured 6 ohms.

Assuming X=0, R-6?

Quote
Is this a suitable way to measure impedance of circuits like this?

Could be a single frequency measurement, or a sweep. 

Quote
It tunes and peaks ok but there sure is a lot of loss in it.

How are you measuring loss?  What is "a lot"?

First thing that comes to mind is it's an output filter intended to transform a power output device impedance as well as filter, but that's a guess.  A schematic/design link would tell more.

Mark K5LXP
Albuquerque, NM
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AC7ZN
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Posts: 83




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« Reply #2 on: October 30, 2017, 05:44:23 AM »

Hi Robert,

LC filters work by passing desired frequencies to the load , and rejecting unwanted frequencies by reflecting them back to the source.  Assuming your filter is intended to work with 50 ohm inputs and outputs and a 50 ohm termination, you should see roughly 50 ohms resistive impedance at passband frequencies, but stopband frequency impedances will be very different: very high or low, and inductive or capacitive.

Your measurement seems to be indicating at that frequency you are in the filter's stopband.   If your measurement frequency is supposed to be the passband, you may have an assembly error or excessive component tolerance error.  Do a plot of the measured impedance over several frequencies to see if you can find a passband.  The shape of this curve may give clues as to what is wrong.  By publishing the numbers you measure here, some filter gurus may be able to help.

73,
Glenn AC7ZN


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KH2BR
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Posts: 239




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« Reply #3 on: October 30, 2017, 08:33:34 AM »

Here is the webpage for the band pass filter.
http://kitsandparts.com/bpf2.php

I was expecting to see 50Ω with the load on the output. Yes, I can sweep it.
 Just got up from my beauty sleep, this will be the first thing I will solve today.
How did I determine the loss? I used a scope.

Robert KH2BR
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KH2BR
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Posts: 239




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« Reply #4 on: October 30, 2017, 11:44:52 AM »

Ok, I did a sweep and found that the pass band was to narrow.  50Ω 's right where I want it.

Now, back to the original question. What do you all think about using this small antenna analyzer to measure circuit impedance? Recommend some other method?

Robert
Kh2br
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AC7ZN
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Posts: 83




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« Reply #5 on: October 31, 2017, 07:16:18 AM »

I think using your antenna analyzer to measure the filter input impedance is a clever and valid technique.  you have already gathered useful information:  Your filter is basically working but is too narrow.  Assuming the coupling capacitor (C5/6) is the correct value (it probably is) you need only tune the two tanks:  The easiest way to do this is to use your antenna analyzer across each tank when they are not coupled:

1.  Choose the center frequency of the filter
2. Remove C5/C6.  With your analyzer connected to the input, set it to the center frequency and adjust the first tank variable capacitor until your impedance is predominantly resistive.  It will be a high resistance value at resonance.
3.  Remove the 50 ohm load from the output and connect your analyzer to the output.  Repeat step 2 for the second tank.
4. Reinstall C5/6.
5.  You should now see a wider bandwidth.  If you don't, increasing C5/C6 will increase coupling and widen the bandwidth.  If that capacitor is too high, you will see a wide bandwidth but the impedance at the center frequency will not be close to 50 ohm resistive.

There are Youtube videos on tuning these filters.   I found this:
https://youtu.be/zZR51MMOUnQ 

It is long: the relevant part seems to start at 28 minutes.

Good luck,

Glenn AC7ZN


   
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KH2BR
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Posts: 239




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« Reply #6 on: October 31, 2017, 11:19:34 PM »

Thank you for validating my question. I am going to play around with this for a while. Just for the heck of it,
I tried it out on the on the antenna input of a Icom IC-R75 with power on and changing to each band.
I guess you cannot expect perfection, some bands were way off and some were right on.
I also found that on other circuits I have  been playing around with, for example a small  rf amp, input , the circuit has to be powered on.
If its off, you get a different reading.

Thanks again,
Robert KH2BR
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G8HQP
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Posts: 611




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« Reply #7 on: November 01, 2017, 05:53:18 AM »

Yes, input and output impedances of most circuits will change when power is applied.
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WD4HXG
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Posts: 298




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« Reply #8 on: November 03, 2017, 09:42:57 AM »

Insert a pot in series with the filter input. Connect the center tap of the pot to the filter input. Connect the remaining leg of pot to your signal source. Set the signal source to fixed frequency. Adjust the pot so the ac voltage from the filter input to ground is 1/2 the ac voltage from the generator to ground. Once you reach that state remove the lead from the generator and measure the dc resistance of the pot from the lead that was connected to the generator to the center tap of the pot. That measured resistance will be the input impedance of the filter.

73

Chuck
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KH2BR
Member

Posts: 239




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« Reply #9 on: November 03, 2017, 06:13:22 PM »

I was excited to find this, thru the years of vanishing technical knowledge, I find this quite useful.

http://electriciantraining.tpub.com
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VK4FFAB
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Posts: 424




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« Reply #10 on: November 11, 2017, 01:17:01 PM »

Ok, I did a sweep and found that the pass band was to narrow.  50Ω 's right where I want it.

Now, back to the original question. What do you all think about using this small antenna analyzer to measure circuit impedance? Recommend some other method?

Robert
Kh2br

Perfectly valid tool for the job in passive circuits, for active circuits I use the 50% waveform and scope method as described by WD4HXG. An antenna analyzer is a very useful tool to have for the homebrewer, i think i have used mine a few dozen times on antennas but 100's of times measuring other things. 
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