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What materials are used for Er/impedance/phase tuning after production?

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Advanced Member level 3
Feb 25, 2012
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I know the method of tuning when microstrips are cutted by knife, or soldering a thin copper plate. But there are also some other cool methods using different materials. I whant to know what are the names of that materials, so i can google them and find them.

1) Some kind of dried glue / paint or something.
Where: Usually it put on the microstrip, white or some gray color.
What it does: I guess it locally changes Er, so can be used for phase tuning without cutting
Experience: Removing it degrades circuit performance :)
Usage: I guess using spectrum analyzer or NA, then put some,remove some glue, put again and so on.

What is the name of this paint? Is there any manufacturers of such things?

2) Some kind of plastic transparent film.
Where: Usually it put above bandpass filters / decoupled microstrip line
What it does: I guess it for less signal losses.
Experience: Removing it degrades output signal level
Usage: Some experience RF designers know how to use

What is the name of this film?

3) Some kind of non-transparent rectangular substance
Where: above decoupled microstrip line, on some stubs, etc.
What it does: I am not sure, some kind of tuning, or maybe resonance?
Experience: removing it does not produced noticeable effect, except it is placed above decoupled microstrip line - degrate output signal level
Usage: Some experience RF designers know how to use

So what is that thing? I guess it is some kind of ceramic or plastic material, but not sure for it futures.

4) Some kind of special RF glue
Where: on DRO-s, etc. glued elements
What it does: not fall off
Experience: i tried two-component glue, some super glue, etc. But never got such good adhesion. Actually original glued DROs is very difficult to unglue without damage. My glue does almost nothing: i can remove puck with my finger.
Usage: glue

I asked for such glue some times ago... But i am not interested in it too much, just need some time to find right glue. But still post it here as PS, maybe some experienced RF designer can suggest me a good glue =)

Are my guesses right? I am reading RF design book, but nothing about such things as paint, transparent film, plastic rectangle or glue. Where i can find info on that?


Tuning after production is generally a no-no. What you do is do pilot production runs, find out where the tuning is manually added, and try to respin the artwork to contain those changes. You also try to get a very repeatable component procurement cycle going for repeatability.

If you have something that needs tuning and can not be engineered to eliminate it, or your run is a low enough volume to preclude are some things you can do:

Add unconnected metal pads near the main lines in the locations where tuning is usually added. Then you can solder over small wires/ribbons between the main line and those pads for a more "repeatable" tune job.

You can take a wood Q-tip, and add a small flag of copper or silver paint the end. Sweep the frequency response as you move this "tuning stick" around, and when you find a magic spot where the tuning stick added a drop of silver paint or solder on a copper flag.

There are variable reactance ICs available. You could add one of them in shunt or series, and use an automated test and then digitally program the value to set the reactance.

You could add digitally tuned frequency equalizers, where a PIN or Varactor diode or fet is switched in/out to provide a gain slope vs frequency.

If you have a lot of ripple, sometimes adding a couple dB of attenuator pad between the two componens with high VSWRs smooths out the ripple enough to not need tuning.

Tuning DRO pucks, a drop of 2 part 5 minute epoxy will lower the frequency. After it is cured, you can use a dremel tool to grind off a little epoxy if you went too far. You can grind the DRO puck too, but you will need a hepa dust mask since you do NOT want to breath in that dust!
Good answer. Is it silver paint or something on microstrips:
(this picture found here
I saw such things few times. One time i was able to use multimeter to check if it some metallic paint, but that one was dielectric material (white color).
How about this?

yes, that looks like silver paint to me. On those two stubs in the upper right of the picture, you can add a little siver paint near the point where the stub attaches to the main line. This moves the "effective location" of the attach point a little. Often the end of the stubs are also painted to tune a null at a certain frequency.

Silver paint is preferred over something like silver epoxy, since the silver paint dries very quickly. You can only see the final effect of the tuning after the paint has dried, so a 30 second dry time is a good thing.
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