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Help me about Cavity-Diplexer

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Junior Member level 3
Nov 22, 2001
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I have read some books about Filter,but none of them talked about Cavity
Diplexer, what's the principle of it,and the prototype of it ? Whether Genesys has a example of it ? How can I master it ? Pls give me some instructions . Thanks a lot !

In general

In general they are two band pass filters which go high impedance out of band. You put one end of each in parallel which is the common port of the diplexer. You can get fancier and have the impedance of each one at the pass band of the other be finite, but compensated for in the input impedance of the other.

You can get a feel for this by using the trial version of Filter Solutions which does diplexers in LC and transmission line elements.

**broken link removed**

Thank you flatulent for your help. I know the basis of diplexer, Ansoft Serenade have a example of diplexer which is based of transmission .and even i know some filter of coaxial lines and microstrip ,but never seen about cavity filter or diplexer. I am just wanting to know something about cavity filter or diplexer.

This is the best book I know on the subject:

I build this type of equipment in my job, but that is the only usefull book I've seen.

I don't think you can get it as an ebook and its far too big to scan, but I will try to help you if I can.

Let me know what channels/frequencies you are trying to combine, what are the peak and mean powers and I will scan the pages which I think are usefull.

Previous post

I have gone through my library and only one book (other than the one suggested in the previous post) makes reference in passing to cavity types and gives 1965 era IEEE journal references. I suspect that a university engineering library might have a book on this.

Genesys will not be able to help you because the E&M simulator in it is only 2 dimensional. (They claim 2.5 dimensions because of being able to do vias.)

Are you operating at such a high frequency or power level that you need a six sided metal box with holes for coupling? You could design the two filters with transmission lines using Genesys and then convert the design to air dielectric coax type construction with round metal bars in rectangular outer conductors.

Try to get the good old book of Matthey, Young, Jones : Microwave Filters and Impedance Matching Networks. (May be that the title a little different, I do not remember exactly).
You can get this book from a lot of libraries. It is a well known reference book.

Thanks for all of you,i will try my best to find the bible of cavity filter.

After giving this some thought, I am not sure that you would be wise to attempt this project.

You will need a good metal working shop, a lot of developement time, and about ten years experience to achieve your goal. These things never work first time and you need the experience to know how to make them work.

I am still willing to help you all I can, but my advice would be to buy your requirement off the shelf somewhere.

Gakusei is right, the project is not a fast one for a beginner and metal workshop is also needed. But with diligence and a lot of works it can be done in 3 - 6 month, if you have test equipments for this band.

Thanks for all of your advice(suggestion),I had built some Microstrip Filter before, and I also know that for a cavity filter,inner wall must
be plated with silver or gold. metal workshop here are better.but firstly
I must acquire the principle .

You need silver or gold plating only if you want to use very high Q and very sharp diplexer. In other cases simpler or no plating will do too.

In most cases we use copper plating on steel. But as this is quite a high Q design (about 6 cavities), you will probably need to use silver plating on brass.

My friend ghostdaemon, please tell me what is the frequency you want to build up your diplexer?


One of the three big filter manufacturers for base stations plates with Al-Si8Cu3Fe :)


DanTheMan said:
One of the three big filter manufacturers for base stations plates with Al-Si8Cu3Fe :)

I thought that Ni/Cu/Ag or Cu/Ag or Ni/Ag was most common.
What's the reason for using Si and Fe?

I maybe have misunderstood your post. How should I read Al-Si8Cu3Fe?
I'm talking of silverplating aluminium, and for example Ni/Cu/Ag are plated on the filter in that order. Ni is used when good corrosion protection is required.

Never ever use nickel in basestation aplications! The passive intermodulation problems are terrible.

Thanks all of you,the feature of the Cavity-diplexer is following:

Freq: 825-835MHz /870-880MHz
Insert loss <1.6 dB
inband delta flat <1dB
offband 2.5Mhz the suppression >25dB
offband 5MHz the suppression >50dB
isolation between receive and transmission >95dB

It seems to be a rather difficult thing.
The offband 2.5 MHz and 5 Mhz are about 0.3 and 0.6 % freq. difference.
With high number of resonators and some temperature change (so mech size change) you will be out of spec.
Seems to be realized only for room temperature.
Think the specs over!

classical paper

I have run across a reference to a classical paper on this subject.

Wenzel, R. J. Application of Exact Synthesis Methods to Multi-Channel Filter Design IEEE Transactions on Microwave Theory and Techniques Vol. MTT-13 January 1965 pages 5-15

This may give you some help if it can be found.

I bludered across a company that makes CAD software for cavity filters. This may work for you. Go to **broken link removed**
and look at the bottom right of the links for the filter program demo download.
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