VHF compact filter design approaches

Hi all,

I need a compact size image rejection filter for the front-end section of a receiver. The center frequency should be 215 MHz with a 1-dB bandwidth of 30 MHz and good out of band rejection with minimum loss. Microstrip filters may be a good choice, however conventional topologies have large sizes in this frequency (a quarter wave length is about 137mm on a substrate with a dielectric of 10.2). What's your suggestion? Is there any way to reduce microstrip filter sizes in this band or is there any other solution to my problem?

Thanks a lot.

PS: Do you know of any document describing different types of microstrip filters (interdigital, combline, hairpin, hairpin-comb and so on) usages, pros and cons?

To my knowledge try to use microstrip at 215 MHz is not easy, due to the large phisical dimensions of the lines and capacitors.
Try with the components that at 215 MHz are still true components ( parasitic effects are small ) and if you use SMD the realization is very small.

Mandi

Right, but lumped element filters are usually lossy due to element's Q limitations as one which I constructed (an elliptical filter) suffer from more than 7 dB loss with the other desired specifications. I used Coilcraft inductances which are of high quality and inductances better than them are not at hand at the moment. So, what do you think?

There are also shortened coaxial transmission lines. One is by coiling the line. The other is by coiling the center conductor which is commonly called helical resonators.

Well, what are the specifications of helical filters? Can they meet my project requirements and with how much loss? If your answer is yes, where can I find design documents about them?
And no Idea about my first posting postscript?

One reference is the book Filters With Helical and Folded Helical Resonators by Peter Vizmuller published by Artech House.

Another reference is chapter 9 0f Handbook of Filter Synthesis by Anatol I. Zverev.

We currently are using some from TOKO company.

Are quite good, the three cell type loss is about 3 db, but they cannot handle high power, say maximum 10 mW following the specifications, tested at 100 mW without any problem, themperature rise on the body about 3°C.

Mandi

Thanks Mandi, I checked Toko products and the 0603 chips inductor specifications was surprisingly the same as Coilcraft similar product. However for a 4th order elliptic filter, which meets the requirements losses as high as 6 dB is what that can be achieved.