775MHz microstrip narrow bandpass filter with 10MH bandwidth

i want to design a microstrip narrow BPF at 775MHz with 10MHz bandwidth. i simulated combline and interdigital filters using genesys eaglewre but the problem with above filters are...
1.interdigital---dimensions are large(10cm)
2.combline---if the capacitors are changad from 10pF to 10.05pF,the response is changing very badly......dey r very sensitive
so which topology to use....i need a filter with minmum dimensions...plz help me......

regards
princy

Traditionally you would use tunable capacitors in a combline design. You get out your screwdriver, and tweak them up for the best response after it is assembled. So, as long as the capacitors do not drift after tuning, the sensitivity is not a big problem.

At those frequencies, it's better to use helical resonators with variable capacitors in series.

thank u very much for ur reply....but i want my filter on a PCB....is this possible using helical resonators and can u plz tell me what would be the approx dimensions of this helical filter


regards
princy

If you're able to use high dielectric substrate such as Alumina, the dimesions will be smaller but quite costly.
Because of low frequency, the wavelength is quite long and therefore the dimensions might not be very practical.
You have not so much choice..
This helical filter should be in cascaded cavities with few sections.The allover dimension may be around 7cmX13cm including housing.

Hi,
Sorry, but a Helical Resonator isnt a PCB type (is not a microstrip/or spiral-L), you can maximum populate/mount it onto a PCB: http://en.wikipedia.org/wiki/Helical_resonator
Helical Resonator Design for Filters: http://www.rfcafe.com/references/ele...-resonator.htm
K.

so what filter can i use for 775MHz that can be fabricated on a PCB with min dimensions....because the filter is a part of my schematic.....

Hi,
Did you experienced with diverse L/C systems pls by the combline structure?
50fF as sensitivty seems me tooo bad, but Im not a UHF-MW filter expert...
I would prefer (personly) the Helicals, you can have some good types (maybe from Coilcraft?), but it must be your decision. In the "www.vhfcomm.co.uk" are some descriptions such years ago.
Here is some (German) collection: http://www.elektronikschule.de/~krau...blication.html, especialy: PUFF_Microwave_Design_Part_05.doc, maybe you can adapt it...
www.ukw-berichte.de_English_index.html
regards!
K.

can i design a helical filter on my own (is it very difficult) or should i buy it.......

Hi,
I think here is a good introducton for your deciding:
http://www.rfcafe.com/references/ele...-resonator.htm
K.

Added after 56 minutes:

Hi,
Read pls my past comment on these parallel topic: http://www.edaboard.com/viewtopic.ph...664&highlight=
"11 Mar 2010 11:12 4ms delay line at 1GHz
Hi,
I found on the pp 345 a filter what, I think, you can use (Center frequency 985 MHz, Fractional bandwidth FBW 10.359%, 40dB Rejection bandwidth 125.5 MHz. On a 120x50x1,27mm substrat, Epsilon 10.8...

Best thanx for "alpuslu", www.edaboard.com/viewtopic.php?t=384786#1247286 Smile"
K.