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Help me design a diplexer which has 850MHz low pass and 950Mhz high pass filter

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scoop2003

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hello guys!

i try to design a diplexer which has 850 mHz low pass and 950 mHz high pass filter behaviour. i use 11th degree chebyshev filters and i get the result but the capacity values are very small like 0.8pF. so i can only simulate, not test my diplexer with real elements. i try to transform these capacities to microstrip lines but in high pass filter part i cant do that. i have min 10 pF capacities so designing with lumped elements is not the best way. any suggestions about that?

thanks..
 

Re: Diplexer Design

At this relative small frequency spacing will be hard to make it work using Chebyshev topology. You have to use Elliptical.
There is a rule in designing diplexers. Never put a shunt capacitor from the common port to the ground.
 

Diplexer Design

do you know a source which has formulas and examples about elliptic filters? or is there any calculators for elliptic filters? i have few days to design and test that diplexer.
 

Re: Diplexer Design

how much attenuation and at what frequency do you need?
 

Diplexer Design

Hello,SCOOP2003,
You can do some simulation with Elliptical Topology in ADS or other tools.
 

Diplexer Design

for low-pass part cut off freq. 850 mHz and at least -40dB insertation loss at 950 mHz,, for high pass part cut off freq. 950 mHz and insertation loss at least -40 dB at 850 mHz. i found a filter design tool called AADE filter design. i design 5-7-9 degree elliptic filters in AADE but the same problem occurs again. capacity values are less than 10 pF.
 

Re: Diplexer Design

Less than 10pF is a normal value for a filter like this.
 

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