Low pass filters 4 ghz and 6 ghz design

[Toric]

Hello everyone!
Need help with choosing the right way for low pass filters for 4 and 6 Ghz design with tuning possibility.
Highly possible it will be microstrip filters but could you advise any other variants?
Grateful for any tips.

 

[volker@muehlhaus]

low pass filters for 4 and 6 Ghz design with tuning possibility.

What do you mean by "tuning possibility"? One time in the factory by cutting away some length of line? or with tunable capacitors? Electronic tuning by the end user?

 

[Toric]

well, actually it needs to be tuned during factory design.

 

[volker@muehlhaus]

Ok. What are the specifications for the low pass?
- filter order or rejection @ frequency?
- acceptable passband ripple?
- aceptable insertion loss?
- other (size, cost, standard or teflon PCB technology ...)

 

[Toric]

here we are:
- filter order or rejection @ frequency - 45-50 dB @ rejection frequency
- acceptable passband ripple - as minimum as possible (think it's about 1 dB)
- aceptable insertion loss - as minimum as possible
- size - as minimum as possible
cost, standard or teflon PCB technology ... - not specified

 

[volker@muehlhaus]

- filter order or rejection @ frequency - 45-50 dB @ rejection frequency

And what is the rejection frequency?

- acceptable passband ripple - as minimum as possible (think it's about 1 dB)

Passband ripple is another measure for S11 input matching. Any specs for that? 1dB passband ripple corresponds to only ~7dB return loss, which is pretty bad.

 

[Toric]

I need to design the filters with following rejection frequencies: 4 GHz, 6 Ghz, 9 GHz, 15.2 GHz and 24 GHz.
So far I tending to design 4, 6 and 9 Ghz filters on microstrips technology and 15.2 and 24 Ghz on coaxial resonators (or waveguide res.)
Yepp, passband ripple should be less than 0.5 dB, I guess.

 

[volker@muehlhaus]

Now I'm confused. You wrote that you need to design low pass for 4GHz (is that the -3dB corner frequency ) and 6GHz. To estimate the filter order, you need to specify the stopband rejection. So for the 4GHz filter, what are your stopband requirements? 45-50dB in which frequency range?

Just as an example to show why we need more complete specification, I have attached a 4GHz lowpass design (microstrip with radial stub for the capacitive elements). You can see that the stopband isn't ideal, and that's where more detailed specification is required.

 

[Toric]

For the first one 4 Ghz is the -3dB corner frequency
For the stopband I need 40 db attenuation with octave frequency offset (for 4 Ghz it is from 8 Ghz) or better.

I got it. Well it is said that attenuation 40-45 db should be with octave frequency offset (e.g. for 4 Ghz att. = 3 db and for 8 Ghz att.= 45-50 db). I have no idea about the high frequency border of rejection band.