Designing a low pass filter working at frequencies higher than cutoff
I am in the process of designing a low pass filter with a cut off frequency of 1 GHz that works up to the 12 GHz region. Exact specification are 30 dB cutoff or more from 3 to 12 Ghz.
Is there a practical way of realizing that filter or am I trying to overachieve? As far as I can tell, discrete components can't be used due to the fact that the values required for the lumped prototype are too large and have small SRFs.
I was thinking of cascading two low pass filters designed for different frequencies, one for lower and one for higher, so that when one filter stops behaving like it is supposed to, the other one can compensate.
I am trying to create an RF ground at those frequencies to connect it to a voltage source, so that the input signal, which is within that region that i require the cutoff, won't be able to affect that voltage source. If I'm going about it the wrong way I would like to know if there is some other way I could try.
Any feedback regarding this matter would be well appreciated. Thank you all in advance.
For a LP filter with 1GHz cut-off frequency, SMD lumped components can absolutely be used.But the problem is there will be magnetic couplings at higher frequencies between the components therefore filter response will not be desired at higher frequencies.Instead, this filter can be realized with distributed elements so Microstip or similar structures are very common on a low loss dielectric substrate.
I couldn't understand what you would do in your second paragraph.
Here is an example of an elliptical 400MHz low pass that I created long ago, with a mixture of SMD components and layout elements (lines and interdigital cap). Not yet the stop band width you are looking for, but it might show a possible starting point. As you can see, there are shunt caps. You might try to replace some of that SMD shunt capacitance by lambda/4 stubs, which gives shunt C at low frequencies and at the same time shorts transmission at higher frequencies.
Thank you @BigBoss and @volker@muehlhaus for your quick replies.
@BigBoss These magnetic couplings is the reason i created this thread. Creating the filter with Richards' Transformation results in a periodic filter with a period of 4 times the frequency the transmission lines are designed at. Designing the transmission lines with a length of λ/8 at 1 GHz which would be the most common doesn't result in the stop band i require, since S(2,1) becomes periodic with a period of 4 GHz. Designing their length in a higher frequency increases their impedance to unachievable levels. A hi-Z, low-Z implementation doesn't result in the stop band i require as well which is again periodic but with a much more complicated period. So with those two implementations out of the way, is there another method that could yield the result i want?
What i'm trying to say in my second paragraph is that i'm trying to create a DC Feed for a voltage source, that shorts my RF signal to ground. I hope that makes it more clear.
@volker@muehlhaus Thank you for the filter! I shall try to see if I can adjust it somehow to my specifications and I shall let you know if i do so.
Once again, thank you both for taking the time to reply.