Can you make a low pass filter a using a variable capacitor?

Thread Starter

talikarng

Joined May 27, 2017
18
I would like to make a low pass filter with a corner frequency of approx. 200MHz which uses a variable capacitor (range 100pF to 2000pF). All I have seen so far are Butterworth or Chebyshev filters with fixed capacitors, but when I look up simple low pass filters I see two component filters. (NOTE: The filter will follow a buffer amplifier, accuracy is not vital because all I will be doing is using a diode detector to roughly determine the value of the variable capacitor)

1) Is there an advantage in using an LC filter over a RLC filter? I
2) Do I need to match the impedance of the filter to the output impedance of a buffer amplifier? If so, can someone please point me to how I calculate and match the impedance of the filter? I'd like to learn about how to do this.

Regards.
 

Papabravo

Joined Feb 24, 2006
12,546
To answer your question the answer is yes, you can do it. Just because you can do something does not make it a good idea. Let us say you three components in a filter and all three are adjustable. Every time you make a small change to improve something something else goes wrong because all of the components interact with each other and with their surroundings. The design of filters all go back to requirements. The reason a Butterworth filter exists is because it guarantees maximum flatness in the passband. The Chebyshev filter on the other hand allows some ripple in the passband , and the stop band, for a steeper rolloff in the transition band. There is no application for adding resistors to a filter, so the LC filter is the basic element. The reason for adding LC sections is to make the transition from the passband to the stopband sharper; that is the transition happens over a smaller frequency range.

Here is an example of a filter specification that I might write:
  1. Butterworth characteristic for maximal flattness with a corner frequency of 172 MHz.
  2. Low pass with a passband from DC to 172 MHz.
  3. Maximum attenuation in the passband 1.5 dB
  4. Transition band from 172 MHz. 178 Mhz.
  5. At least 45 dB attenuation in the stopband from 178 MHz to blue light
  6. Input and output impedance at 170 MHz. is to be 50 ohms.
From this specification I can tell you how many LC sections you will need and what component values to use. Just sitting here in my PJs I can't tell you if this filter can be realized with practical circuit components; that determination comes later. I give you this example so you will have some idea how the design process works. I expect you will have more questions.

To answer your second question, the impedance matching is done by frequency scaling. You do the design for a filter with a corner frequency of 1 Hz. When you are all done you scale all the components to the frequency of interest eg. 172 MHz.
 
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