I'm trying to design a Low-Pass transmission line stub filter, and I have a few question regarding it.

I'm rather new to RF-circuit design, so please bear with me.

If my desired specs include the pass-frequencies and the block-frequencies, does that let me figure out a suitable \$N\$ somehow (does bigger \$N\$ make the slope more steep? How is this determined?)

How do I go about limiting the size of the reflection coefficient, or what counts as \$Z_S\$ in the circuit?

Any useful literature I should dive into in the topic of designing a transmission line stub filter and figuring out the details?

Thanks in advance.

Also, how should I consider the desired attenuation decibel amounts at pass/stop frequencies?

  • \$\begingroup\$ N is the number of stubs? All I know about stubs is they act as a lumped inductor or cap. So use the resulting transfer function. \$\endgroup\$ – PJazz Feb 24 at 15:33
  • \$\begingroup\$ I know how to perform the transformations from a basic circuit into stubs, but I'm not sure how to approach the circuitry problem regarding this \$\endgroup\$ – Grak Feb 24 at 16:21
  • \$\begingroup\$ So I've found formulas for Buttersworth's filter where I can decide the cut-off frequency and use that to calculate the component values. I do not know how the order will affect the actual filter's performance, apart from the assumption I made in the OP, or how I can affect the attenuation in pass/stop regions \$\endgroup\$ – Grak Feb 24 at 18:45

Yes, the order of your filter (i.e. number of sections) is determined by the stopband attenuation.

You may use the formulae from reference books (1), but I strongly recommend to use free software tools like (2) for that. You'll save a lot of time.

In order to implement a distributed lowpass filter, you should previously design a LC Chebyshev prototype. Then replace inductors by high impedance transmission lines and shunt capacitors by low impedance stubs according to (3):

enter image description here

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The same procedure applies to lowpass mininum cap. elliptic filters.

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Say, Z0L ~ 20Ohm and Z0H = 400Ohm. YMMV

Hope this helps


Example (Chebyshev) enter image description here

Example (elliptic) enter image description here

(1) Microwave engineering. David M. Pozar. 4th Edition. Section 8.3

(2) http://www.iowahills.com/9RFFiltersPage.html

(3) Microstrip Filters for RF/Microwave Applications. JIA-SHENG HONG. M. J. LANCASTER. JOHN WILEY & SONS, INC. 2001. page 119. Eq. 5.9

  • \$\begingroup\$ It does, thank you and especially thanks for including a few sources to boot \$\endgroup\$ – Grak Feb 25 at 7:04

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