# designing Low-Pass Transmission Line Stub Filter

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?

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

• 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. – PJazz Feb 24 at 15:33
• 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 – Grak Feb 24 at 16:21
• 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 – 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):

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

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

Hope this helps

[UPDATE]

Example (Chebyshev)

Example (elliptic)

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

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

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