λ/4 transmission line and bandwidth

[senmeis]

Hi,

λ/4 transmission line is often used to transform the impedance in RF circuits such as Doherty amplifier. What about the bandwidth of such applications? I mean λ/4 is strictly accurate only for one single frequency.

 

[FvM]

The bandwidth depends on the transformed impedance ratio. Why don't you try with impedance values of your choice?

 

[JohnBG]

A single lambda/4 transmission line is called narrow band because if applied correctly it inverts impedance between ports, with reasonably large enough |s11|, but |s11| quickly rises on both sides of the single frequency chosen to calculate the characteristic impedance.

lambda/4 transmission line sections are also called transmission line impedance transformers. Example

What one can do is for instance combine more than one transmission line sections, for instance shut-series, like this:

exercise 5.3 | µ~ Eng Matlab

jgb2012.wixsite.com

Shut-series-shunt:

exercise 5.10 | µ~ Eng Matlab

jgb2012.wixsite.com

or cascaded sections, called binomial or Chebyshev impedance transformers according to what function used, example binomial 4 lambda/4 sections:

exercise 5.16 | µ~ Eng Matlab

jgb2012.wixsite.com

I don’t know if it’s your case for this question, but instead, it’s important to consider programming a DSP for instance, to do such impedance transforming / impedance matching / filtering, saving considerable PCB space. After all, smart phones do a lot of RF/microwave signal processing with just a few, as small as possible, components, as National Instruments motto used to read some years ago: Software is the Instrument.

 

[vfone]

To be more explicit, in case of λ/4 TL transformer, higher the transformation ratio, narrower the bandwidth.
For example, if match 50 ohms to 60 ohms gets a higher bandwidth compared to bandwidth of 50 ohms to 90 ohms transformation.

Yes, the term λ/4 is strictly accurate only for one single frequency, because λ = phase velocity/frequency.

 

[Georgy.Moshkin]

for increasing bandwidth and reducing physical size you may consider nonuniform line impedance transformer

 

[Georgy.Moshkin]

https://jpier.org/PIERC/pierc44/01.13080807.pdf

 

[biff44]

Thru mathematical trickery, if you use MULTIPLE quarter wave sections, you get bigger bandwidth. Try 3 sections