Wideband Filters

Thread Starter

Antonio

Joined May 29, 2007
27
Can anyone tell me:

Why the Return Loss performance is poor in Wideband Filters?

Or

Why is it difficult to get good Return Loss performance in Wideband Filters?
 

Papabravo

Joined Feb 24, 2006
14,399
Because it is difficult to maintain a proper match with frequency dependent impedances over a wide bandwidth. Return loss is not the only problem, insertion loss is also a problem.
 

Papabravo

Joined Feb 24, 2006
14,399
Certainly. Consider a voltage source with some internal resistance, called small r. Consider an external load resistance called big R. Question, for any voltage V and internal resistance small r, when will the maximum power be transferred to the load resistor big R? The answer is, when small r equals big R.

Simple right? Now replace small r and big R with impedances small z and Big Z which are frequency dependent. Question, for any AC voltage source V=Ae^(jωt) and source impedance small z, when will the maximum power be transferred to the load impedance big Z?.

If you said when small z is equal to Big Z....AANNNGH! loose 2 points. This is complex algebra here. The correct answer is when Big Z is equal to the complex conjugate of small z. This is called a "complex conjugate match" or sometimes just a "conjugate match".

Inductors have impedances with positve imaginary parts whose value is
ωL, a linear function of frequency. Capacitors have impedances with negative imaginary parts whose value is (ωC)^-1 which is NOT a linear function of frequency. Over a narrow range of frequencies it is possible to do a conjugate match. Over a wide band it becomes increasingly difficult as the bandwidth increases.

At RF frequencies a small return loss corresponds precisely to power being delivered to the load. A big return loss corresponds to power being reflected from the load and returning to the source. 25 dB down is a pretty good return loss, 3 dB down is a not so good return loss.
 
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