Balanced Bandpass Filter, necessity?

[kumar07]

Could anyone tell me what is the necessity of balanced bandpass filter at high frequencies (GHz)? What s the need of suppressing common mode signal?

[Sch3mat1c]

Well, if you need common mode suppression, then you need a filter with common mode rejection...

In general, I see no "necessity" for balanced signals or bandpass filters in relation to unspecified applications at GHz frequencies. You'll have to get a lot more specific.

This sounds like a homework question: has your instructor explained any of these terms or given any context? If not, set up a meeting immediately (if you can't set up a meeting, contact the registrar or other administrative services that your instructor sucks!).

Tim

[kumar07]

Hi Tim,

I'm doing my project in UWB bandpass filter. When I made a literature survey on the types of RF filters designed so far, there are a very few filters with such balanced structure.

So, please tell me why would one prefer balanced bandpass filter at the RF front end?

[Sch3mat1c]

Well, balanced filters are bigger -- you need double the components or board area. From what little I've seen, balanced signals aren't all that common; most GHz applications use single ended sources, like 1/4 wave whip antennas, or coax connectors. It would seem, one would not prefer balanced circuitry.

Most high bandwidth ADCs are differential input, but SDR applications generally use baluns to interface a single-ended signal to the differential inputs. One could use balanced circuitry all the way, but again, it would take up more space.

I suppose one could argue waveguide filters with bilateral symmetry are "balanced", but that's kind of silly.

Balanced signals are, however, used extensively for time-domain applications with similar bandwidth (LVDS and the like); these don't want filtering, though!

Tim