Balanced SAW filters

Jun 16, 2011 4 Replies

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rich 15 years ago

Hi All, I am designing a RF front end for a single frequency receiver. (My RF knowledge is ok, but limited)

My design utilizes SAW filters as bandpass filters. I have a design that uses two filters, one filter (connected to antenna) is unbalanced and the second filter is after an amplifier and it feeds a log amp, it is balanced.

While I like this design I do not want to order two types of saw filters, balanced and unbalanced. I can find the filter I want in balanced form only.

Can a balanced filter be used as unbalanced? Perhaps a balun? Would this still be suitable at the antenna where the impact of losses would be significant?

Thanks

Rich

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whit3rd 15 years ago

The conversion from unbalanced to balanced can be (if you have already conditioned the signal) just a transistor with equal collector and emitter resistors. The conversion from balanced to unbalanced is easier (short or disconnect one wire...).

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Tim Wescott 15 years ago

You should be able to make or buy a fairly low loss balun. A more valid question to ask may be "how much does it cost?".

Bandwidth? Cost drivers? Space constraints? Etc.?

Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html

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rich 15 years ago

Are you saying that one connection of a balanced filter (one per side) can be left unconnected? Short? To what?

Thanks

Rich

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whit3rd 15 years ago

Depends on the item in question. If the differential output is a pair of collectors (like in a Gilbert cell) just short the collector resistor (or, if parasitics matter) use the same impedance for the unused collector as the signal path presents. If the differential output is a transformer winding, and the center tap is NOT present and grounded, ground the unused output. If there's a center tap connection, leave the unused output open, of course.

The signal may be smaller, and the noise on bias sources will=20 be more important to consider with a single-ended signal, of course.

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