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Engineer's Log of Mark W. Persons
RF Cavity Filters

Thursday, April 25, 2013: 

This was a project to retune an FM band Bext FDDPTC05 Double Balanced Bridge Combiner, which has one narrow band input and a broadband input.

I added a Telewave TWPC1005 triple-cavity band pass filter to the broadband port on the combiner. 

There is more than tuning for the least loss at the desired frequency. 

A flat band pass over the frequency of interest is very important.  In this case I set the filters for +/- 200 KHz at each FM frequency to minimize audio distortion and AM noise.

Return loss at each transmitter input port is also important so that reflected power at the carrier frequencies is low as well as at the upper and lower limits of the modulation frequencies.

It is a balancing act to get all the factors to come out right.

Band pass chrematistics and return loss measurement plots are shown below.    

It's all in a day's work.

The stories go on and on.  Stop in again sometime.  I'll leave the soldering iron on for you. 
Mark W. Persons

Questions?  Email Mark Persons:  teki@mwpersons.com        

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page last edited 02/21/2016