After Microstrip Adjustments

I have made the modifications to the 738 filter PC board layout.  I shortened the strips to bring the center frequency to 738 MHz. The original length from the Ansoft calculations was 61mm.  I had to shorten these to 52mm.  That is a reduction of 14.75%.  My guess the reduction is required because of stray capacitance on the board.   The final 3DB bandwidth comes out to 15.1 MHz.  The final gain with the +20 DB MMIC is 0DB.  My insertion loss is 20DB behind that amp.  This is in line with Ansoft’s calculations using FR4 board.  Rogers PC board would be a better choice because of the dielectric loss tangent (TAND).  It is 0.02 on FR4 but only 0.0009 for Duroid 5880.

 

The input return loss is interesting.  Using FR4’s TAND value Ansoft shows the best return loss as only 10DB but above Fo at 756 MHz.  This is less than the original 18DB I had before mucking with the strips.  I see this on my tracking generator.  I was able to improve this to 15DB by adjusting the tap point on the first microstrip element.  I moved it a millimeter or two toward ground.  I don’t know that it improved return loss at 738 as that still looks to be 10DB.