First Microstrip Bandpass Filter Results

I etched the board for the microstrip band pass filter.  The desired center frequency is 738 MHz.  The problem with designing such a filter on FR4 PC board stock is the variation of the dielectric constant at the desired frequency.  The second source of errors in the layout of the board itself.  At UHF and above it is difficult to get the necessary precision where a quarter of a millimeter could make a big difference.  My results of the first filter look promising.  The filter response was about what I wanted except the frequency was off by 100 MHz.  The center of the bandpass was 630 MHz and not 738.  I think the insertion loss may be high but it is masked by the output MMIC amp I have on the board.  The overall gain is +2 DB because of the amplifier.  While this does not sound bad remember that the MSA-0886 amp has about 20 DB of gain at 600 MHz. 

 

The plot of the filter response shows a “step” on the upper frequency side of the curve.  This indicates one of the stages is not aligned on the same frequency as the others. 

 

Anyway, the initial measurements:

 

Input return loss at center of bandpass:  18 DB

Overall gain with MMIC amp: +2 DB

Center frequency of band pass:  630.68 MHz

3db bandwidth:  638.33 – 623.64 MHz = 14.7 MHz

 

The filter was designed using Ansoft Designer SV (student version). I will now go back to the design and modify the FR4 dielectric constant to see if I can account for the 100 MHz frequency error.  The idea is to use the modified constant to rework the board dimensions to see if I can get closer to 738 MHz and improve the insertion loss. 

 

As for this board it has served it purpose.  I can try to shorten the microstrips and use capacitors to tune the sections.  I can not adjust the coupling other than to solder metal tabs onto the microstrips and use them to increase coupling to the next strip.