9MHz IF Filters

 

     The radio will have selectable IF filters for 2.4kHz and 5kHz bandwidths.  The first will be an INRAD 8-pole SSB filter and the latter a 4-pole crystal filter.  

 

SSB Filter Module

    The 8-pole crystal filter is a bit expensive for a kit, but I had one and wanted to use it in a design.  The impedance of the filter is 500 Ω // 30pF.  I used an L network to impedance match each side of the filter with the actual termination impedances. 

 

     For the filter response measurement below, I used 5% resistors to impedance match the filter to my 50 Ω analyzer.  The slight impedance mismatch is causing passband ripple and creates ~28dB of loss.  I’ve had better results with L/C matching networks but I didn’t save a picture.

Inrad_Center.TIF

 

Cohn Filter

     A 4-pole crystal filter using 9MHz crystals was created using the architecture below.  The crystal parameters were determined using the G3UUR method.  With these parameters, I used AADE filter design software to create a Cohn Filter for my AM filter.  The terminating impedance of the filter is ~338 Ω with all capacitors being 52pF.

 

 

    I decided to impedance match the filter with a simple L-network rather than the matching network proposed by the AADE software. To start, I converted the 52pF end capacitor and 338 Ω load resistance into its parallel equivalent combination at 9MHz.  (676 Ω // 26.2pF).  Where Cx is 26.2pF and all other capacitors are still 52pF.

   

 

     Finally, I had to transform the 676 ohm impedance to the actual impedance seen on each end of the filter.  I used the L-network equations to find appropriate L/C values.  At this point, I was able to combine Cy from the L network with Cx from my filter into a single capacitor value.

 

 

     For the filter response measurement, I again used 5% resistors to impedance match the ~338 Ω filter to my 50 Ω analyzer.  Again, it creates ~28dB of loss due to matching networks.

9M.TIF