RX Bandpass Filters

 

     A receive bandpass filter is used to prevent out of band signals from entering the mixer and to also eliminate the possibility of LO radiation getting to the antenna.   

 

Coupled Resonator

     The coupled resonator architecture is reasonable for a narrow bandwidth (<10% center frequency) BPF.  The input/output are AC coupled and all inductors are the same value.  A triple tuned circuit schematic is shown below.

 

 

 

     The plot below shows my first attempt at a 28MHz filter.  Whoops!  Component lead length is having an impact and also my standard capacitor values are not close enough for what the filter needs.  On the bright side, it works better on CB!

 

BPF1.TIF

 

Third Order Butterworth

    The second type of BPF used is a third order butterworth filter.  Odd order T configuration designs have the advantage of AC coupling at both ends. 

 

 

Filter Switching

     The best way to switch between these filters would be with relays, but I am going to use a BAR63 pin diode.  This diode will degrade noise performance but it is simple, small, and cheap.