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Making a 472 to 479 kHz bandpass receive filter for 630m.
Receive only design.

This project involved the construction of a bandpass filter, suitable for receive only use, on the 630m amateur radio band.
 
 
What's in the box.

This first photo shows the interior layout of the filter. Insulated bnc sockets provide both 50 ohm input and output connections. This unit has been designed to match 50 ohm input and output impedances. Utilising five cheap 455kHz IF transformers and four good quality 2-18pF trimmer capacitors, produced surprisingly good results.

 
 
Finished filter project interior
 
 
Circuit layout.

The schematic of this project is shown below. This circuit is designed to give more than 50dB and up to 80 dB rejection of broadcast stations in the AM broadcast frequency range. The 472 to 479 kHz passband is only reduced by 3.8 - 3.9dB. The filter does require initial tuning to get the best performance. Access to proper test equipment for measuring insertion and through loss, helps considerably. Once tuned, no further tuning is required. Hope you have similar success.

 
 
Schematic of 475 BC filter
 
 
Test results.

This next screenshot shows the frequency response of the filter as seen on a HP spectrum analyser. The centre frequency is 475kHz and the insertion loss is 3.9dB and has since been reduced with further tweaking.

 
 
Test results
 
       
 
Here is a simplified interpretation of the results that have been extrapolated from some sampled points taken from previous screenshot.

 
 
Interpolated results
 
       
    2006 - 2013 S.Spirat 
Another SPIRAT initiative