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VK5ZVS |
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| Making a 472 to 479 kHz bandpass receive filter for 630m. |
This project involved the construction of a bandpass filter,
suitable for receive only use, on the 630m amateur radio band.
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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.
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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.
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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.
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Here is a simplified interpretation of the results
that have been extrapolated from some sampled points taken from previous screenshot.
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