1 transistor CW – filter

You cannot get it more simple than this, a 1 transistor CW filter. This filter is build according to the (surprise !) Multiple Feedback BandPass Filter (MFBPF) topology. Below, you see the circuit, originally designed by KE3IJ (alas now SK).

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40m DC Receiver – CW Filter

So after studying some designs of multiple feedback band-pass filters for CW by other designers (see my previous post), I’ve decided to try my hand at designing and building one myself. Actually, I’ve made two different versions. The first one is designed by myself and is a two stage filter with an fc of 750 Hz. The second one is the filter designed by Tom Hall GM3HBT and published in Ham Radio Today 1987. I selected this filter because the gain ( ~1x), the center frequency ( ~750 Hz) and the Q / bandwidth ( ~5x / 100Hz).

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13. GM3HBT2-3390128202.21.17404.20176
Overview of MFBP CW filter found in radio literature – PA3COR.nl

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Soundcard Interface

Coming soon

Multiple Feedback Band-Pass filter

Band-pass filters are filters that are designed to let only a certain set of frequencies pass through. Overtime a myriad of different topologies have been developed by some very creative designers. Passive vs active, RC-filters vs LC filters, combinations of low pass and high pass filters, with gain and without gain, etc. One of the topologies that are often used for CW filters in receivers is the multiple feedback band-pass filter :

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40m DC-receiver – What’s next?

Now that the basic receiver is working, what do I want to add to consider the receiver ‘complete’ ? This is the current set-up :

The following things I would like to add:

  • S-meter : shows the strength of the received signal. Based on AF signal
  • CW filter : Multiple Feedback Band-Pass filter, 1 or 2 stages, centered around 750Hz
  • Audio Amp – current the bench utility amp is used with a gain of 20 dB.
  • Keypad – Add 3×4 Keypad much like the K-225 keypad as found in Lowe HF-255 receivers