CW Lowpass Filter

This circuit is an easy to build peaked lowpass filter and works great in the audio of the B&E Receiver.

The filter is from "Simple active filters for direct-conversion receivers", subtitled "How to design simple active filters for radio communications", by Wes Hayward, Ham Radio Magazine, W7ZOI, April l974, Page 12.

Four sections were used in my prototype and the selectivity for CW was excellent for casual operating. The feature that really makes this circuit work is that the resonate frequency is amplified 28dB. This gain puts a weak signal above the QRM for easy copy.

None of the component values are critical. Wes states "the low Q of each pole pair would allow the use of 20% components with a minimal degradation in performance. Indeed, the slight stagger-tuning effect that could result might be quite desirable."

Npn transistors are 2N3565, 2N3904, or similar; Pnp transistors are 2N3638, 2N3906, or similar.

Specifications (See Chart) on the filter are as follows: "The Q of each active filter section is about 1.9 which yields a net 6-dB bandwidth of about 200 Hz. Skirt response, however, is not lacking. With a center frequency of 540 Hz, the attenuation is 75 dB at 1200 Hz." (Page 13)

"The moderately wide bandwidth makes the unit easy to use,... the steep skirts insure adequate rejection of adjacent channels. When used with even the most simple direct-conversion receiver, performance is suitable for the majority of amateur communications." (Page 13-14)

I wholeheartedly confirm this statement as I "Worked All States" with the B&E and this filter. Highly recommended!

I found four sections would fit on a small Radio Shack general purpose printed circuit board. I did not try to fit this on the prototype section. The filter is switched with a couple of DPDT relays.

The filter is connected right before the volume control at R4, after the NE602 BFO. Speaker volume can be altered according to how much stronger the signal appears after the filter is switched in the circuit.

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Last Update: 4/6/10
Web Author: David White, WN5Y