QRPGuys Simple Lowpass CW SCAF Filter – $20
The QRPGuys Simple Lowpass SCAF Filter is an adjustable lowpass audio filter for cw that can be very helpful for a variety of receivers that lack the selectivity, or receive both sidebands simultaneously to narrow the audio pass band so individual cw stations can be heard more easily. It uses the MAX7427, 5th order, low pass, elliptic switched-capacitor filter IC. The filter is adjustable from a cutoff frequency of ~300Hz to ~800Hz, using the voltage across a varactor diode. The filter can be used externally with the 3.5mm jacks for input and output, or configured to mount internally, with a smaller footprint using the auxiliary connection pads, and power from your receiver. There is a board mounted potentiometer for low pass frequency adjustment, audio gain, and a push button DPDT latching switch to enable or bypass the SCAF filter. It is powered by two CR2032 coin batteries, and draws only 1.25mA. The board measures just over 2.00″ x 2.38″., has #4 holes in the corners, and rubber feet for external use. If powered in an internal setting, you must provide 2.7-3.6V max. to power the unit. It is a through hole component kit. On a difficulty scale of 1 to 5, 5 being the most difficult, this is rated at 2.