VARIABLE BANDWIDTH CW FILTER


VARIABLE
BANDWIDTH CW FILTER

BY

As the sunspots return and DX
propagation conditions to improve, the age old problem with QRM
begins to show up on the amateur bands. Fortunately, help is at
hand. The circuit in figure 1 is a variable bandwidth CW filter
using two IC’s. The first IC is a 741 op amp used as an active
filter and the second IC is a 386 audio amp IC. This circuit
connects to the headphone or speaker jack of your HF rig. The
volume is controlled by the volume control of your rig and R2
controls the bandwidth. There is plenty of audio to drive a small
speaker or a pair of headphones.

This circuit is different from
other active filter circuits because it provides an audio boost
which more than compensates for any attenuation in the filter
stage. Many active filter circuits do not have enough volume to
make them useful for most operators. Capacitor C5 and R7 were
added to prevent the LM386 IC from oscillating which is common
with this family of IC’s. Use good construction practices since
this circuit contains a very large amount of gain at the selected
bandwidth. To make life easier, only Radio Shack parts were used
for those who hate mail order.

The circuit in Figure 1 has been
shown to be most effective for older radios and QRP sets which
lack adequate filtering.

DE

Parts List

R1
 
100K resistor (brown,
black, yellow)

R2
 
10K potentiometer

R3
 
330 ohm resistor (orange,
orange, brown)

R4
 
220K resistor (red, red,
yellow)

R5,R6
 
47K resistor (yellow,
violet, orange)

R7
 
10 ohm resistor (brown,
black, black)

C1,C2,C3
 
.01 microfarad capacitor
(103 marking)

C4
 
10 microfarad
electrolytic capacitor (observe polarity)

C5,C6
 
220 microfarad
electrolytic capacitor (observe polarity)

IC1
 
741 op amp IC (use 8 pin
socket)

IC2
 
LM386 audio amp IC (use 8
pin socket)

Note: All resistors are � watt 5%
tolerance unless otherwise specified.

VARIABLE BANDWIDTH CW FILTER

VARIABLE BANDWIDTH CW FILTER #2


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