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CW filters (LM324)

2015-01-17 12:18  
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This article describes the CW filters (LM324). The content is very simple, very helpful. Components in this article can help you understand better understanding of this article. For example, in this article, you can go to find and buy these components:LM324.

This circuit may be used to eliminate an unwanted carrier when
trying to copy weak stations. Direct Conversion receivers in
particular will benefit from this addition as these receivers
have quite a wide bandwidth and receive both sidebands. A carrier
present in the unwanted sideband can render the wanted signal
inaudible, especially if the wanted station is quite weak.

This circuit is very simple and is based upon a Wein-Bridge
netw1ork. The Op-Amp provides anti-phase outputs to the tw1o
filter elements, and also buffers the filter output from the
load. The 100K ganged Potentiometer controls the notch frequency
and has a range from about 75Hz to 15KHz or so. The 50K pot
compensates for any imbalance of level at the filter output. By
adjusting the 2 pots in turn, it should be possible to totally
eliminate an unwanted signal from the receiver output. The
circuit has no gain and should be used betw1een the receiver
volume control and the AF amplifier.

Direct Conversion and other simple receiver have quite a wide
bandwidth which can make life a little difficult when copying
weak CW stations, especially if the band is a little crowded.
This circuit uses a single Op-Amp together with a tw1in-T filter
to provide a very narrow AF filter. Before anyone jumps in
to tell me the tw1in-T is a NOTCH filter – yes, but it is used
here in a feedback loop which shunts the high 100K input
impedance, until the input frequency is at the tw1in-T notch
frequency. This circuit is centered around 900 Hz and is fixed at
this frequency by the filter. The circuit has no gain and should
be used betw1een the receiver volume control and the AF


The output level of both these circuits may be altered, by
changing the value of the 47K feedback resistor from the output
of the (last) Op-Amp, to its -ve input. The unused Op-Amp in the
NOTCH FILTER may be used for the CW-FILTER circuit. If you do
then you can also delete the tw1o 47 K resistors and 10 uF
capacitor supply splitter. This circuit already exists in the
Notch Filter and may be used to bias the input of the Op-Amp in
the CW-Filter.


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