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AMATEUR RADIO
By GARRY CRATT, VK2YBX
Active CW filter for weak
signal reception
This simple circuit will clean-up those
difficult to read CW signals. It provides
active bandpass filtering and tone decoding
of the incoming signal, and then regenerates
the signal using an oscillator.
Considerable concentration is required by both experienced and inexperienced CW operators when
called upon to copy a wea~ signal
affected by strong background
noise. This in turn can lead to
fatigue and may result in errors. By
using this active filter, the ease of
reception of weak CW signals is
substantially improved.
Active filters have long been used to improve the readability of
weak CW signals but they still depend to a large degree upon
receiver tuning, as the "beat"
signal must fall within the narrow
filter passband. This type of filter is
most useful when the receiver being used has neither AF (audio frequency) or IF (intermediate frequency) filtering designed for
dedicated CW reception.
The design presented here is
slightly more complicated in that it
comprises two stages of active
bandpass filtering, each having a
slightly different centre frequency.
This combination provides an adequate passband (400Hz or so) and
rapid rolloff, in this case around
20dB per octave. We could have used a single high Q bandpass filter
but this would have given a very
narrow bandwidth, making it difficult to use.
Fig .1 shows the combine d
response of the bandpass filters
while Fig.2 is a block diagram of the
Active CW Filter. As shown, one
bandpass filter is centred on
1.15kHz and the other on 1.35kHz.
The output from the filter stages is
then fed to a PLL tone detector.
When a valid tone is detected, the
PLL "keys" (ie, switches on) an
audio oscillator to regenerate the
signal.
Circuit details
All the parts for the Active CW Filter are mounted on a small PC board. We
used PC stakes to terminate the external wiring connections.
94
SILICON CHIP
Now take a look at Fig.3 which
shows all the circuit details.
The bandpass filters are two
stages of an LM1458 op amp (ICla
& IClb) and these provide an
overall gain of around 40dB. Bias
for the non-inverting inputs of the
op amps is provided by a voltage
divider consisting of two 22k0
resistors connected across the supply, while the 47kn and 470
resistors form an input attenuator
which ensures that the filter is not
overdriven.
The output of the filter (pin 7 of
IC1 b) is fed to pin 3 of ICZ via a
47kQ resistor and 1µ,F capacitor.
IC2 is an LM567 phase locked loop
tone decoder, commonly used in
+10
\
-10
i
-2U
I\
J
V
/
-40
/
-50
-60
100
\
I
-30
Fig.1: this graph shows the
combined response of the
two bandpass filters. The
response is centred on
1.2kHz and the passband is
about 400Hz.
/
400
200
600
\
2k
1k
I\
4k
I\
6k
10k
HERTZ
INPUT
1.15kHz
1.35kHz
BPF
BPF
PLL TONE
DETECTOR
LPF
AUDIO
OSCILLATOR
OUTPUT
Fig.2: block diagram of the Active CW Filter. The output from the bandpass
filter stages is decoded by a PLL tone detector which then switches an audio
oscillator to regenerate the signal.
telephone circuits for decoding
signalling tones. When a valid tone
(ie, a 1.ZkHz signal) is received, the
PLL locks on and pin 8 switches
low.
VRl, in conjunction with the
2.7k0 resistor and O.lµF capacitor,
sets the centre frequency of the
tone decoder. In practice, VRl is
set to give a centre frequency of
1.2kHz. The O. lµF capacitor on pin
2 sets the bandwidth (ie, the locking
range) of the tone d.:::coder to about
130Hz.
This narrow locking range prevents two adjacent signals from activating the unit.
The output of the LM567 (pin 8)
drives PNP transistor Ql, a BC557
which operates as a DC switch.
This in turn controls phase shift
oscillator stage IC3. When pin 8 of
IC2 switches low, Ql turns on and
supplies power to pin 7 of IC3 and
also bias to pin 3 via two 33k0
resistors.
IC3 is an LF351 BIFET amplifier
and this generates a reasonably
pure audio tone which the operator
can listen to directly with a set of
high impedance headphones. VRZ
adjusts the oscillator for minimum
distortion.
Construction
The circuit is built up on a small
PC board coded SC 06204901 and
measuring 106 x 66mm. Fig.4 shows
the parts layout. You can install the
---------------------<11-----+9V
PARTS LIST
47k
5
1
1
2
1
1
ON/OFF
+ 9 V - - - - - - - - - - + - - -......-------o>---o+9V
022I.
10
16VW
.,.
+
-
33k
01
BC557
C
IC2
LM567
VR1
10k
O.ll
.,. 2.2
+
16VW+
B
01+
1~.
,
1
"~'
"'!"'
EOC
VIEWED FROM
BELOW
VR2
470k
+
F-o
OUTPUT
~
33k
.,.
-t
ACTIVE CW FILTER
Fig.3: the circuit uses IC1a & IC1b as the bandpass filters and these feed tone
decoder stage IC2. When a valid tone is detected, pin 8 of IC2 goes low and
turns on Q1 and phase oscillator stage IC3.
PC board, code SC
06204901, 106 x 66mm
PC stakes
battery snap connector
SPOT toggle switch
RCA output sockets
1OkQ trim pot (VR 1 )
4 70kQ trimpot (VR2)
Semiconductors
1 LM1458 op amp (IC1)
1 LM567 tone decoder (IC2)
1 LF351 op amp (IC3)
1 BC557 PNP transistor (01)
Capacitors
2 1 OµF 16VW electrolytic
1 2.2µF 16VW electrolytic
2 1µF 1 6VW electrolytic
1 0.47µF metallised polyester
2 0 . 1- µF metallised polyester
1 .022µF metallised polyester
3 .01 µF metallised polyester
4 .0033µF metallised polyester
Resistors (0 .25W,
1 470k0
3
1 390k0
1
2 47k0
1
3 33k0
1
2 22k0
1
5%)
5.6k0
4. 7k0
3.3kQ
2. 7kQ
470
APRIL 1990
95
D.DAUNER
ELECTRONIC
COMPONENTS
WE STOCK A WIDE RANGE
OF ELECTRONIC PARTS
•
i-_,_______.
0
. 47k
--.✓
GND
INPUT
Fig.4: here's how to install the parts on the PCB. The finished board
can be housed in a plastic jiffy box and powered from a 9V DC
plugpack or from batteries.
-----.----7
rXo z..,
-
a,a--
0
! I oooa.;
~-0?
Jo
L
Fig.5: here is a full-size artwork for the PC board.
parts in any order you wish bu~ be
sure to install the ICs, transistor
and electrolytic capacitors with
correct polarity.
you will need a digital multimeter
(to monitor the voltage on pin 8 of
IC2) and either an audio oscillator
or a frequency meter. The first job
is to set the centre frequency of IC2.
Adjustments
If you have an audio oscillator,
apply a 1.2kHz signal to the input
and adjust VRl until pin 8 switches
low. Note the setting on VR 1, then
continue to rotate the trimpot until
pin 8 switches high. Set VRl to the
midpoint of these two settings.
Alternatively, if you have a frequency meter, you can set the centre frequency of IC2 by monitoring
pin 5. Make the measurement with
the probe in the xlO position and
adjust VRl to give a reading of
1.2kHz.
Check the completed PC board
carefully for possible errors before
applying power. To adjust the unit,
CAPACITOR CODES
D
D
D
D
D
96
IEC
No
Value
1
2
1
3
4
470n
0.47µ,F
100n
0 .1µ,F
.022µ,F
22n
.01 µ,F
10n
.0033µ,F 3n3
SILICON CHIP
EIA
474
104
223
103
332
for
Development • Repair
• Radio Amateur
• Industrial Electronic
• Analog and Digital
WHILE STOCKS LAST
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US Filter capacitor 4/.tF 3kV.
. $15.00
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Timer Motor 240VAC 6RPH.
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Come and see.
Showroom:
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(Behind Caltex Service Station In Blrdwood Road)
Phone 724 6982
TRADING HOURS:
Monday to Friday 9.00 a.m. to 4.00 p.m.
Saturday from 9.00 a.m. to 12.00 noon.
Now, whenever a 1.2kHz tone is
applied to the input, IC3 should
generate a new tone at the output.
This can be monitored using a pair
of headphones. Adjust VR2 so that
the output has minimum distortion
content.
Using the CW filter
The entire project could be housed in a plastic jiffy box and run
from either a DC plugpack or batteries (9V). There are no controls to
operate other than the ON/OFF
switch. To use low impedance headphones, it will be necessary to add
a simple attenuator to the output of
the audio oscillator. This can take
the form of a lkO potentiometer
wired as a voltage divider.
The filter can be driven from the
headphone or "record" output of
most receivers.
In fact by using the record output, which allows the receiver
loudspeaker to operate in the normal fashion, it is possible to make
an instant comparison between
filtered and unfiltered audio. You'll
be amazed at the results!
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