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By LEON WILLIAMS
VK2DOB
ACTIVE
FILTER cleans up
weak CW reception
Dig out those weak CW signals from the noise
and interference with this simple filter unit. It
easily connects between any receiver and an
external speaker. Unlike other designs that use
a fixed narrow filter this unit has a variable
filter control for obtaining optimum reception.
Most radio receivers are only intended to receive voice signals and are
required to have an audio bandwidth
of several kilohertz.
A typical amateur band receiver
fitted with a SSB filter has an audio
frequency response from 300Hz
to 2700Hz, giving a bandwidth of
2400Hz. By contrast, the frequency
compon
ents of a CW signal only
occupy a bandwidth of 100Hz or so,
depending on the sending speed.
It is quite obvious that there is plenty of room in the receiver bandwidth
24 Silicon Chip
to fit a little CW signal. This situation
is quite OK until another CW signal
or some interfering signal is received
perhaps only a few hundred Hertz
away. We are now in the situation of
trying to decipher a CW signal amongst
a whole lot of other sounds. It’s a bit
like trying to listen to a small voice in
a noisy crowd of people.
The solution is to narrow the audio
frequency response so that the interfering signal is filtered out, leaving the
wanted signal on its own.
There are two main ways of doing
this. First, a narrow crystal filter can
be switched in to the intermediate
frequency (IF) circuits of a superheterodyne receiver. A typical bandwidth
might be 500Hz. When normal voice
signals are to be received, the narrow
filter can be switched out and a wider
(2400Hz) filter switched in.
Filters such as this are quite expensive and you would have to be a
keen CW operator to consider this.
The second alternative is to switch in
and out a narrow audio bandpass filter
somewhere in the audio stages of the
receiver. This technique can be used
in both superheterodyne and direct
conversion receivers.
Direct conversion receivers do not
have IF stages. Once again, the filter
needs to be switched out when voice
signals are received, because the narrow filter would eliminate too many
frequency components of the voice
and probably make it unintelligible.
A recent innovation is the use of out
board DSP processors which digitise
the voice signals and through digital
manipulation result in various filter
responses. These units are expensive,
costing hundreds of dollars.
This Adjustable CW Filter has a
number of advantages over the methods mentioned above.
Firstly, it is self-contained, is
powered from a DC plugpack and
no modifications need to be done to
the receiver. Also it is inexpensive
to build, uses standard parts and it
simply connects between the speaker
socket or audio output of the receiver
and an external speaker.
The front panel has two controls,
Volume and Filter. The filter control
can be adjusted to give any bandwidth
between wide open (no filtering) and a
very narrow band pass response centred on 800Hz. The filter control can
also be used to good effect with voice
signals by acting to reduce the level of
high frequency noise and interference.
If 800Hz is not your favourite frequency this can be changed, as explained
later on.
Fig.2 shows the range of filter responses provided by the unit.
A special feature is a LED on the
front panel that is turned on when
800Hz is detected. This gives an indication that you are tuned correctly
to the CW signal and also it is an
opportunity to learn to “read” CW by
decoding the flashes with the volume
turned down.
Circuit description
The complete circuit is shown in
Fig.1. Audio signals from the receiver
are fed to the 10kΩ trimpot VR1. The
minimum input level that the tone decoder will function correctly is about
70mV RMS or 200mV peak-peak.
Following VR1 is op amp IC1a
which has a gain of two. A 560pF
capacitor connected across the 100kΩ
feedback resistor from pin 2 to pin 1
provides some low pass filtering. As
we are not using separate positive
and negative power supply rails, the
non-inverting input (pin 3) is biased
Fig.1 (right): the circuit is a variable
bandpass filter centred on 800Hz
and includes a tone decoder (IC2) to
indicate when the receiver is correctly
tuned for CW transmissions.
December 1996 25
PARTS LIST
1 metal case, 102 x 62mm x
148mm (W x H x D)
1 PC board, code 06112961, 97
x 68mm
1 9-12V DC plugpack
1 DC panel socket
1 6.5mm mono jack socket
1 RCA panel socket
2 20mm knobs
1 LED bezel clip
1 10kΩ log potentiometer (VR4)
1 10kΩ dual linear potentiometer
(VR2a,VR2b)
2 10kΩ horizontal trimpots
(VR1,VR3)
17 PC pins
Semiconductors
1 TL074, LF347 quad op amp
(IC1)
1 LM567 PLL tone decoder (IC2)
1 LM386 audio amplifier (IC3)
1 1N4004 diode (D1)
1 5.6V 1W zener diode (ZD1)
1 5mm green LED (LED1)
Capacitors
1 1000µF 25VW electrolytic
1 470µF 25VW electrolytic
3 100µF 25V electrolytic
6 2.2µF 63V electrolytic
5 0.1µF MKT polyester
1 .047µF MKT polyester
4 .022µF MKT polyester
2 560pF ceramic
Resistors (0.25W, 1%)
6 100kΩ
2 820Ω
4 47kΩ
1 180Ω
2 10kΩ
1 100Ω
1 5.6kΩ
2 10Ω
1 1kΩ
Miscellaneous
Screws, nuts, spacers, hook-up
wire
to half the supply voltage by two
10kΩ resistors. A 100µF capacitor
helps filter out noise. This half supply
voltage point is also connected to the
non-inverting inputs of the other three
op amps in IC1.
The output of IC1a is split into two
paths. First, it is applied to the input
of an audio bandpass filter using IC1b.
This stage has been designed for a
centre frequency of 800Hz, unity gain
in the passband and a -3dB bandwidth
of 150Hz.
26 Silicon Chip
AUDIO PRECISION SCFREQRE AMPL(dBr) vs FREQ(Hz)
5.0000
01 NOV 96 13:53:59
0.0
-5.000
-10.00
-15.00
-20.00
-25.00
-30.00
-35.00
-40.00
-45.00
20
100
1k
10k
20k
Fig.2: this diagram shows some of the bandpass responses available from a
filter, ranging from a deep notch to quite wide.
When calculations are done for any
audio filter circuit, it’s almost certain
that non-standard value components
will be called for. The components
specified in the parts list are close
enough to the calculated values without greatly affecting the performance.
The output of this stage is connected
to an identical stage using IC1c. The
combination of these two bandpass
stages provides a narrow response
with high attenuation of frequencies
either side of the passband. Fig.3
shows the general filter configuration
and the formulas used in the design
calculations.
The output of IC1c is coupled to
one half of a dual-gang potentiometer
(VR2b) by a 2.2µF capacitor while the
output of IC1a is coupled via another
2.2µF capacitor to the second gang
(VR2a). The mixer stage uses IC1d and
has unity gain.
The wipers of VR2a and VR2b are
each connected to the mixer stage by a
100kΩ resistor and a 0.1µF capacitor in
series. The point to note is that when
VR2 is fully anticlockwise, no signal is
passed from the bandpass filter output
to the mixer, while the full output of
the unfiltered signal from IC1a is fed
through. When VR2 is rotated fully
clockwise, the reverse occurs and
only the output of the bandpass filter
is fed to the mixer. By varying the
position of VR2 we can obtain any
degree of filtering between narrow in
the clockwise position and wide in the
anticlockwise position.
The output of IC1d is coupled to
the Volume control (VR4) via a 2.2µF
capacitor. VR4 varies the signal level
applied to the audio power amplifier
IC3, an LM386. This drives an external
speaker. The 2.2µF capacitor from pin
7 to 0V is included to reduce the level
of hum to an acceptable level when a
plugpack power supply is used. The
10Ω resistor and .047µF capacitor
help keep the amplifier stable at high
frequencies.
800Hz indicator
IC2 is an LM567 tone decoder,
which turns on the LED (LED1) when
800Hz is applied to its input. The
LM567 is actually a phase locked loop
circuit which compares an internal
oscillator to an external signal at pin
3. When they are within a few hundred
Hertz of each other, the open collector
output at pin 8 switches to 0V and
lights LED1.
The frequency of the internal oscillator is determined by the resistance
between pins 5 & 6 and the capacitor
from pin 6 to ground. With the values
shown on the circuit the frequency can
be varied between about 600Hz and
1500Hz by VR3. The capacitors from
pins 1 & 2 to ground provide filtering
for the internal circuits and also affect
Fig.3: the general filter configuration & the
formulas used in the design calculations.
the detection bandwidth. The values
shown on the circuit were arrived at
through experimentation by monitoring LED1 for correct operation under
various signal conditions.
A 180Ω resistor, a 5.6V zener diode
ZD1 and a 100µF capacitor provide
a regulated 5.6V power supply for
IC2. The full circuit is powered by a
9-15V 500mA DC plugpack. Typically
this will be a 12V 500mA plugpack.
Diode D1 provides protection against
the power supply being connected
with reverse polarity. A 10Ω resistor
and a 1000µF capacitor decouple the
power supply line and reduce the
level of hum.
Construction
The prototype Adjustable CW Filter was housed in a standard metal
case measuring 102mm wide, 62mm
high and 148mm deep. The parts are
mounted on a PC board measuring 97
x 68mm and coded 06112961. The
PC component overlay and wiring
diagram is shown in Fig.4.
Start construction by assembling the
Fig.4: the PC component overlay and wiring diagram for the CW filter.
Check your work carefully before applying power.
PC board. Inspect the board for shorts
between tracks and correct hole sizes.
The holes for the trimpots, PC pins
and diodes may need enlarging. Use
the component overlay as a guide and
solder in the resistors, trimpots and
diodes first.
In some cases, a low impedance
termination will be required by the
amplifier in the receiver; eg, 8Ω or
10Ω. If this is the case, an 8Ω or 10Ω
0.5W resistor can be connected across
VR1; provision has been made for this
on the PC board,
Install the PC pins next, followed by
the capacitors. Double check the polarity of the electrolytic capacitors to
make sure they are installed correctly.
Finally, solder in the three integrated
circuits, again checking that they are
in the correct positions. The parts list
details the components associated
with the bandpass filters for a centre
frequency of 800Hz. If you want to
change this, use the equations in Fig.3
to calculate the new values.
When it is complete, put the PC
board aside and mark out and drill
December 1996 27
The completed CW filter
is easy to set up and
adjust, provided you have
access to a multimeter
and an audio signal
generator. Use cable ties
to keep the wiring neat
and tidy.
the holes in the metal case. There are
three holes required on the front and
back panels and four mounting holes
in the base. Install the sockets on the
rear panel and the pots and LED on the
front panel. The LED is held in place
with a plastic bezel clip.
Mount the PC board in the case with
3mm screws and nuts and 6mm spacers. This done, connect the sockets,
pots and LED to the PC board with
hook-up wire. The prototype used
separated cores from rainbow ribbon
cable which makes the job of tracing
wires easy. If you have trouble identi
fying the tags of the DC socket, plug
the plugpack into the socket, turn on
the power and check the voltage and
polarity with a multimeter.
Finally, fit the two knobs and adjust them so that when the pointer is
vertical the pots are at mid-position.
Fig.5: actual size artwork for the PC board.
Testing
To test and adjust the filter you
should have a multimeter and an audio
generator. If you don’t have an audio
generator, try to borrow one as it makes
the setting up process easy.
Turn the filter and volume pots fully
anticlockwise and the input trimpot
VR1 and the decoder trimpot VR3
fully clockwise. Plug a speaker into the
speaker socket, connect the DC plug
pack and connect the audio generator
to the receiver socket.
Power up the CW filter and measure
the voltage between pin 6 of IC3 and
0V. If you are using a 12V plugpack
28 Silicon Chip
Fig.6: actual size artwork for the front panel.
this reading will probably be around +14V. Most plugpacks
are unregulated and only really get to their rated voltage
at full load, so don’t get too alarmed if the reading seems
high. If the reading is 0V or close to 0V, check to see if the
plugpack polarity is reversed or that diode D1 is around
the wrong way.
If these check out OK there may be a short circuit in
the PC board. Turn off the power straight away and check
the PC board for problems. There could be a component
in the wrong way, shorts between tracks or a wiring error.
If everything appears OK, set the audio generator to
800Hz (or the alternate frequency chosen for the bandpass
filters) with an output of around 100mV RMS. Adjust the
volume pot so that you can hear the tone in the speaker.
Now, using a small screwdriver, turn the decoder trimpot
VR4 until the CW LED turns on and note VR4’s position.
Keep rotating VR4 until the CW LED turns off, again noting
the position. This done, return VR4 to the midpoint between these two positions, disconnect the audio generator
and check that the CW LED turns off.
That’s all there is to setting up the CW filter. The lid can
now be fitted to the base using four self-tapping screws.
Using the filter
The CW filter is designed to sit alongside your receiver
and be permanently connected. The method of connection to the receiver will depend on the specific receiver.
Probably it will have an external speaker socket. When
using this option you will need to check that the internal
speaker is disconnected otherwise you will hear both the
unfiltered and filtered signal at the same time.
Some receivers have an output that is not directly from
the speaker but from a low level audio signal point before
the speaker. If you use this option you will need to once
again make sure the speaker in the receiver is turned off.
As mentioned previously, if the output of your receiver
overdrives the CW filter, adjust the input trimpot VR1 until
the sound from the speaker is undistorted.
If you don’t have an external speaker for your receiver,
now is the time to get one. There are commercially available speaker boxes specifically suited for this purpose or
you could buy yourself a cheap speaker and build a box
to mount it in. The cheapest solution could be to use a
small speaker box from a discarded mini stereo system.
In any event, the sound from an external speaker will
almost always be better than that obtained from the little
speakers found in most receivers.
To operate the CW filter, turn the filter control to the
wide position and adjust the volume control to suit. Tune
in a CW signal on the receiver and watch the CW LED
until it flashes in time with the bursts of tone. Turn the
filter control clockwise until the CW signal is clear of any
interference and easy to listen to.
If the interfering signal is wideband it won’t be possible
to completely filter it out as some of it will lie in the filter
passband but a significant improvement will be heard
anyway. You will be amazed how effective the CW filter
can be when listening to a noisy crowded band. A small
CW signal can sometimes hardly be heard but when the
filter control is turned to narrow, the CW signal seems to
jump out from the noise.
Of course, if the band conditions are good the filter con
SC
trol can be left in the wide position.
If you are seeing a blank page
here, it is more than likely that
it contained advertising which
is now out of date and the
advertiser has requested that
the page be removed to prevent
misunderstandings. Please feel
free to visit the advertiser’s
website:
www.winradio.com/
December 1996 29
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