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Audio Filter
The PA0LQ Audio CW Filter
When Harry Grimbergen, PA0LQ started receiving noise
on 136 kHz from thyristors and triacs, he developed a very
effective AF Adaptor, with built-in noise blanker and 30 Hz CW
filter modules. The circuit below (see Figure 1) shows
the four-pole active filter section of the adaptor (built around the
TL072 dual opamp IC), and centred on 1000Hz. The circuit and
details have been extracted from Harry's original schematics
and notes of the complete AF Adaptor which Harry sent to me in
October 1999.
Figure 1 - The PA0LQ Active
Filter
![Original PA0LQ Filter](Audio Filter_files/filter1.gif)
R1: Approximately 180
ohms, adjust for 966 Hz R2: Approximately 180 ohms, adjust
for 982 Hz R3: Approximately 180 ohms, adjust for 1018
Hz R4: Approximately 180 ohms, adjust for 1034
Hz
In his letter, Harry wrote, "For the alignment of the
four pole 30 Hz opamp filters, a stable audio generator with
frequency counter and an oscilloscope is needed as the adjust
frequencies given above must be accurate to within 5 Hz in order to
get the flat phase response for ringing-free reception.
Adjusting R1 to R4 for the proper frequencies also compensate for
the C tolerances, which can be up to 10%. But the components
must be stable. So use either polystyrene or mica capacitors,
and metal film resistors."
Results
I am absolutely delighted with the performance of my PA0LQ
filter! In my case, I have centred my filter at about 800 Hz
(not 1000 Hz), and it is indeed possible to adjust the filter to
provide a very narrow passband, which is free of ringing. It
is certainly the best audio filter I have ever used.
Providing that the receiver AGC is not being hit too hard by a
strong signal, the filter readily separates stations which are only
100 Hz apart to yield a very pleasing CW tone that is easy to read -
even at 25 wpm!
When I worked Ossi, OE5ODL at 09:45 UT on Saturday 26th February
2000, this was another new 'first' on 136 kHz (first OE-GW 2-way
QSO). Of course, the 136 kHz band was quite busy (it always is
on a Saturday) and I have no doubt that this QSO would not have been
possible without my newly constructed audio filter. Whenever I
switched the filter out, Ossi's signal simply disappeared into the
QRM!
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Computer Simulations
The graphs below show the computer predictions of filter
performance forwarded to me by Harry PA0LQ. Click on the thumbnail
sketch to show the full-size graph. |
Performance: Frequency
Domain
![Performance: Frequency Domain](Audio Filter_files/freq_res1_small.gif)
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Performance: Time Domain
![Performance: Time Domain](Audio Filter_files/time_res2_small.gif)
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Alternative Alignment Method
One major problem in aligning the filter was that I did not (and
still do not) have access to a frequency counter having a resolution
better than 100 Hz. Given the choice, I would have followed
Harry's alignment instructions exactly. However, without the
ability to make accurate frequency measurements, I had to devise an
alternative method. This section describes my approach, using
a dual beam [two channel] oscilloscope.
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Figure 2 - Adjustable Resistor
Network
![Adjustable Resistor Network](Audio Filter_files/filter_tune.gif)
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The first thing I did was to
construct four adjustable resistor networks with which to find the
required values for R1 - R4.
Each of the four networks used resistor values as shown in Figure
2. Once the filter had been finally adjusted, a digital
multimeter was used to measure the required value and hence select
the required combination of fixed value
components. |
Initially, I peaked all four stages of the filter to
the frequency of the FT707 sidetone generator, at about 800
Hz. This resulted in a very narrow bandwidth - perhaps about
20 Hz. But this was far too narrow for practical purposes,
because: - the backlash in the FT707 VFO mechanism prevented
accurate tuning; - the IRT (Clarifier control) adjustment was too
coarse; - the amount of ringing meant that normal-speed CW would
not pass through the filter.
Of course, Harry's design
necessitated that the filter was aligned using staggered tuning of
the four stages.
Having decided to align the filter to the
frequency of my sidetone generator, I used the oscilloscope's
Channel 1 input to monitor the input to the filter (from the FT707).
The output of the filter was monitored using Channel 2, and also fed
to a separate amplifier & loudspeaker.
With all four
stages of the filter still peaked to the frequency of the FT707
sidetone generator, the ringing could be seen quite clearly: when
keying the FT707 with 'dots' from my electronic keyer - the output
of the filter never went to 'zero'!
I then replaced the
resistor network on the first stage (R1) with equivalent fixed-value
components.
Then, I set the electronic keyer to send dots at
about 30 wpm rate. Using the remaining three variable resistors, I
adjusted them with 'skill and dexterity' [or was I guided by good
fortune?] until the output waveform 'looked right' (see below). I
then replaced the three adjustable resistor networks with fixed
value components.
Output
Waveform Once I had soldered in fixed resistors for
R1, the approach I used was to set the tuning of the remaining poles
(R2-R4) for minimum ringing, as seen on Channel 2 of the
oscilloscope. All adjustments were made while keying 'dots' on
the FT707 at about 30 wpm.
Once aligned, the filter output signal had reduced to about 25 %
of the maximum output voltage (which had occurred when all four
poles were tuned to the same frequency). The final waveform
showed an exponential increase in the keyed envelope, until, at peak
amplitude, an exponential decrease to zero then followed. A
hand-drawn illustration of what I saw on the oscilloscope is shown
in Figure 3.
Figure 3 - Comparison of filter input and
output waveform
![Input and Output Waveforms](Audio Filter_files/time_res1.gif)
I do not know how close I got to Harry's intended
alignment points, but the results are superb! I can receive 25
wpm easily through the filter, and the bandwidth would seem to be
less than 45 Hz, but I cannot measure it. Also, the VFO drive
mechanism & IRT of my aging FT707 cope very well with the
bandwidth & shape factor.
My procedure for aligning the filter may not result in optimum
performance, but it produces much better results than just ‘tuning
for maximum smoke’. The final values used are shown in Figure
4, and the filter bypass switching is shown in Figure 5.
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Figure 4 - Audio Filter, following
alternative alignment procedure
![Final version of the filter](Audio Filter_files/filter2.gif)
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Figure 5 - Filter IN/OUT switching
arrangement
![Filter IN/OUT Switch](Audio Filter_files/filter3.gif)
The two 470 ohm resistors serve to hold the positive
end of the 1 uF capacitor at about half rail - about the same
potential as pin 7 of IC2b. This helps to reduce the level of
the 'click' that occurs when switching the filter in and out of
circuit.
The circuit of Figures 4 & 5 was built on plain
matrix board into a modified 'multi-media' PC speaker unit
('Creative' model SBS20 - £15 from PC World).
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