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
Figure 1 - The PA0LQ Active Filter
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."
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!
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.
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.
Figure 2 - Adjustable Resistor Network
The first thing I did was to construct four
adjustable resistor networks with which to find the required values for R1 - R4.
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
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.
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
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
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 copes 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 4 - Audio Filter, following
alternative alignment procedure
Figure 5 - Filter IN/OUT switching
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
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).