af filter

Posted on Feb 6, 2014

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. 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 foll

af filter
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owed 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. 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: 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`! 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...

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