af filter

Description: The graphs below illustrate the computer predictions of filter performance provided by Harry PA0LQ. Click on the thumbnail to view the full-size graph. A significant challenge in aligning the filter was the lack of access to a frequency counter with a resolution better than 100 Hz. Ideally, the alignment instructions from Harry would have been followed precisely. However, due to the inability to make accurate frequency measurements, an alternative method was devised. This section outlines the approach, which utilized a dual beam (two-channel) oscilloscope. Each of the four networks used resistor values as depicted in Figure 2. Once the filter was finally adjusted, a digital multimeter was employed to measure the required values and select the appropriate combination of fixed-value components. Initially, all four stages of the filter were peaked to the frequency of the FT707 sidetone generator, approximately 800 Hz. This resulted in an excessively narrow bandwidth of about 20 Hz, which was impractical. After deciding to align the filter to the sidetone generator frequency, Channel 1 of the oscilloscope monitored the input to the filter (from the FT707), while Channel 2 monitored the filter output, which was also directed to a separate amplifier and loudspeaker. With all four filter stages peaked to the FT707 sidetone frequency, the ringing was clearly observed; when keying the FT707 with dots from an electronic keyer, the filter output never reached zero. The electronic keyer was set to send dots at a rate of approximately 30 wpm. The remaining three variable resistors were adjusted skillfully until the output waveform appeared correct. Subsequently, the three adjustable resistor networks were replaced with fixed-value components. After soldering in fixed resistors for R1, the tuning of the remaining poles (R2-R4) was adjusted for minimal ringing, as observed on Channel 2 of the oscilloscope. All adjustments were made while keying dots on the FT707 at around 30 wpm. Once aligned, the filter output signal was reduced to approximately 25% of the maximum output voltage, which occurred when all four poles were tuned to the same frequency. The final waveform exhibited an exponential increase in the keyed envelope, followed by an exponential decrease to zero at peak amplitude. A hand-drawn illustration of the oscilloscope output is shown in Figure 3. The filter successfully received 25 wpm, with a bandwidth estimated to be less than 45 Hz, although this could not be measured. The VFO drive mechanism and IRT of the aging FT707 managed well with the bandwidth and shape factor. While the alignment procedure may not yield optimum performance, it produced significantly better results than merely tuning for maximum output. The final values used are displayed in Figure 4, and the filter bypass switching is illustrated in Figure 5. The two 470-ohm resistors maintain the positive end of the 1 µF capacitor at approximately half rail voltage, similar to the potential at pin 7 of IC2b. This configuration helps to mitigate the clicking sound that occurs when switching the filter in and out of the circuit. The circuits shown in Figures 4 and 5 were constructed on a plain matrix board and integrated into a modified multimedia PC speaker unit (Creative model SBS20, purchased for £15 from PC World).

The described filter alignment procedure utilizes a dual beam oscilloscope to monitor both the input and output signals of the filter, enabling real-time adjustments to achieve the desired frequency response. The approach begins with the initial tuning of the filter stages to the sidetone frequency of 800 Hz, which is critical for ensuring that the filter operates effectively within the intended bandwidth. The use of variable resistors allows for fine-tuning of the filter characteristics, specifically targeting the reduction of ringing, which can adversely affect signal clarity.

The adjustment process is methodical, requiring the operator to observe the waveform output on the oscilloscope while simultaneously sending keying signals. This real-time feedback loop is essential for achieving optimal performance, particularly as the filter stages are progressively converted from adjustable to fixed resistors. The final configuration minimizes bandwidth while maintaining sufficient signal integrity, allowing for effective operation at the specified 25 wpm.

The implementation of the two 470-ohm resistors serves an important role in stabilizing the capacitor voltage, thereby enhancing the overall performance of the circuit by reducing transient noise during switching operations. The integration of the filter into a modified multimedia PC speaker unit indicates a practical approach to utilizing existing components for custom applications, demonstrating both ingenuity and resourcefulness in circuit design. The resulting filter circuit not only meets the operational requirements but also exemplifies effective engineering practices in electronic design and implementation.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 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 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 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. 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 5. 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).