CW / SSB headphone filter
Many operators use lightweight walkmen, which are cheap, available, ergonomic, but not the most appropriate. Their frequency range is very wide, which can lead to fatigue. By narrowing the transmitted band to a minimum width, we will additionally gain an increase in communication efficiency.
Especially in cheaper devices, the bandwidth is too much, in addition, a CW filter on some intermediate frequency can be purchased as an add-on only for a lot of money. Narrowing the width of the transmitted band is also possible in the low-frequency part. The described filter works as a low-pass filter with a break-off frequency 1,6 kHz (-3 dB) or bandpass with width 200 Hz (-3 dB) at a frequency of approx 700 Hz. The six-part filter is connected to the speaker output and due to the simplicity of the connection, it is also suitable for complete beginners, while its qualities will be appreciated even by experienced amateurs.
The principle of the filter is very simple – it is a voltage divider. The CW part of the filter consists of 47 mH choke and a pair of capacitors 1 or change the color of the signal 100 nF connected in parallel. They form a series resonant circuit with a frequency of approx 700 Hz. At lower frequencies, the reactance of the capacitors is used, at higher reactance chokes. If we will (for simplicity) consider the headphone impedance as constant, it is based on the voltage divider principle, that the largest current will pass precisely at the resonance frequency, when the impedance of the series LC circuit will be the lowest.
Of course, each operator is comfortable with a different frequency, to which the filter needs to be set. We achieve a change in resonance frequency by changing the capacity of the capacitors. We calculate their capacity according to the formula:
C = 540 000 x (1/f)2 [uF;Hz]
The SSB part of the filter consists of a 100-ohm resistor and a 1-microfarad capacitor. These components again form a loaded voltage divider. As the frequency increases, more and more of the signal is routed through the capacitor to ground. Theoretically, at infinite frequency, there will be no signal at the output.
The attenuation of both the SSB and CW parts of the filter is almost the same. Therefore, it is not necessary to adjust the volume on the device when switching. However, the headphones also contribute to the filter's own attenuation as well as the bandwidth. The filter is designed for load impedance 16 ohm, which we get by connecting stereo headphones with impedance 2 in parallel×32 ohm. Although the filter also works with a different impedance, however, its properties are worse.
The choke is a commercially available type (e.g.. SHOP), capacitors are high quality polyester. The resistor is miniature, it is enough for a load of 0.25W. It is convenient to build the filter into a plastic box.
There is another mechanical solution, which is suitable for a CW filter. Two or three parts (according to the number of capacitors) can be connected to a short cable with connectors after soldering. At one end we fly, for example, 6.3″ jack, 3.5 for the second female″ jack. Place these parts in a small box between them.
