Transmitters and Receivers
A CW, or Morse code, transmitter only requires the absence or presence of signal, and can be the simplest transmitter. Usually the oscillator is not turned off and on by the key, as this can cause the frequency to drift or "chirp". Rather an amplifier, or final amplifier stage even, is "keyed" off and on. For simple circuits, a stable oscillator might be a crystal-controlled (single frequency) oscillator or a variable frequency oscillator (VFO – a tunable oscillator). For more complex circuits, phased-locked loop (PLL) or direct digital synthesis (DDS) oscillators might be used. An amplitude modulated (AM) transmitter requires that the RF output level follows the pattern of the voice input, and this is usually accomplished by imposing the voice signal on the power lead to the final amplifier.
The resulting AM modulated carrier can be seen above left. A simple AM receiver is nothing more than a rectifier (envelope detector) which passes only the positive voltage on and a low pass filter that smooths the gap between the remaining pulses to reproduce the original modulating signal, shown in red. The integrator is often not shown, and can be nothing more than the circuit capacitance, or the effects of the audio frequency amplifier, which cannot respond to the higher RF frequencies. The simplest form of this receiver has no amplifier at all, but rather just a set of high impedance headphones, and is remembered (by some of us) as a crystal radio, where the term crystal applied to the detector, which was often a galena crystal, probed by a metallic "cat whisker" for a point that produced the desired non-linear response. Any rectifying junction will do, and an old standby in the Boy Scouts used to be a graphite pencil lead and a steel razor blade. Now-a-days, a semiconductor diode would be used, with the desireable feature being that the forward conduction threshold be as low as possible, so germanium or point-contact diodes are often used.
Receiving the CW signal is a little more complex. We have to have a wave to signify the difference between the lack or presence of the signal. Let me introduce the mixer, also known as a multiplier or a product (i.e. multiplying) detector. A mixer is a typically made from a non-linear device that takes two signal as inputs and can produce (amongst other things) an output containing the original frequencies as well as their sums and differences. If we place an oscillator at one of the mixer inputs that is close in frequency to the desired CW signal, then the difference, or "beat", frequency between the two signals can be placed in the audio range. Thus, whenever the CW signal is present we will hear a tone, and when it isn't, we will not hear that tone.
