Meissner-Circuit
Amplitude Modulation
Superheterodyne Receiver
Crystal Oscillator
Frequency Modulation

Early Electronic for Radio Transmission

The Triode

Until the early twentieth century, electrical signals could not be amplified. For that reason the most important invention for any progress in communication was the amplifier tube. This indicated the dawn of the age of electronics.

In the vacuum of a tube, electrons flow from a heated anode to a cathode. But only in one direction. This effect can be used for a diode/rectifier function. This is the Edison-Richardson effect.

It is possible to influence the electrode flow of a tube diode. Another cathode (the so-called grid) can be used to regulate the current. The application is the amplification of electrical signals. Using a weak electrical signal to control the flow of electrons could amplify the signal.

Main application of the triode was of course the amplification of weak signals as it was required for telephone long distance transmission. But it became also usefull to generate continuous oscillation. This is done with the so-called Meissner Circuit.

Meissner Circuit

A Meissner circuit consists of an oscillating circuit which determines the frequency. This oscillating circuit is inductively coupled to an amplifier circuit. Normally the triode is not conductive and the oscillator is free. Only at one certain phase, when the capacitor is holding the charge, the triode becomes conductive and is therefore charging the capacitor fully. This is like pushing a pendulum always at the right time in its oscillation.

Amplitude Modulation

In amplitude modulation, a low-frequency signal changes the amplitude of a high-frequency signal (carrier signal).

The amplitude modulation creates frequency ranges to the right and left of the carrier frequency, which correspond to the modulation spectrum. These have the bandwidth of the modulating signal.

Superheterodyne receiver

How is it possible to demodulate an AM signal, especially if multiple AM signals can be received? This is done with a special receiver, the Superheterodyne receiver.

The received radio (RF) signal is amplified after it is received with an antenna. Then it is „mixed down“ with an electronic element called a „mixer“. A mixer multiplies two signals. In this case, a high-frequency signal is multiplied by an intermediate-frequency signal. This intermediate frequency signal is generated with a tunable oscillator. This brings the carrier signal to a fixed intermediate frequency IF (e.g. at 400 kHz). At this intermediate frequency, a fixed bandpass filter can ensure that all frequencies except the upper sideband are filtered out. Only the upper sideband then remains, which can now easily be demodulated, for example with a diode, i.e. made audible.

Crystal Oscillator

The central element of a crystal oscillator is a quartz. A quartz has piezoelectric properties, which means that it contracts when an electrical voltage is connected. This allows a crystal to vibrate. These „natural oscillations“ are very precise. With electronic feedback e.g. via a Meissner circuit, a quartz oscillation can be kept running continuously and thus becomes usable.

The main application at the beginning of the 20th century was the creation of precise clocks. In the 1920’s crystal oscillators were used to create stable frequencies for radio stations. The first crystal oscillators were developed in the Bell Laboratories above all from 1923 onwards.

Frequency Modulation

Frequency modulation can be generated by using controllable capacitances. In modern circuit technology, these are varactor diodes, for example. Frequency modulation has clear advantages over amplitude modulation:

• Disturbances, above all atmospheric disturbances, primarily affect the amplitudes but not the frequencies. A frequency-modulated signal is therefore significantly less susceptible to interference.
• Frequency modulated signals are also less susceptible to interference from nearby transmitters of the same or close frequency.
• Frequency-modulated signals enable a larger dynamic range and are therefore of a much higher quality than amplitude-modulated signals.
• Frequency modulated signals allow a high frequency range (up to 15 kHz).

The main disadvantage of frequency modulation is the much higher bandwidth. Therefore frequency modulation requires high frequencies (ultra short wave).