Demodulator

As demodulator is known in the radio art means in the radio - receiver to recover the modulation signal from a previously transmitter modulated carrier wave. A demodulator for frequency modulation is z. B. in every VHF receiver to recover the audio signal from the high-frequency, frequency-modulated signal (87.5-108 MHz).

Different methods are used depending on the type of modulation.

Amplitude modulation (AM)

To demodulate amplitude-modulated vibrations (e.g. radio broadcasting long, medium and short wave, but also the picture content in television broadcasting), the signal is rectified and then sent through an RC element to suppress high-frequency residual vibrations ( envelope curve demodulation ). To put it precisely, it is a mixture of the sidebands with the carrier signal on the non-linear characteristic of the diode .

Another possibility is coherent demodulation , in which the received signal is multiplied by a locally generated carrier signal that is matched in phase and frequency. This shifts the high-frequency oscillation to the baseband. The prerequisite, however, is an exact match of the phase of the two carrier signals.

Single sideband modulation (SSB)

Although SSB is a special form of AM, it cannot be recovered (demodulated) from the high frequency signal by envelope demodulation. The reason is that the carrier frequency is suppressed in the transmitter, but is required again for demodulation in the receiver. This must be generated in the receiver by an auxiliary oscillator and combined with the high-frequency signal in a mixer . Then the original modulation content is created again. An exact match of the phase with the original carrier signal is not necessary if low distortions are accepted.

Morse code (CW)

In the very original Morse code , the transmitter is switched on and off in the rhythm of the characters. In the receiver, with most demodulators, you would only hear short clicks that could hardly be deciphered. If the received signal is superimposed in a mixer with an oscillator signal whose frequency deviates by around 800 Hz, the familiar whistling tones can be heard.

Slow Scan Television (SSTV)

The transmission of images requires a very small bandwidth because the brightness of the image points is coded as sound frequencies in the range from 1000 Hz to 3000 Hz. These low frequencies are reconstructed from the high frequency signal by superimposing an oscillator signal of a suitable frequency, which is generated in the receiver in a similar way to SSB. This superposition takes place in a mixer, the output signal is the original tone sequence of the camera.

Frequency modulation (FM)

By converting the frequency deviation into a voltage that is as proportional as possible, FM-modulated signals can be demodulated. FM demodulation usually requires a reference variable that is available in the form of a reference frequency or the natural frequency of an oscillating circuit.

FM demodulators are designed to be narrow or broadband depending on the signal bandwidth. A narrowband demodulator (e.g. voice radio) only records a frequency deviation of approx. 10 kHz, while a broadband demodulator (as is common in VHF broadcasting) has to process a frequency deviation of approx. 150 kHz.

With the phase discriminator , ratio detector and coincidence demodulator , the FM is first converted into a phase modulation before it is then demodulated.

With edge demodulation (also edge rectification, now obsolete), the FM received signal is sent to a somewhat detuned resonant circuit and converted into an amplitude-modulated RF signal at its resonance edge, which after subsequent AM demodulation is only approximately linear to the frequency change.

Often, the FM demodulation is a PLL - circuit achieved. The PLL tries to tune a VCO (voltage controlled oscillator) to the same frequency as the input signal. The control voltage corresponds to the frequency deviation and thus to the demodulated signal.

With digital angle modulations such as QPSK , instead of a PLL, the Costas loop is primarily used for demodulation.

Further procedures are:

Repeatedly measuring the frequency for a short period of time in a frequency counter ; the result (number) is subtracted from a reference value and corresponds to the original signal after passing through a digital-to-analog converter. In measurement technology, the process is referred to as a frequency-voltage converter .
Periodic triggering of a pulse of constant length ( monoflop ) by each zero crossing of the alternating voltage: afterwards there is a pulse width modulation . This PWM must now pass through a low-pass filter to eliminate the high-frequency components (smoothing) in order to receive the analog demodulation signal.

Frequency demodulation also provides the signal for an AFC (automatic tuning to the transmitter frequency).

A summary of both current and outdated methods of FM demodulation can be found in.