At A, envelope detection of AM, shown in the time domain.
At B, slope detection of FM, shown in the frequency domain.
At C, a ratio detector circuit for demodulating FM signals.
At D, a product detector using diodes.
At E, a product detector using an NPN bipolar transistor biased as a class B amplifier. Both of these circuits can also be used as signal mixers.
Detection, also called demodulation, is the recovery of information such as audio, images, or printed data from a signal.

Detection of AM

The modulating waveform can be extracted from an AM signal by rectifying the carrier wave. A simplified time-domain view of this is shown in above figure A. The rapid pulsations occur at the car rier frequency; the slower fluctuation is a duplication of the modulating data. The carrier pulsations are smoothed out by passing the output through a capacitor large enough to hold the charge for one carrier current cycle, but not so large that it smoothes out the cycles of the modulating signal. This scheme is known as envelope detection.

Detection of CW

For detection of CW signals, it is necessary to inject a signal into the receiver a few hundred hertz from the carrier. The injected signal is produced by a tunable beat-frequency oscillator (BFO). The BFO signal and the desired CW signal are mixed, or heterodyned, to produce audio output at the difference frequency. The BFO is tuned to a frequency that results in a comfortable listening pitch, usually 500 to 1000 Hz. This is called heterodyne detection.

Detection of FSK

The detection of FSK signals can be done using the same method as CW detection. The carrier beats against the BFO in the mixer, producing an audio tone that alternates between two different pitches.
With FSK, the BFO frequency is set a few hundred hertz above or below both of the carrier frequencies that is, of both the mark frequency and the space frequency. The frequency offset, or difference between the BFO and signal frequencies, determines the audio output frequencies, and must be set so certain standard tone pitches result (such as 2125 Hz and 2295 Hz in the case of 170-Hz shift). Unlike the situation with CW reception, there is little tolerance for BFO adjustment variation or error.

Detection of FM

Frequency-modulated (FM) signals can be detected in various ways. These methods also work for phase modulation.
Slope detection: An AM receiver can detect FM in a crude manner by setting the receiver frequency near, but not on, the FM unmodulated-carrier frequency. An AM receiver has a filter with a passband of a few kilohertz, having a selectivity curve such as that shown in above figure B. If the FM unmodulated-carrier frequency is near either edge, or skirt, of the filter response, frequency variations in the incoming signal cause it to swing in and out of the receiver passband. This causes the instantaneous receiver output to vary. The relationship between the instantaneous FM deviation and the instantaneous output amplitude is not linear, however, because the skirt of the passband is not a straight line, as is apparent in the figure. The result is an unnatural-sounding received signal.
PLL: If an FM signal is injected into a PLL, the loop produces an error voltage that is a duplicate of the modulating waveform. A limiter, which keeps the signal amplitude from varying, can be placed ahead of the PLL so the receiver does not respond to AM. Weak signals tend to abruptly appear and disappear, rather than fading, in an FM receiver that employs limiting.
Discriminator: This type of FM detector produces an output voltage that depends on the instantaneous signal frequency. When the signal is at the center of the passband, the output voltage is zero. If the frequency falls below center, the output voltage becomes positive. If the frequency rises above center, the output becomes negative. The relationship between the instantaneous FM deviation and the instantaneous output amplitude is linear, so the output is a faithful reproduction of the incoming signal data. A discriminator is sensitive to amplitude variations, but this can be overcome by a limiter.
Ratio detector: This type of FM detector is a discriminator with a built-in limiter. The original design was developed by RCA (Radio Corporation of America), and is used in high-fidelity receivers and in the audio portions of TV receivers. A simple ratio detector circuit is shown in above figure C. The balance potentiometer should be adjusted for the best received signal quality.

Detection of SSB

For reception of SSB signals, a product detector is preferred, although a direct-conversion receiver can also do the job. A product detector also works well for the reception of CW and FSK. The incoming signal combines with the output of an unmodulated LO, producing audio or video. Product detection is done at a single frequency, rather than at a variable frequency as in direct-conversion reception. The single, constant frequency is obtained by mixing the incoming signal with the output of the LO.
Two product-detector circuits, which are also representative of the mixers used in superhet receivers, are shown in above figure D and E. At D, diodes are used; there is no amplification. At E, a bipolar transistor is employed; this circuit provides some gain. The essential characteristic of either circuit is the nonlinearity of the semiconductor devices. This generates the sum and difference frequency signals that result in audio or video output.