Upon completion of this chapter you will be able to:
1. Describe cw detector circuit operations for the heterodyne and regenerative detectors.
2. Discuss the requirements for recovery of intelligence from an AM signal and describe the theory
of operation of the following AM demodulators: series-diode, shunt-diode, common-emitter, and
3. Describe fm demodulation circuit operation for the phase-shift and gated-beam discriminators
and the ratio-detector demodulator.
4. Describe phase demodulation circuit operation for the peak, low-pass filter, and conversion
In chapters 1 and 2 you studied how to apply intelligence (modulation) to an rf-carrier wave. Carrier
modulation allows the transmission of modulating frequencies without the use of transmission wire as a
medium. However, for the communication process to be completed or to be useful, the intelligence must
be recovered in its original form at the receiving site. The process of re-creating original modulating
frequencies (intelligence) from the rf carrier is referred to as DEMODULATION or DETECTION. Each
type of modulation is different and requires different techniques to recover (demodulate) the intelligence.
In this chapter we will discuss ways of demodulating AM, cw, fm, phase, and pulse modulation.
The circuit in which restoration is achieved is called the DETECTOR or DEMODULATOR (both of
these terms are used in NEETS). The term demodulator is used because the demodulation process is
considered to be the opposite of modulation. The output of an ideal detector must be an exact
reproduction of the modulation existing on the rf wave. Failure to accurately recover this intelligence will
result in distortion and degradation of the demodulated signal and intelligence will be lost. The distortion
may be in amplitude, frequency, or phase, depending on the nature of the demodulator. A nonlinear
device is required for demodulation. This nonlinear device is required to recover the modulating
frequencies from the rf envelope. Solid-state detector circuits may be either a pn junction diode or the
input junction of a transistor. In electron-tube circuits, either a diode or the grid or plate circuits of a
triode electron tube may be used as the nonlinear device.
What is demodulation?
What is a demodulator?