otal transmitted power. For smaller modulation depths, it is even less. One method of producing AM is to add the baseband signal to the (18.10) carrier and apply them to a non-linear amplifier, as shown in Figure 18.2. If the input/output characteristic of the non-linear circuit is The first method is used for low level modulators before the power amplifier of a transmitter. The second method is used for high level Non -Linear A.M. output modulators in the final stage of amplification. Amplifier An AM wave can be demodulated by the simple diode circuit shown in Figure 18.4. The rectified output voltage across the load resistor follows the envelope of the modulated input signal as in Figure 18.2 Low level amplitude modulator given by Equation 18.6 and its input voltage by Equation 18.7, then the output voltage is as in Equation 18.8. (18.6) (18.7) Figure 18.4 Envelope demodulator circuit Figure 18.5. The time constant CR must be large compared with the period of the carrier to prevent the output voltage decaying substan- tially between the peaks of the carrier. However, time constant CR in Figure 18.4 must be sufficiently small for the output voltage to decay as rapidly as the envelope changes when the baseband signal has its maximum frequency, F_.Iff.>>F_, this is easily arranged. (18.8) Other demodulators, which give a better performance at poor input signal/noise ratios, are the coherent demodulator (described The bandpass filter is required to remove all components except later) and the phase locked loop AM demodulator (Gardner, 1979; those which comprise the AM wave. These are shown in bold in the Gosling, 1986). above equation. Another form of modulator is shown in Figure 18.3. This uses a 18.2.2 Suppressed carrier modulation gain controlled amplifier whose input signal is the carrier and whose control voltage is the modulating signal. For example, the carrier It is possible, by using a balanced modulator (Tucker, 1953) to may be applied to the base of a transistor and the modulating signal eliminate the car