 around the wanted one and rejecting the others. As the inductor and variable capacitor form a parallel tuned circuit, the impedance reaches a maximum at resonance. This means that signals on frequencies either side of the resonant frequency pass to earth. At resonance the signals do not pass through the tuned circuit. Instead they pass into the diode. This rectifies the signal SO that the amplitude variations from the signal can be obtained. The small smoothing capacitor C1 acts to remove any remaining radio frequency components and smoothes the signal as shown in Figure 5.3. The rectified signals from the diode can be heard by connecting a pair of headphones to the output of the set. However, it is soon discovered Receivers 121 Figure 5.2 A circuit of a crystal set Rectified signal - the capacitor removes Radio frequency the radio frequency elements signal to leave just the envelope, i.e. the modulation Figure 5.3 Demodulating an AM signal that the signal strengths are weak even when a good antenna is used. Also the tuning is very broad and a single, strong station may be heard over a large part of the tuning range. TRF receiver If a crystal set is built, its limitations very quickly become obvious. The low signal strengths can be overcome by adding amplification. This can be placed after the detector as an audio amplifier, but it can also be added in the radio frequency stages prior to the detector. A receiver of this nature is called a tuned radio frequency (TRF) receiver because the tuning takes place at the radio frequency. This distinguishes it from other types mentioned later. 122 Newnes Guide to Radio and Communications Technology The lack of selectivity in the crystal set is mainly caused by the fact that there is only one tuning circuit. To overcome this problem and increase the gain still further a circuit known as a regenerative detector can be used. This type of circuit was very popular in the early days of radio when the number of valves had to be kept to a minimum to keep the cost of circuits down