nal Frequency Figure 5.40 Concept of a slope detector Receivers Input Demodulated signal output Figure 5.41 Stages of demodulation when using a slope detector or demodulator from the carrier of the signal. As the frequency of the signals varies up and down in frequency according to its modulation, SO the signal moves up and down the slope of the tuned circuit. This causes the amplitude of the signal to vary in line with the frequency variations. In fact at this point the signal has both frequency and amplitude variations. The final stage in the process is to demodulate the amplitude modulation and this can be achieved using a simple diode circuit. One of the most obvious disadvantages of this simple approach is the fact that both amplitude and frequency variations in the incoming signal appear at the output. However, the amplitude variations can be removed by placing a limiter before the detector. Additionally the circuit is not particularly efficient as it operates down the slope of the tuned circuit. It is also unlikely to be particularly linear, especially if it is operated close to the resonant point to minimize the signal loss. For many years circuits employing discrete components were used and the two most popular circuits were the ratio detector or demodulator and the Foster-Seeley demodulator. Of these the ratio detector was the most popular as it offered a better level of rejection of amplitude modulation, and hence it would operate satisfactorily with poorer levels of limiting in the preceding IF stages of the receiver. The circuit of the ratio detector is shown in Figure 5.42 and essentially operates by using a frequency sensitive phase shift. It is characterized by the special transformer and the diodes that are effectively in series with one another. When a steady carrier is applied to the circuit the diodes act to produce a steady voltage across the resistors R1 and R2, and the capacitor C3 charges up as a result. 158 Newnes Guide to Radio and Communications Technology The transformer enables the ci