FM Slope Detector

According to the principle of the slope detector, the received FM signal is applied to an LC circuit whose output is an amplitude and frequency-modulated signal. This signal is then passed to an AM detector, which uses a detector diode, D, as shown in Figure (a) to recover the modulating signal, V_o.

The circuit diagram of a slope detector is shown in Figure (a). This circuit is also known as a single-tuned slope detector.

The transformer, T, shown in Figure (a), passes the received signal to the diode D. The secondary winding of the transformer T used as the inductor, and a capacitor. C_T is connected in parallel to constitute an LC resonating circuit. The secondary winding is tuned to a frequency slightly less than the resonating frequency of the LC resonating circuit the resonating frequency of the resonating circuit. fo is greater than the central frequency of the input signal f_c.

Symbolically. f_o > f_c as shown in below Figure (b).

If the maximum frequency deviation in the input FM signal is ±df. the operating frequency range of the voltage versus frequency curve of Figure (b) will be (f_c -f_d) to (f_c +  f_d), as clearly shown in Figure (b). This range covers the linear region of the curve. The frequency variation is converted into the corresponding voltage variation, and the voltage available at the anode of the diode D carries both the amplitude variation and the frequency variation in direct proportion to the modulating signal. This is marked as V_o in Figure (a).

The detector diode D rectifies the secondary voltage VD, which is marked as r(t) in Figure (a). The rectified voltage is used to charge the capacitor C up to the peak values.

The capacitor discharges through the resistance R to develop the modulating, voltage V_o. This modulating voltage is the voltage em, as marked in Figure (a). As a result, the slope-detector circuit demodulates the received signal and recovers the original modulating signal.

Drawbacks of Slope Detector

A slope detector has the following drawbacks:

• Although a slope detector is a very simple and low-cost circuit, it has a severe drawback. Which amounts to harmonic distortion. The frequency range over which the voltage versus frequency curve, shown in Figure (b), remains linear, is very small. Therefore, all frequency deviations are not linearly translated into their corresponding voltage variations. When the non-linear portion of the curve comes into effect, the output of the resonating circuit contains the corresponding voltage variation and the higher harmonics of this voltage. This introduces distortion into the requisite output.
• A slope detector transfers even the slightest amplitude variation of the incoming FM signal to the output because it does not reject the amplitude variation of the incoming FM signal and responds equally to this variation. Therefore, if there is an increase or decrease in the amplitude of the received FM signal which is unwanted, the same change is reflected at the output along with the amplitude variations due to the frequency deviations, which is desirable. This alters the original modulating signal and the true modulating, signal is not recovered.

Due to these drawbacks, the slope detector is hardly used in practice. The drawbacks of the slope detector are removed in the balanced slope detector another frequency discriminator.