The Demodulation of Frequency-Modulated Waves

The Limiter and Discriminator.² The limiter is a device that "clips off" the received and amplified frequency-modulated signal so that no amplitude variations (such as might be caused by static) occur in the wave envelope. Several types of limiters are used, singly or in combination; among these are the crystal detector (page 272), circuits using vacuum tubes that limit because of grid-current flow through a resistor, and circuits using tuned amplifiers that saturate because of low plate voltage.⁴³ Because it removes unwanted amplitude variations, the limiter is very important.

The discriminator is the demodulating device that is actuated by the frequency-modulated radio signal and produces a replica of the original modulating signal. The discriminator is an outgrowth of frequency-control methods.^44,45 Modifications of the discriminator circuit of Fig. 26 are used.

The operation of the discriminator when the received signal is at midfrequency is as follows: After passing through the limiter, the radio-frequency signal that is frequency modulated is impressed as indicated in Fig. 26. The primary is tuned to the midfrequency, and, if at the instant under consideration the frequency is at midfrequency, the vector diagram at the left in (6) applies. The impressed voltage E_p causes the 90° lagging current (approximately) to flow in the primary coil. This current induces a 90° lagging voltage E_i in series in the secondary coil that also is tuned to the midfrequency. The secondary current I_S will be in phase with this voltage when the signal is at midfrequency. A voltage drop will be caused across the secondary capacitor, and this will lag the secondary current by 90°. The connections are assumed to be such that the voltage between the anode and the cathode of each tube is the vector sum of the primary voltage E_p and one-half of the secondary voltage E_s The capacitor connecting the primary and the center of the secondary is sufficiently large to have negligible reactance. When the signal is at midfrequency E₁ and E₂ are equal, the rectified currents I₁ and I₂ are equal, and, since R₁ equals R₂, no resultant voltage will exist at the audio-output terminals.

The operation of the discriminator when the received signal is below midfrequency is as follows: For this frequency the secondary will be out of resonance, capacitive reactance will predominate, I_s will lead E_i, and the secondary voltage E_s will be as indicated. This causes diode voltage E₁ to exceed diode voltage E₂, rectified current I₁ will exceed

Figure 26. Circuit and vector diagrams for a discriminator used to demodulate frequency-modulated signals.

I₂, and, since R₁ equals R₂, the lower audio-output terminal will be positive and the upper terminal will be negative.

The operation of the discriminator when the received signal is above midfrequency is as follows: The secondary will be out of resonance for this frequency and inductive reactance will predominate, causing the secondary current I_s to lag the voltage E_i. This results in diode voltage E₂ exceeding E₁; also, rectified current I₂ will exceed I₁, and the upper audio-output terminal will be positive and the lower negative.

At the frequency-modulation transmitter, one cycle of modulating audio signal voltage causes a frequency shift below midfrequency and a shift above midfrequency. This will cause one cycle of audio-output discriminator voltage. At the transmitter a strong modulating voltage will cause a large frequency shift, and, at the receiver, a large frequency shift will result in a greater lead, or lag, and a stronger audio output.

In some circuits the capacitor connecting the primary and the secondary of Fig. 26 is omitted, and a coil is placed in the wire leading to the center of the secondary. This coil is coupled to the primary and serves to pick up a reference voltage that corresponds to E_P of Fig. 26(b) and (c).

The discriminator is satisfactory, particularly if it is modified as just explained, but it must be preceded by a limiter. Because the limiter reduces the signal to the point at which all normal amplitude variations are eliminated, the intermediate-frequency amplifier preceding the limiter must have much gain; also, two limiter stages are sometimes used for more complete limiting action.

The Ratio Detector. The ratio-detector of Fig. 27 requires no limiter. Note that one diode is reversed and that other significant changes have been made from Fig. 26. Also, the capacitor between primary and secondary is usually omitted, and, as for some discriminators, another secondary coil is placed in series with the ware to the center of the secondary.⁵⁰ The vector diagrams of Fig. 26 apply to the ratio detector.

Figure 27. Circuit of the ratio detector, a demodulator for frequency-modulated signals that need not be preceded by a limiter.

Capacitor C₃ of Fig. 27 is often about 8 microfarads, and the circuit is such that the voltage across C₃ can vary only slowly. Assume that a station is unmodulated and is, accordingly, on center frequency. If the station produces a strong signal (the limiter is omitted when a ratio detector is used), a large rectified current will flow as shown by the arrows, and capacitor C₃ will charge slowly to a relatively large voltage value. If the station produces a weak signal, a small rectified current will flow and capacitor C₃ will discharge slowly to a low value. This makes available an automatic volume-control voltage between the terminal AVC and ground. Note that at the center frequency no current flows through the R-F choke or resistor R₁.

If the transmitter is frequency modulated and the signal falls below the center frequency, the radio-frequency voltage across diode 1 will increase and that across diode 2 will decrease (Fig. 27). This will cause the current in diode 2 to fall and that in diode 1 to rise; this causes flow through the R-F choke and through resistor R₁. The voltage across capacitor C₂ will fall, and that across C₁ will rise, but the sum of these voltages will equal that across C₃. When the frequency rises above the midfrequency, the action is reversed. Thus, an audio voltage, which corresponds to the frequency shifts in the received-modulated signal, exists across resistor R₁. If a sudden change occurs in the magnitude of the received frequency-modulated signal, the magnitude of the demodulated output voltage remains fixed because capacitor C₃ holds the voltage constant across capacitors C₁ and C₂. Only the ratio of these voltages can change. Thus a limiter need not be used.