Demodulators

Demodulators or detectors measure phase or amplitude variations which are used to convey intelligence or control another device. A circuit often used for phase detection is shown in Fig. 199.

Fig. 199. Phase-difference demodulator.

Transformer T₁ has a balanced secondary with e₁ volts per side. In frequency or phase demodulators, the circuit is called a discriminator, and uses air-core transformers. This circuit is used in phase demodulation to produce a d-c voltage proportional to phase shift. In this case e₂ leads e₁ by 90° for zero output. This condition is shown in the upper vector diagram. If e₂ should lead the upper e₁ by less than 90°, e_A would increase and e_B decrease, causing a net d-c voltage to appear across the output. If e₂ should lead the lower e₁ by less than 90° the d-c output voltage would change polarity. The plate voltage of one diode is the vector sum of e₁ and e₂, and is 90° out of phase with the plate voltage of the other diode (which is the vector difference of e₁ and e₂). If the phase angle Φ between e₁ and e₂ changes either positively or negatively, output voltage E_dc is very nearly proportional to Φ, up to Φ = ±60°. For positive phase shift, the output E_dc is negative; for negative phase shift, E_dc is positive. For good linearity, load resistances R_L should be large compared to the diode resistances.

Modifications of this circuit made to eliminate the vector difference voltage or to reduce the degree of amplitude modulation are known as balanced and ratio detectors.⁽¹⁾

As used in servo control, the demodulator produces d-c output which changes polarity when one voltage reverses with respect to the other; in this case e₂ adds directly to one voltage e₁ and subtracts from the other. The lower vector diagram shows how a voltage e₂ - e₁ causes a low difference voltage on diode A and a large sum voltage on B to produce a net d-c output voltage. Transformers in phase-reversal demodulators are usually iron-cored. Both transformers T₁ and T₂ should have low exciting current so that the phase angle between the voltages to be measured is not appreciably increased. For the same reason, the source impedances should be small; when this is not possible, the transformers should be matched. Figure 131 shows how necessary this is. With equal source and load resistances, several degrees of phase shift are introduced even if the ratio of transformer reactance to source resistance is 10 : 1. Variations in this reactance cause errors in the phase-demodulator output.

In a-m receivers, the received signal is modulated radio frequency. If 100 per cent amplitude modulation is used, the r-f amplitude is varied from 0 to 200 per cent at an audio-frequency rate. The detector, or demodulator, of the receiver first rectifies the r-f signal and then eliminates the r-f component, leaving only audio frequencies in the output. A rectified signal, before the r-f is eliminated, is shown in Fig. 200.

Fig. 200. Rectified amplitude-modulated wave.

Demodulation is accomplished in the circuit of Fig. 201 by means of diode 6H6-2.

Fig. 201. Demodulator and automatic gain control circuits.

Each half of the r-f cycle is rectified and the d-c power is absorbed in resistor R₃. Audio power is bypassed around R₃ by capacitor C₂, and the voltage is impressed upon the primary of transformer T₂. If an amplitude-modulated wave is used in this amplifier, the output voltage of winding S₁ on transformer T₁ has the form shown in Fig. 200. The first few cycles are shown as full-wave rectified loops with constant amplitude, that is, with no modulation. The audio output for this section of the wave is zero. A sine-wave envelope of 100 per cent modulation is shown in the rest of the figure. Average voltage left after the carrier frequency half-loops have been absorbed by the r-f filter L₁C₁ is the audio voltage impressed on transformer T₂.

The method of demodulation just described is known as diode demodulation. It is often accomplished by means of a single diode, and then every other lobe of the wave in Fig. 200 is omitted. Methods are in use also for demodulation with a triode, in which some amplification of the demodulated wave is obtained.

(1)	See "Diode Phase-Discriminators," by R. H. Dishington, Proc. I.R.E., 37, 1401 (December, 1949) ; also, Radiotron Designer's Handbook, F. Langford-Smith, RCA Victor Division, Harrison, N. J., p. 1088.