Radio today (Apr-Dec 1939)

Dynamic Testing — Part XV An explanation of the basic principles involved in detection, with emphasis on ways of determining whether or not the detector is operating properly. By Vinton K. Ulrich Hytronic Laboratories Formerly Managing Editor of Radio Today No part of a modern radio or television receiver is more important than the detector; yet on the other hand, it is one of the biggest mysteries insofar as servicemen are concerned. While its purpose is fairly-well known, there have been very few tests for determining just how well the detector is working. In present-day receivers there are the following common types of detectors: diode, grid-leak, plate, and infinite impedance. Because a diode detector can be used to supply an AVC bias, it is by far the most popular circuit. The one purpose of all types of detectors is to rectify the received radio signal in such a manner that the audio modulation is not distorted. In Fig. 1 are shown carriers with sine-wave modulations of 25%, 50%, 75% and 100%. The variations in the envelope (dotted lines) of the carrier represent the modulation which is superimposed upon the carrier wave. (This is known as amplitude type modulation, and this article will be limited to modulation of this type.) MODULATED WAVES There are certain things about the modulated wave that should be noted. First of all, there is the unmodulated carrier wave which is shown at the left as being of continuous or having constant peak amplitude. The modulation in the form of a sine wave causes Fig. 2 — Typical diode detector circuit used in superhet receiver. the peak amplitude of the radio-frequency wave within the envelope to vary. When the modulation increases and decreases the peak amplitude of the carrier by 25% of its unmodulated value, the modulation is said to be 25 %. If the peak carrier voltage is instantaneously increased to 100% of its normal value and reduced to zero, the percentage of modulation is 100% as illustrated in Fig. 1. The frequency of the modulation for a sound receiver generally ranges from 50 to 7,500 cycles for high-fidelity reception and is considerably less for average sets. For video work in television, the frequency of modulation ranges up to 3 or 4 megacycles. REQUIREMENTS OF A DETECTOR For perfect reproduction the detector must be able to: 1. Handle incoming signals from 25%'MODULATION 7 3% MODULATION 1 50% MODULATION 10 0% MODULATION I Fig. 1 — When carrier is modulated, peak amplitudes of carrier waves vary in accordance with modulating signal. The greater the percentage modulation, the larger is the change in amplitude. Dotted lines represent modulation envelope. the minimum to the maximum carrier voltages without objectionable distortion. 2. Rectify signals having modulation frequencies ranging from the desired minimum to the desired maximum without introducing noticeable distortion. 3. Provide distortionless detection of signals having modulation percentages from zero to well over 80%. (Actually up to 100% is desirable, but it is hard to achieve in commercial practice.) In order to test a detector for these requirements, the serviceman must know how the detector operates; and to correct any defects that cause improper detection, it is necessary to understand of a few of the simpler basic design factors. In view of the fact that the diode is most commonly used and is the simplest form of detector, it is natural that it come first in this discussion. In Fig. 2 is shown a typical 2nd detector as used in a superhet receiver employing the diode tube. The values of diode load resistor, AVC filter network, and coupling components to the grid of the first audio are those commonly employed. DIODE DETECTION CHARACTERISTICS The family of characteristic curves of a diode detector is shown in Fig. 3, which is technically termed, a transrectification diagram. Each curve, representing a specific value of RMS carrier signal input, was obtained by varying the DC load resistance of the diode while maintaining a constant carrier signal input. The diode DC currents and DC voltages were measured and curves plotted. The plots show how the tube behaves for different values of unmodulated carrier voltages, which is useful in calculating the performance of the detector under dynamic operating conditions. Since each radio uses a specific value of diode load resistance, the proper value should be drawn in on the diagram. 1,000,000 ohms is a very common value and is shown in Fig. 3. Starting from zero (point A) at the right, the load line is erected having a slope equal to 1,000,000 ohms. For 40 volts a current of 40 microamperes would flow through 1,000,000 ohms. Consequently, the second point (B) that determines the load line is 40 volts and 40 microamps. This procedure of drawing in the load line is similar to that employed with amplifiers and was discussed on pages 50 and 51 of the October issue and pages 52 and 53 of the September issue of Radio Today. Assuming a carrier voltage of 15 volts RMS, point "O" becomes the op 46 RADIO TODAY