Radio today (Apr-Dec 1939)

for the modulation frequency being considered. Again assuming a carrier of 15 volts RMS, point "0" is the operating point for the detector. For 5,000-cycle modulation, the 270,000 ohm curve should be used. Note the absence of linearity in the 270,000 ohm curve and that it cuts off before zero voltage is reached. That means that the diode will not be conducting for low values of instantaneous carrier voltage. This cut-off causes serious distortion for high values of modulation percentage, as will be shown. Below the plot in Fig. 5 showing the characteristics of the detector tube, are sample modulation waves having percentage modulations of 75% and 25% as marked. Since only the positive half-cycles of the carrier wave are rectified, the zero axis of the input wave coincides with the zero line on the detector characteristics plot. NON-LINEAR DIODE DETECTION Assuming a modulation frequency of 5,000 cycles, the characteristic for 270,000-ohm load impedance is used. It is on this basis that the projections of the input waveform have been made and transferred to the right. The wave at the right is the output current from the detector. Note how for extremely low values of instantaneous RMS carrier voltage (75% modulation), the output current is distorted, due to the cut-off characteristic. Because the 1 megohm load impedance is fairly linear and continues to zero RMS carrier voltage before cut-off, detection with such a load would have no distortion. Values of Fig. 6— As modulation capability of detector is decreased, the detection distortion increases. load impedance between 1 meg. and 270,000 ohms would create distortion values lying between the two extremes illustrated. In Fig. 5, there is no distortion for low values of modulation percentage as is shown for 25% modulation. Since the distortion that is created is a function of the percentage modulation, detectors are rated in terms of their modulation capability. A detector that will handle 75% modulation without appreciable distortion, is said to have a modulation capability of 75%. Since present-day broadcasting stations use automatic modulation control for maintaining a high percentage modulation, it is not uncommon for the average modulation to be 60% with numerous peaks that range from 80% to 95%. It is these peaks that will be distorted, unless the receiver has an adequate modulation capability. Fig. 6 shows the amount of detector distortion that exists when a 100% modulated signal is applied to detectors having modulation capabilities from 70% to 95%. A detector with 95% modulation capability produces less than 1% distortion. If, however, the capability is only 75%, the distortion is 10% for a 100% modulated signal. Naturally this distortion is fed into the audio amplifier and reproduced by the loudspeaker. Because of the fact that this distortion is present only at high modulation levels, ordinary tests with the conventional service oscillators having a modulation of 400 cycles at 30% to 50% are valueless. Permeability push-button tuning in Belmont model 677 Inductance type tuning is employed in some of the newer Belmont auto radio sets. Housed in a compact remote unit with push-button tuning, the high-frequency circuits are tuned by iron-core units that move inside antenna, radio frequency, and oscillator coils. The use of these ganged permeability-tuned circuits does away with the need of a variable tuning condenser. In addition tuning by means of the mechanical push-button tuning unit, full manual tuning is always available. The high-frequency circuits which are separated from the chassis are shown (Continued on page 56)