Radio Broadcast (May-Oct 1922)

Sharpness of Tuning in a Radio Receiver 351 Antenna Tuning Condenser Primary Coil A Earth mu Coupling r Detector Secondary By pass Coiie Condenser 1 Telephone Receivers Fig. 2: The inductively coupled receiver with broadly-tuned secondary circuit coupling is to be maintained. Probably the most convenient way to adjust the circuits natural frequency to resonance with an arriving radio wave is by using the tuning condenser shown, since the coupling is not appreciably changed by the variations of capacitance of this instrument. It is entirely feasible to use the arrangement of Fig. 2 with a non-adjustable primary coil, the tuning being then entirely dependent upon the variable condenser. CONCERNING DETECTOR VOLTAGE HOW will changing the detector voltage or coupling affect the practical operation of the two circuits shown? If the circuit of Fig. I. is arranged so that with the tuning condenser at a mid-scale setting and each of the two inductors A and B at one half its maximum value, the natural frequency of the antenna agrees with the frequency of an arriving stream of radio waves, the receiver will have the capability of tuning considerably above or below this frequency. When such a stream of waves arrives, it will generate radiofrequency currents in the antenna-to-ground circuit. Corresponding voltage impulses at the points Y and Z will be impressed upon the detector; by its characteristic rectifying action the voltages which occur in one direction will produce a larger current through the detector circuit than will those in the opposite direction; thus a rectified or direct current will flow through the meter and cause a deflection of the indicating needle, the scale reading of course being larger as the applied voltage is increased. Let us suppose that with the inductance evenly divided between the two coils A and B and with the condenser scale at 90°, the receiver is tuned to 360 meters wavelength (833,000 cycles per second). Let us further assume that some near-by station is transmitting a constant, unmodulated (or unvarying) stream of waves at this resonant frequency, of such intensity as will cause the current meter to show a deflection of 50 milliamperes. If now the tuning condenser is altered to a scale reading of 100°, the natural frequency of the antenna will no longer be 833,000 cycles, but something less; the antenna current will no longer attain its resonant maximum, the detector voltage will fall, and the meter reading will drop off to, perhaps, 10 milliamperes of current. The same reduction of current may be secured if the tuning condenser is turned away from the resonant point in the opposite direction, say to 80° on the scale, since the natural frequency will then be higher than the frequency of the arriving wave. WHY VARIABLE UNITS ARE VALUABLE NOW suppose that instead of dividing the total inductance equally between coils A and B, we put one quarter of it in A and the remaining three quarters in coil B. Since the total inductance remains as before, the resonant setting of the condenser will again fall at 90°. However, we may expect to find that the increased resistance effect of the detector will decrease the antenna current so much that the meter reading even at resonance will be less than before, or, say, 40 milliamperes. Moreover, the sharpness of tuning will be decreased, so that when the condenser is moved off resonance to either 80° or 100° the current indicated by the meter will not fall so greatly but may remain as large as 25 milliamperes. For the opposite case, in which coil A is given three quarters of the inductance and only one quarter placed in coil B, the resonant antenna current will be greater than before. The part of the total voltage applied to the detector may be less, however, so that the meter reading will reach only 45 milliamperes as a tuned maximum. Nevertheless, reduction of the resistance effect will sharpen the tuning very greatly, so that even if the condenser is moved only 5° on either side of the resonant point (i. e., to 85° or to 95°), the current reading may drop as low as only one or two milliamperes. An entirely similar set of effects will be found with the arrangement of Fig. 2 The signals and tuning will both be bad if the coupling between primary and secondary is too close; loosening the coupling somewhat will improve both signal strength and sharpness of tuning; further separation of the coils will cause some weakening of signals but will allow still sharper tuning. The inductively coupled circuit in