Radio Broadcast (May-Oct 1922)

The Selective Double-Circuit Receiver BY JOHN V. L. HOGAN Consulting Engineer, New York; Fellow and Past President, Institute of Radio Engineers; Member, American Institute of Electrical Engineers w HEN we use a receiver embodying a type of detector sucli as that including the crystal variety, which is capable of absorbing a relatively large amount of electrical energy, it is necessary to arrange some way of controlling and restricting the voltage applied to the detector system if sharp tuning is to be secured.* As the proportion of voltage applied to the detector (in comparison with the total voltage developed in the tuning system) is reduced, less energy is drawn from the persistently oscillating circuits and the anti-resonating resistance effect of the detector assembly is made smaller. To secure maximum selectivity by radio-frequency tuning, we must provide condenser and coil circuits which can oscillate freely and in which resistance is minimized. The receiving circuits illustrated in the earlier articles of this series contained only a single tuned or resonating circuit, i. e., that which included the aerial itself. By coupling the detector system somewhat loosely to this tuned antenna circuit it is possible to sharpen its resonant selection considerably, as has been explained. But the aerial and ground resistances, as well as the re-radiation resistance effect, remain in the circuit and put a limit to the improvement in tuning sharpness which can be secured by reducing the detector voltage; even under the best conditions of adjustment this single circuit tuner is hardly selective enough for working through severe interference. It is entirely feasible to add to the receiver a second tuned circuit in which resistance or damping effects are further reduced, and which consequently adds materially to the sharpness of tuning in the system. Fig. i shows the simplest way in which this tuned secondary circuit may be arranged with a crystal detector. The usual antenna circuit *See "Sharpness of Tuning in a Radio Receiver," by John V. L. Hogan, Radio Broadcast for August, 1922, pp. 348. contains the primary tuning condenser and the primary coil; inductively coupled to this latter is the secondary tuning coil, and across its terminals is directly connected the new element, a secondary tuning condenser. Suitable choice of the sizes of secondary coil and secondary condenser (which should be variable) produces a closed resonating circuit in which the antenna and ground resistances appear only to the small degree reflected through the inductive transformer. Thus the sharpness of tuning in this secondary circuit, and its resonant selectivity, will be very high. The only serious limitation to the selection power of the simple two-circuit receiver of Fig. I is the effect of detector resistance; as may easily be seen, the entire secondary voltage is applied to the crystal branch and hence damping due to the detector will be a maximum. However, if the by-pass or telephone-shunting condenser is made of moderately small capacitance (say 0.005 microfarad) fairly sharp tuning will be had. The selectivity of the double-circuit tuner may be greatly increased by reducing the proportion of the secondary voltage applied to the detector. A simple way to do this is /Infenna Pn 'm. tun/nff cani^enser Smtcfi X Detector i tumng' coil tuniry co/l Sec. tuning ca/rden^r Bypass conaepser tetep/jones W^fartf? FIG. I The Double-circuit Tuner with Crystal Detector: the tuned secondary is shown in heavy lines