Radio Broadcast (Nov. 1925-Apr 1926)

Record Details:

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APRIL, 1926 CUTTING OUT THE LOCALS 687 the receiver. At the same time, these wave traps offer a very low impedance to all other frequencies. In Fig. I there has been plotted the change in impedance of a wave trap as the frequency is varied, and this gives a good idea how a wave trap functions. At the frequency marked / on the diagram, the impedance as read on the ordinate of the curve is very high and since this circuit would be connected in the antenna system of a receiver, it is evident that at this frequency the impedance of the antenna system to this particular frequency would be very high, and for that reason practically no energy could be received at this frequency. At the same time, the impedance of the trap to any other fre A i y ! — E c2 ; Vacuum Tube Voltmeter SETTING OF CONDENSER C2 FIG. 5 A curve showing the decrease in interference obtained by the use of a wave trap quencies, either above or below the frequency /, is very low. The efficiency with which the trap operates depends upon the steepness of the sides of the curve, and in order to obtain satisfactory operation, it is essential that a sharp resonance curve be obtained. This idea of high impedance at resonance may "T"a FIG. 6 The small arrows indicate the circulating current set up in the wave trap by the interfering signal require some explanation since we are accustomed to think of resonant circuits as having a low impedance. At resonance, a circuit consisting of a coil and a condenser has a low impedance to the flow of current around the path FIG. 4 The circuit diagram of the test apparatus used to obtain some of the data given in this article indicated in Fig. 6, by the small arrows. This is important current with regard to the various tuned circuits of a receiver. In a wave trap, however, we wish to impede the flow of current in the circuit AGCa, and at resonance, the trap circuit offers high impedance to the flow of current in this circuit. There are several methods of connecting these traps. Fig. 2 shows the most common method. In this drawing Lx Ci constitutes the trap circuit, and L2 C2 the antenna coupler and tuning condenser of the receiving set. It is seen that the trap is connected between the antenna post of the receiver and the antenna lead-in. Fig. 3 represents a slightly different method of connecting the trap in the circuit. In this latter method, the wave trap is inductively coupled to the antenna. This inductive coupling is obtained by winding a few turns of wire about one end of the coil One end of this new winding connects to the antenna and the other to the antenna post of the receiver. This circuit is prac _ tically equivalent to that of Fig. 2 with the difference that somewhat sharper tuning is obtained. WHY THE CIRCUIT PREVENTS INTERFERENCE IN ORDER to give an idea of the effectiveness of these traps, a series of experiments were carried out in the Radio Broadcast Laboratory to illustrate how interfering signals are eliminated by the use of such a filter. The circuit illustrated in Fig. 4 was excited by means of an oscillator. The output of the oscillator was fed into the coil L3 which was inductively coupled to coil L4. This coupling was very loose so that variation in the test circuit caused no change in the oscillator output. Lx Ci is the trap circuit and Ls C2 represents the input circuit of the receiver. As shown in the diagram, a vacuum tube voltmeter was placed across the L2 C2 circuit so as to measure the voltage induced across this circuit. This would be the voltage that would ordinarily be applied to the grid of the first tube of a receiving set and the extent to which this voltage is reduced by the wave trap is a measure of the trap's efficiency. With the trap circuit Lj C, detuned from the incoming frequency produced by the oscillator, the condenser C= was adjusted until maximum voltage was read on the vacuum tube voltmeter. This indicated that this circuit was adjusted to resonance. The frequency of the oscillator was then changed by 10,000 cycles but no change was made in L2 C=. This circuit was, therefore, tuned to a wave 10,000 cycles (10 kc.) different in frequency from that being supplied by the oscillator. However, a certain amount of voltage was still to be measured on the vacuum tube voltmeter but since the circuit was not tuned to the oscillator frequency, the voltage which was measurable represented an interfering signal. This voltage read on the vacuum tube voltmeter under these conditions is represented as Er in Fig. 5, The trap was then adjusted and as condenser Ci was varied, the voltage across L„ C2 was recorded and a curve Fig. 5, plotted, showing the variation of voltage as the trap condenser Ci was changed. This curve shows a large decrease in voltage as the trap circuit is brought into resonance with the incoming frequency. With the trap in resonance the voltage decreased to about 1 5 per cent, of its former value. This whole test was analogous to the case of a Radio Broadcast Photograph FIG. 7 Space-wound solenoid coils can be used to construct a very efficient wave trap receiver tuned to a particular station and at the same time receiving a certain amount of energy from another station differing in frequency by 10 kilocycles (10,000 cycles). Under such conditions, the use of a trap would have caused a decrease of about 85 per cent, in the strength of the interfering signal. Let us take a numeri FIG. A simplified diagram of a receiver employing three stages of radio frequency amplification, really successive wave traps cal example of such a case. Suppose it is desired to receive a signal having a frequency of 500 kilocycles and to eliminate the interference from another station operating on 510 kilocycles. The antenna circuit of the receiver would be