Radio Broadcast (May 1928-Apr 1929)

SOME EXPERIMENTS WITH BAND PASS FILTERS 105 However, they actually tune to two separate frequencies when operated with coupling between the two circuits as shown. In other words, if the coils and capacities are of the proper size to tune both circuits, Li, Q, and L> Q>, to a frequency of Fi kc. independently, the combination will not have any resonant peak at Fi kc, but will have two resonant peaks at other frequencies. One of these peaks will be Fm kc. above the frequency of resonance, the other will be Fm below the frequency of resonance and this interval of Fm is defined by the equation: ux-222 ■ 0) This equation says in simple terms that, if the mutual inductance between the coils is A per cent, of the inductance of either coil, there will be a peak of resonance A/2 per cent, either side of the frequency of resonance of the circuits considered independently. Thus, in the circuit of Fig. 4, if we select two coils of 230 microhenries each, tune them independently to 1000 kc, and then couple them with a mutual inductance of 2.3 microhenries, there will not be a resonant peak FIG. 4 Mr. Claugb chose to study this variation of the V reeland scheme — he uses the mutual inductance between two coils as the coupling impedance. at 1000 kc, but there will be two other peaks, one at 995 kc, and the other at 1005 kc. Anticipating that this arrangement would approximate ideal selectivity, the circuit was set up for measurement and curves were run. ANALYSIS OF CURVES THE circuit used for these curves is shown in Fig. 5. Two commercial coils of very low-loss construction, the specifications of which are given on the circuit, were chosen for the tuner. Two mechanical placements are given in Fig. 5A which resulted in a measured mutual inductance of 'i per cent, of the coil inductance. In order to compensate the tube and other capacities of the circuit, the tuning of the condensers was accom lit Is p 5V 72 Turns No.25 P.E.wire-32 turns per inch L, = L2 (diagram a) + 45 FIG. 5 How the circuit actually was set-up to he measured. A screen-grid tube fed the vacuum-tube voltmeter by means of the band-pass coupling mechanism. Diagram A — Two possible methods of obtaining one per cent, coupling between the two Silver Marshall coils used by Mr. Clough. FIG. 3 Dr. Vreeland's circuit started all the discussion regarding hand-pass tuning for broadcast-frequency amplifiers. It consists of two identical coils and condensers tuned to the same frequency and coupled together by an inductance. plished by disconnecting the primary condenser, Ci, and tuning the secondary to resonance with the desired wave, then disconnecting the secondary condenser, C2, and tuning the primary. It was necessary to use a strong signal from the oscillator while tuning in this manner as the transfer from one coil to the other was very low. After tuning each circuit independently, both condensers were connected and the curve for a particular frequency measured. These curves were taken at 600, 1000, and 1500 kc, and are shown in the full lines of Fig. 6. It will be noted that they check very well with the theory of the circuit, for in the 600 kc. curve Fig. 6a, we have the two resonant humps lying very close to 3 kc. off resonance on either side while in the 1000 kc. curve, Fig. 6b, they are 5 kc. off the normal resonance of the individual circuits. The 1 500 kc. curve did not turn out as well, although the separate resonances can still be distinguished. It will be noted that the curve at 1000 kc. compares very favorably with the ideal curve as the amplification varies very little in the 5 kc. pass band either side of the carrier. Attention is called to the variation with frequency in the width of the band passed in the three curves, which was predicted in equation (1) to which we will refer later. The low amplification obtained with the screengrid tube in the above curves is of no moment for our discussion. It was due to the low value of coupling used between the tube and primary circuit (20 turns at the base of the coil), which was employed in order to prevent the tube circuit from affecting the coil circuit until the curve shapes were assured. In order to compare the shapes of the band curves of Fig. 6 with the performance that would be obtained when using the same tube, coupling, and coils in Fig. 7, the secondary circuit was removed and the measurement repeated with the single circuit shown in Fig. 7. These curves are plotted in dotted lines in Fig. 6 so they may be compared with the band-pass filter performance. It will be noted that at each measured frequency the selectivity of the band circuit is greater than the ordinary resonant circuit and that the amplification is about on a par, one with the other. Other curves were checked using the whole primary coil for coupling to the tube. It was found that the curve shapes remained substantially the same as shown in the full lines of Fig. 6, but the amplification went up to an average of 30 per stage. NEW FILTER UNIT ZITHER studies of this circuit indicate that ^-^ the coil resistance must be kept very low in order to maintain the desired shape of curve and amplification for the stage. A similar type of band-pass filter having two sections has appeared recently upon the market and the writer had an opportunity of running a curve on the selector feature. The size of the coils was such that a high resistance could be predicted and the resulting curve is shown in Fig. 8. It will be noted that the amplitude varies very badly with the frequency and the band effect, while better than the average tuner, is far from the desired shape. To check by actual observation the effect of the band passed on the reception, a receiver was made in breadboard style with two of the band circuits and screen-grid tubes. Feeling that some readers may desire to hear this circuit for them 1.7 1.6 1.5 1.4 1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 ! -1 f r— 1 1 A \ 1 I — H V L 1 1 1 i 1 — 1 — \ / I i 1 \}50C KC Hi woo KC Pt U 1 -50 -40 -30 -20 -10 0 +10 +20 +30 +40 +50 KC OFF RESONANCE FIG . 8 A commercial receiver uses a system somewhat similar to the one described in this article. The result obtained in measuring the ratio between output and input voltages of such a band-pass selector is shown in this figure.