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Tuned Bandpass Filters
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Tuned Bandpass Filters for High-Fidelity- t-r-f Tuners
Here a special form of tunable bandpass filters is presented which in general are used only in expensive receivers built for professional use, because their use greatly increases the cost of manufacture.
However, this type of bandpass filter is also realized in the Philips A5X93A set for the input filter of the AM broadcast band.
Ghirardi, A.A.: Receiver Circuitry and Operation, Rinehart, 1951, pp. 104 – 106
Sec. 3-21 R-F AMPLIFIERS AND T-R-F RECEIVERS
A brief discussion of band-pass tuning-circuit characteristics is required here. The frequency-response characteristic of a single tuned circuit depends largely upon the Q of the coil, but otherwise follows a fairly standard form (see Fig. 3-3).
However, when two resonant circuits tuned to the same frequency are coupled together either magnetically, capacitively, or directly, the response characteristic of the combination is different from that of either of the circuits taken separately and depends very largely upon the degree of coupling that exists.
For example, at very low coefficients of coupling, the response curve approximates closely that of either of the tuned circuits considered alone, as shown at (A) in Fig. 3-21. As the coupling is increased, the curve becomes broader andthe peak value of the responseis reduced. These trends continue as the coupling between the coils is increased further [see Fig. 3-21 (B) ]. When a coupling value known as the critical coupling is reached, the curve develops into a double-peaked curve with a dip in the response at the center or resonance frequency, as shown at Fig. 3-21 (C). Further increase in the coupling causes the two peaks in the response to occur farther and farther apart, and the dip in response at the resonance frequency becomes more pronounced.
Observe that a response curve of the general form shown at Fig. 3-21 (C) is the type desired to provide (1) sensibly equal response to both side-bands of the AM signal that it is desired to receive and (2) sharp rejection of all frequencies above and below them. If the coupling between the two tuned circuits is properly designed, an over-all response curve which will pass a band sufficiently wide to fulfill wide-range AM broadcast transmission requirements can be realized in practice – a total bandwidth of 20 kc commonly being considered to be most satisfactory since this will pass the 10-kc sideband components corresponding to a modulating frequency of 10,000 cps, which is an adequate working upper limit for good fidelity.
The two coupled tuned circuits form the band-pass tuning circuit. Early band-pass arrangements utilized either inductive or capacitive tuning between the two tuned circuits, with the result that the response bandwidth was not constant but varied with the frequency to which the resonant circuits were tuned within the AM broadcast range. The receivers employing them therefore had differing degrees of selectivity and change in tone quality caused by sideband cutting.
The improved band-pass circuit shown in Fig. 3-22and used in the high-fidelity tuner of Fig. 3-20 consists of two tuned circuits L1-C1 and L2-C2 coupled together magnetically by coil L and capacitively by capacitor C. Proper design of the size of coupling inductance L and capacitor C provides the desired response characteristic and this is maintained quite constant throughout the AM broadcast-frequency range.
In the tuner circuit of Fig. 3-20, a 6SJ7 tube is employed as an infiniteimpedance detector, because of the good fidelity characteristic of this type of detector (see See. Fig. 3-15). In order to provide automatic volume control for the r-f amplifier tubes, a 1N34 Sylvania Germanium Crystal Diode is employed.