The r-f phase-shift system illustrated above is convenient in a case where it is desired to make small changes In the operating frequency of the system without the necessity of being precise in the ad/ustment of two coupled circuits as used for r-f phase shift in the circuit of figure 21.
Radio-Frequency A single-sideband genera-Phasing tor of the phasing type requires that the two balanced modulators be fed with r-f signals having a 90-degree phase difference. This r-f phase difference may be obtained through the use of two loosely coupled resonant circuits, such as illustrated in figures 21A and 2IB. The r-f signal is coupled directly or inductively to one of the tuned circuits, and the coupling between the two circuits is varied until, at resonance of both circuits, the r-f voltages developed across each circuit have the same amplitude and a 90-degree phase difference.
The 90-degree r-f phase difference also may be obtained through the use of a low-Q phase shifting network, such as illustrated in figure 23; or it may be obtained through the use of a lumped-constant quarter-wave line. The low-Q phase-shifting system has proved quite practicable for use in single-sideband systems, particularly on the lower frequencies. In such an arrangement the two resistances R have the same value, usually in the range between 100 and a few thousand ohms. Capacitor C, in shunt with the input capacitances of the tubes and circuit capacitances, has a reactance at the operating frequency equal to the value of the resistor R. Also, inductor L has a net inductive reactance equal in value at the operating frequency to resistance R.
The inductance chosen for use at L must take into account the cancelling effect of the input capacitance of the tubes and the circuit capacitance; hence the inductance should be
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