4-2. Use of Reactance Tube for Frequency Sweep The reactance tube is one of the main methods used for providing frequency modulation in sweep generators. Let us review briefly how a reactance tube works. The reactance tube produces artificially, by electronic means, the effect of capacitance or inductance. More important, it provides the means of varying the value of that capacitance or inductance by variation of a d-c control voltage applied. If the effective capacitance or inductance produced by the reactance tube is made to form an appreciable part of the capacitance or inductance of the tuned circuit of an oscillator, the frequency of that oscillator's signal can be made to vary by the variation of a d-c control voltage applied to the reactance tube. If the d-c control voltage is made to alternate (that is, become low-frequency alternating current) the oscillator frequency also alternates and thus becomes a "sweep" frequency. This is the method used in some sweep generators to provide the desired frequency sweep. A typical reactance tube circuit is shown in Fig. 4-1. The operation is as follows: 1. R-f is coupled to the plate and cathode of the reactance tube from the oscillator tank through C2 and C3. 2. This causes the reactance tube plate-cathode circuit to act as a load across the oscillator tank. The current drawn through this load depends upon the value of the grid bias, which in this case is the control voltage across Rl. 3. Also across the oscillator tank circuit is the series combination Cl-Rl. This series circuit is designed so that the reactance of CJ is much greater than the resistance of Rl. The current through this circuit is therefore leading the oscillator voltage applied by nearly 90 degrees. This current through Rl produces a voltage drop which is also nearly 90 degrees ahead of the oscillator voltage. This Rl voltage is applied to the grid of the tube and there acts to control the current in the plate circuit. The plate current variations are in phase with the grid voltage variations. Since the latter are almost 90 degrees leading with respect to the r-f voltage applied from the oscillator, the plate current is also almost 90 degrees leading with respect to the oscillator r-f voltage. The result of all this is that, looking from the oscillator toward the reactance tube, the oscillator sees a load which draws current that leads the applied voltage by nearly 90 degrees. Since this is exactly what would happen if a capacitor were connected in place of the reactance tube, the oscillator does not distinguish it from a capacitor, and its frequency is controlled accordingly. The larger the capacitance, the greater r-f current it will draw from the oscillator; in the same way, the more positive (or less negative) is the control voltage on the reactance tube, the more current the tube draws. Thus the more positive the control voltage, the larger will be the capacitance exhibited by the reactance tube; the more negative the control voltage, the less the capacitance. Now if the control voltage is made to vary rapidly back and forth, we produce the same effect as though we were rapidly rotating a variable capacitor across the oscillator tank circuit. This effect causes the oscillator to change frequency rapidly in accordance with the control voltage changes. In other words, the oscillator frequency "sweeps" back and forth. In sweep generators of the reactance-tube type, a small voltage derived from the power line is applied as control voltage. This voltage is ordinarily a 60 cps a-c sine wave, and thus causes the oscillator frequency to vary sinusoidally. In sweep generators, the sweep width is controlled by variation of the a-c control voltage applied to the reactance tube. The oscillator whose frequency is being swept is usually operated at a rather high frequency (the values in Fig. 4-1 are for an oscillator at 40 mc) so that a given percentage of frequency deviation can produce as high as possible a sweep in megacycles. For constant sweep width with varying output centre frequency, and for reasons of stability, the oscillator which is thus frequency modulated is usually kept at a fixed centre frequency, while variable output centre frequencies are obtained by heterodyning with another, unmodulated, variable frequency oscillator. This is explained more fully later in this chapter.