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.