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negative to cut off the current flow. L3 tends to keep the current moving and, as its field collapses,
discharges C3. When C3 discharges sufficiently, the quadrature grid becomes positive, grid current flows,
and the cycle repeats itself. This tank circuit (L3 and C3) is tuned to the center frequency of the received
fm signal so that it will oscillate at that frequency.
The waveforms for the circuit are shown in figure 3-18. View (A) is the fm input signal. The
limiter-grid gate action creates a wave shape like view (B) because the tube is either cut off or saturated
very quickly by the input wave. Note that this is a square wave and is the current waveform passing the
limiter grid.
Figure 3-18A.—Gated-beam detector waveforms.
Figure 3-18B.—Gated-beam detector waveforms.
At the quadrature grid the voltage across C3 lags the current which produces it [view (C)]. The result
is a series of pulses, shown in view (D), appearing on the quadrature grid at the center frequency, but
lagging the limiter-grid voltage by 90 degrees. Because the quadrature grid has the same conduction and
cutoff levels as the limiter grid, the resultant current waveform will be transformed into a square wave.
Figure 3-18C.—Gated-beam detector waveforms.
Figure 3-18D.—Gated-beam detector waveforms.
Both the limiter and quadrature grids must be positive at the same time to have plate current. You
can see how much conduction time occurs for each cycle of the input by overlaying the current
waveforms in views (B) and (D), as shown in view (E). The times when both grids are positive are
shown by the shaded area of view (E). These plate current pulses are shown for operation at resonance in
view (F).
