Figure 3 is a combined block and ele mentary schematic diagram. The input signal is clipped or limited, amplified, clipped a second time, and amplified again. The resulting wave-shape approximates a square wave or a series of pulses, depending upon whether the input signal is a sine wave or pulses, respectively.
The pulses trigger a Schmitt circuit, which produces positive pulses of 0.1-/xsec duration, coinciding in time with the positive-slope zero crossings of the input signal. These pulses are amplified to trigger the monostable multivibrator, or timing stage. Adjustment of. the input waveform control makes possible a constant Schmitt-circuit sensitivity regardless of the duty ratio and polarity of the pulses.
The monostable multivibrator produces a pulse of constant amplitude and duration for each input pulse. The pulse duration is determined by resistor-capacitor combinations, with accuracy and stability assured by the use of precision, temperature-compensated capacitors and General Radio wire-wound resistors. Range switching changes the timing resistors and capacitors to produce decade changes in pulse duration.
The standardized pulse is then fed to the output stage (to simplify the diagram, two meters are shown in place of one). The left-hand triode, which is normally at cut-off, is turned on by the standardized pulse and remains on for precisely the duration of this pulse. The current flowing at this time is determined by the regulated voltage, V4, and the cathode resistor — a stable, wire-wound resistor in series with the direct cal potentiometer. The average current through the plate-load resistors is, therefore, directly proportional to the number of input pulses per second, and, hence, to the input frequency. A meter across a portion of the load resistor indicates this frequency.
The average dc voltage developed between the plate and ground, i.e., at the fm terminals, is precisely 15 volts at full-scale deflection (1.5). A high-impedance recorder or dc voltmeter can be connected to these terminals as an additional readout.
The right-hand triode is a constant-current source with a plate load consisting of 15 equal-value, 0.05%, wire-wound resistors. Its plate current is adjusted so that the total voltage drop across these resistors is a constant 15 volts. When used for interpolation the meter is connected between the plate of the left-hand triode, which is between 0 and —15 volts, dc, depending upon the input frequency and the appropriate "bucking" voltage from the precision voltage divider, and full-scale sensitivity is increased 10:1 by removal of its shunt. End-of-scale readings are eliminated by the provision of a 50% overlap on each of the 15 interpolation ranges; hence, as indicated in Figure 1, any frequency indicated between 1.0 and 1.5 may be read also on the meter between 0 and 0.5 if the interpolation offset frequency switch is set one digit higher.
With the shorting link removed, a maximum of 7 ma is available at the direct record terminals, so that most standard 1-ma and 5-ma recorders can be used. The record current potentiometer is a convenient sensitivity control for these recorders, and has no visible interaction on the direct meter indication.
An additional recorder output at the record interpolation" terminals makes available the interpolation signal from the meter circuit. Full-scale voltage is 0.64 volt behind 4800 ohms. Use of a high-impedance recorder eliminates any interaction on the meter indication.
Operation of the frequency meter circuits requires the generation of a constant-amplitude, c onstant-du rat i on pulse for each input cycle. These standardized pulses have a fixed-time relation with respect to the input signal and when cw cw
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