The Constant Waveform Frequency Meter

• IN THE COMMUNICATIONS INDUSTRY there has been a steadily growing demand for direct-reading frequency-meters. Various counting type circuits utilizing thyratrons, multivibrators, etc. have been designed to provide direct frequency indications on a meter scale. General Radio Type 834-A Frequency Meter, 1'or instance, which has been a very popular instrument, is of the thyratron type. The range of this instrument, however, extends only up to 5 kilocycles which, for many applications, is inadequate.

In many laboratory and production measurements, as well as in the monitoring of high-frequency radio transmitters, much higher frequencies must be measured, and, for a general-purpose instrument, a range of about 50 kilocycles is desirable. To provide this greater range the General Radio Company has developed a new circuit* which is considerably simpler than those commonly used.

One of the simplest circuits that can be used for measuring frequency is a resistance-reactance combination, which has a frequency -dependent transmission characteristic. Figure 2 shows the elementary form of such a circuit with a rectifying diode included. When the resistance, R (including the diode resistance), is made small compared to the reactance of the capacitor, C, the current, and hence the voltage drop across R, will be directly proportional to the capacitance and to

Figure 1. Panel view of the Type 1176-A Electronic Frequency Meter.

the frequency. Such a device can be made to provide a substantially linear variation in transmission as a function of frequency, which gives a desirable linear meter scale. It has also the important advantage that the calibration will depend mainly upon a single capacitance-resistance combination, and consequently will have a high degree of stability. If, therefore, provision is made to impress upon such a circuit a waveform which is constant in amplitude and wave shape, regardless of frequency, a simple vacuum-tube voltmeter connected across the resistor will give an accurate indication of the transmission and consequently of frequency.

The simplest waveform to generate is a square wave. The problem becomes, therefore, mainly one of designing suitable limiting and wave shape circuits so as to reduce to a minimum any error resulting from changes in amplitude or waveform of the signal applied to the RC circuit.

The new Type 1176-A Frequency Meter operates on this principle. As shown in the elementary schematic diagram of Figure 3, the circuit consists of a series of limiting amplifiers and diodes, with automatic biasing circuits which provide satisfactory operation over a wide range of input voltages ranging from ^ volt to 150 volts, and for practically any type of waveform which will ordinarily be encountered. A push button is provided on the panel to check that the input voltage is sufficient to insure accurate readings.

Figure 2. Simple frequency-dependent circuit, consisting of a capacitor and a resistor in series.

The RC circuit is switched to provide six ranges giving full-scale meter deflection of 200, 600, 2000, 6000, 20,000, and 60,000 cycles. The indicating meter is actuated by a full wave vacuum-tube meter circuit, which provides additional compensation for any dissymmetry in the waveform after limiting. The circuit is provided with voltage regulation, and the heater current for the vacuum-tube voltmeter is also regulated. Thus a high degree of stability and accuracy is assured, and the permanence of the calibration is such that no trimmer adjustments are required on the front panel.

The meter scale is linear, and provision is made for the addition of an external extension meter or recorder through a multipoint connector at the rear. Two sets of input terminals are provided on the panel and another set on the multipoint connector at the rear. Plugging into the W. E. panel jacks automatically disconnects the rear terminals. The full-scale current is 0.2 milliamperes.

In a time of rising prices, the sim-

Figure 3. Elementary schematic diagram of the Type 1176-A Frequency Meter.

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