Frequency and Period Measurement using Universal Counter

(a) Measurement of Frequency:

The part of the circuit of the universal counter that is utilized for frequency measurement is presented in figure.

The input signal whose frequency is to be measured (counted) is given to the first channel of the universal counter. It reaches the preamplifier through the attenuator. The amplified signal reaches the Schmitt trigger where it is converted in to a square wave. A differentiator circuit, along with a clipper (not shown in the diagram) produces a train of pulses. This train of pulses is separated by the period of the input signal. These pulses are applied to the A input of the AND gate.

The time base selector switch is selected to the appropriate range. The time base pulses are given to the Schmitt trigger from where they reach the gate control flip-flop. The first time base pulse triggers the gate control flip-flop and its output goes high. Therefore the B input of the AND gate will be held at high state, thus enabling the AND gate. Therefore the input signal pulses present at the A input of the AND gate are allowed to reach the decimal counter. Therefore the count starts. The AND gate will be enabled as long as its B input is high. For the second time base pulse, the gate control flip-flop toggles and its output goes low. Therefore the B input of the AND gate goes low. Hence the AND gate is disabled, preventing the input signal pulses from reaching the decimal counter.

Thus the decimal counter will count the input pulses only for the period between the first time base pulse and the second time base pulse. If we select the time base pulse with 1 second duration, the decimal counter counts the number of input pulses presented during that 1 second. This count will be displayed on the display which is the frequency of the input signal.

(b) Time Period Measurement:

From the time period measurement block diagram, it can be seen that the gate control flip-flop is controlled by the input signal presenting the start and stop signals through the two input channels. That is the gating signal is obtained from the input signal whose time period is not known. This input signal controls the enabling and disabling of the main gate.

The other input to the main gate is derived from the decade dividers connected through the Schmitt trigger to the clock oscillator. Now the gate will pass the trigger pulses from the divider to the decimal counter, which counts and displays. The decimal counter counts the number of pulses that occur during one period of the unknown input signal.

(c) Measurement of Frequency Ratio:

The frequency ratio measurement section is presented in the figure. The aim is to measure the ratio of two frequencies. Normally one frequency will be higher than the other. The low frequency will be employed as the gating signal and high frequency signal is used for counting.

From the block diagram it is clear that the low frequency signal, f_L is applied to channel 1. The high frequency signal f_H is given to channel 2. The channel 1 preamplifier's output reaches as period trigger to the gate control flip-flop. The channel 2 preamplifier's output reaches the A input of the Main gate. The gate control flip-flop is triggered by the input from channel 1. The output of this flip-flop is applied at the B input of the main gate.

The number of cycles of the high frequency signal f_H, that occur during the period of the low frequency signal f_L are counted and displayed by the instrument. In case of multiple ratio measurement the period of the low frequency signal can be enhanced by the use of a chain of decimal dividers.