Input circuit: Because the input signal can be sine wave or triangular wave. However, the following gate circuit or counting circuit requires the measured signal to be a rectangular wave, so it is necessary to design a shaping circuit. When measuring, sine wave or triangular wave is first converted into rectangular wave by shaping circuit. Because the measured signal strength is not clear. Therefore, before shaping, it should be amplified and then attenuated. When the voltage amplitude of the input signal is large, the voltage amplitude is reduced by the input attenuation circuit. When the voltage amplitude of the input signal is small, if the attenuation of the previous input is zero and the latter shaping circuit cannot be driven, the measured signal can be amplified by adjusting the gain of the input amplification.
Time base and gate circuit: the gate circuit is the standard time signal to control the counting of the counter. The number of pulses that the measured signal enters the counter through the gate is determined by the gate signal, and the accuracy of the gate signal largely determines the frequency measurement accuracy of the frequency meter. When high frequency measurement accuracy is needed, it should be obtained by frequency division using a crystal oscillator. The time base signal can form a relatively stable multivibrator through a 555 timer, and after shaping and frequency division, a standard time base signal can be generated as the reference time for opening the gate. The measurement signal passes through the gate as the clock signal of the counter.
Counting display circuit: when the gate circuit is turned on, it begins to count how many rising edges the measured signal has. The digital tube does not display numbers when counting. After the counting is completed, the digital tube should display the counted numbers at this time.
Control circuit: In the control circuit, a count clearing signal and a latch control signal are generated. The schematic diagram of the working waveform of the control circuit is as follows.
After shaping, the measured signal becomes a pulse signal (rectangular wave or square wave) and is sent to the gate circuit to wait for the arrival of the time base signal. The time base signal has a 555 timer to form a relatively stable multivibrator. After shaping and frequency division, a standard time base signal is generated as the reference time for gate opening. The measured signal passes through the gate circuit as the clock signal of the counter, and the counter begins to record the number of clocks, thus achieving the purpose of measuring the frequency.