Simple digital frequency meter circuit design

! ! Notice! ! : The maximum measurement frequency of this design is close to 10MHZ. In order to ensure successful measurement, the operational amplifier needs to use a high-speed operational amplifier. The video test was measured when the operational amplifier model is RS8752XK, the input signal is a square wave with a peak-to-peak value of 2V (due to the waveform generator model, the actual output is already close to a sine wave), and the power supply voltage is 5V. The operational amplifier in the schematic diagram The model cannot measure frequencies above 5MHZ.

Design background : A digital frequency meter is an instrument that uses numbers to display the frequency of a measured signal. The measured signal can be a sine wave, square wave or other periodically changing signal. If equipped with appropriate sensors, a variety of physical quantities can be tested, such as the frequency of mechanical vibration, rotational speed, frequency of sound, and product piece count, etc. Therefore, digital frequency meter is a widely used instrument.

Design purposes : 1) Understand the basic principles of digital frequency meter measurement frequency and measurement period; 2) Be proficient in the design and debugging methods of digital frequency meters and methods of reducing measurement errors.

Design tasks and requirements : It is required to design a simple digital frequency meter to measure the frequency of a given signal and display it in decimal digits. The specific indicators are: 1) Measuring range: 1H _Z —9.999KH _Z , gate time 1s; 10 H _Z — 99.99KH _Z , gate time 0.1s; 100 H _Z —999.9KH _Z , gate time 10ms; 1 KH _Z —9999KH _Z , gate time 1ms;

2) Display mode: four-digit decimal number

3) When the frequency of the measured signal exceeds the measurement range, an alarm will occur.

Test method : 1) Turn on the power and observe the current. Under normal circumstances according to this schematic diagram, the current will not be higher than 0.2A. If it is too high, please pay attention to whether there is a short circuit. At this time, the four digital tubes should have the number 0, and the decimal point is controlled by the DIP switch. 2) Use the oscilloscope to check whether the crystal oscillator is oscillating. 3) Use the oscilloscope to measure the 555 output and change the structure of the precision potentiometer so that the output frequency is 1KHZ (calibration). 4) Add the signal to be measured. It will work normally. You can’t check the logic slowly. .

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