Based on the frequency meter hardware circuit AT89C51

The circuit diagram is a frequency counter based hardware circuit AT89C51.Specifically, as shown in FIG. The frequency meter to achieve the following functions:

　　1 test function

　　It shows that digital frequency meter possess all the test functions, typically include measuring frequency, period, the cumulative number of pulses, frequency ratio, time interval, and since than the other functions.

　　2 measurement range

　　It shows the effective measurement range of different functions. As measured frequency measurement range is a digital frequency meter is under normal operating conditions, the measured signal frequency range, generally with a frequency lower limit, said most of the low-end from 10HZ start;-end due to different frequency meter and different. Therefore, the high-end frequency is determined low, medium, high-speed counter basis. When the measurement cycle, the period of the maximum measurement range used, minimum FIG.

　　Enter the properties

　　Digital frequency meter are generally two to three input channels, when testing different items, the signal can be input via a different channel instrument. Input characteristics are show digital frequency meter to a group of characteristic parameters of the signal source connected, usually include the following aspects.

　　(1) Input sensitivity. Usually refers to the RMS instrument can work properly minimum input voltage. Common digital frequency meter sensitivity around 100mV.

　　(2) the maximum input voltage. Refers to the maximum permissible instrument input voltage, the measured signal exceeds this value, the instrument is not guaranteed to work, or even damaged.

　　(3) Input coupling. Instrument settings both AC and DC coupling. When AC coupling, the signal input through a DC blocking capacitor, DC coupling, the measured signal directly into the input circuit. AC signal is applied to measure the coupling with a DC level, DC coupling is suitable for measuring the low-frequency square wave pulse or step signal.

　　(4) input impedance. To reduce the load on the signal source, digital frequency meter generally use high-frequency input impedance. Input impedance input resistance and input capacitance by two parts.

　　The display and work

　　It shows that the content can be displayed, showing the number of digits, the use of a display device and a measurement is completed the display duration measurement results.Some also described the electronic counter is "no memory" display mode or "memory" is displayed.

　　5 Output

　　The instrument can output standard frequency signal directly There are several, but can indicate encoding output measurement data and the output power equal.