Small frequency counter. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Small frequency counter. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Measuring technology

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The frequency meter measures the frequency of the input signal in the range of 10 Hz...50 MHz with a counting time of 0,1 s and 1 s, frequency deviation within ±10 MHz, and also counts pulses with display of the counting interval (up to 99 s). The input impedance is 50 ... 100 ohms at a frequency of 50 MHz and increases to several kilo-ohms at the lowest frequency of the range.

Small-sized frequency meter
Fig. 1

The frequency meter circuit is shown in fig. 1. The main element is the PIC12F629 (DD1) microcontroller, working according to the program, the codes of which are given in the table. Frequency measurement is carried out by counting the number of pulses for a fixed time interval. Two intervals are used - 0,1 si 1 s. In the first case, to obtain the frequency, the number of pulses is multiplied by 10, in the second, the values of the number of pulses and frequency are the same.

Small-sized frequency meter

The microcontroller contains two timer-counters (TMR0 and TMR1), the first of which is used for counting pulses, and the second for counting time intervals. Thanks to the built-in asynchronous eight-bit prescaler, the maximum measured frequency is limited from above only by the speed of its triggers and does not depend on the clock frequency of the microcontroller. However, the contents of the prescaler cannot be read programmatically, and in order to "extract" it, the method described in the article by D. Yablokov and V. Ulrich "Frequency meter on a PIC controller" was used (Radio, 2001, No. 1, p. 21, 22).

The input signal amplifier is assembled on a transistor VT1, from the collector of which the pulse signal is fed to the T0CKI input (pin 5) of the DDI microcontroller. To display information, a digital indicator NT1610 (HG1) with a built-in controller is used. When operating in the slave mode, the input of the NK indicator HG1 is connected to a common wire, and the data is transmitted sequentially in 4-bit packages along the DI and CLK lines. The limited number of I / O lines of the DD1 microcontroller did not allow us to select two of them for the implementation of the standard data transfer mode, so data and clock pulses had to be transmitted from the GP0 output of the DD1 microcontroller through resistive dividers. The pulses are fed to the CLK input of the HG1 indicator through the R7R9 divider, and to the DI input - through the R6R8C8 integrating divider. To transfer a low logic level (logic 0) at the output GP0 of the microcontroller DD1, a voltage pulse with a duration of 5 μs is generated. In this case, the capacitor C8 does not have time to charge, and on the decline of the pulse at the DI input, a logical 1 will be written to the HG0 indicator. To transmit a logical 1, the pulse duration is much longer than the time constant of the R6R8C8 circuit, and the capacitor C8 has time to charge to a high logic level, so a logical 1 will be written The pause between pulses must also be greater than the time constant of the R6R8C8 circuit so that the capacitor C8 has time to discharge.

The frequency meter is powered by a galvanic or rechargeable battery with a voltage of 8 ... 9 V. The power supply voltage of the amplifier and the microcontroller is stabilized by the integral stabilizer DA1. The supply voltage is supplied to the HG1 indicator from the engine of the tuning resistor R5, it must be in the range of 1,4 ... 1,6 V.

After the power is turned on, the microcontroller executes a frequency measurement routine with a counting time of 0,1 s. By briefly pressing the SB1 button, the frequency value is fixed and the microcontroller measures the frequency deviation from the fixed value, followed by displaying this deviation on the HG1 indicator display. A second short press on the SB1 button returns the device to its original state. To switch to the frequency measurement mode and its deviation with a counting time of 1 s, press the SB1 button and hold it for at least 2 s. Another long press on the SB1 button puts the device into pulse counting mode. In this mode, short presses on the button sequentially start, stop and reset the counter and the measurement time indicator.

The frequency and its deviation are displayed on the display of the frequency meter in hertz. With a measurement interval of 0,1 s, the reading looks like this: "1Fxxxxxxxx" for frequency or "1 Fi_xxxxxxx" ("1 F-xxxxxxx") for frequency deviation, where xxxxxxxxx is the frequency or its change, and the sign indicates its increase or decrease . Since the indicator does not provide for the output of the "+" sign, it is displayed as "When the measurement interval is 1 s, the number 2 is present at the first position of the indicator. In the pulse counting mode before the start, the indicator display will be zeros, in the counting mode - SS uuuuuu, where SS is counting time in seconds, uuuuuu - number of pulses.

Small-sized frequency meter
Fig. 2

At the end of the count, the readings are recorded.

Most of the parts are mounted on a printed circuit board made of one-sided foil fiberglass with a thickness of 1 ... 1,5 mm, the drawing of which is shown in Fig. 2. The device uses a tuning resistor SPZ-19, fixed resistors C2-23, MLT, a tuning capacitor KT4-25, the rest - K10-17. The LM2931Z-5.0 chip can be replaced with 78L05, the KT3102A transistor - with transistors of the KT316, KT342, KT368 series with any letter indices. The board, together with the battery, is placed in a plastic case measuring 30x50x70 mm. The indicator and power switch are fixed on the front panel, where holes of the appropriate size are made for them. To power the device, you can use batteries "Krona", "Korund", 6F22, the current consumption is about 9 mA. The microcontroller can be programmed using Pony Prog, 1C Prog programs.

Setting up the device is reduced to adjusting the frequency measurement accuracy. To do this, a continuous signal with a frequency of about 1 MHz, an amplitude of 0,5 V, and a trimmer capacitor C5 are supplied from a reference generator to match the indicator readings with the frequency of the input signal. Then, by selecting the resistor R1, the maximum sensitivity of the frequency meter is set.

The text and codes of the microcontroller program can be downloaded hence.

Author: I. Kotov, Krasnoarmeysk, Donetsk region, Ukraine; Publication: radioradar.net

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