Economical multifunctional frequency meter. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Economical multifunctional frequency meter. 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 main advantage of the proposed device is its simplicity, combined with efficiency and ease of operation. The increase in efficiency is achieved by the use of a liquid crystal display (LCD) and an automatic power off unit after a certain period of inactivity. For convenience, the measurement mode control buttons are located on the back of the indicator and their functions are displayed on it, i.e. by changing the microcontroller program, additional features can be introduced into the device.

The main technical characteristics of the described frequency meter are as follows: frequency measurement range - 0,1 Hz ... 60 MHz (actually the upper limit is higher); input voltage sensitivity threshold - 0,08..0,15 V (peak value); the minimum value of the frequency of a sinusoidal signal reliably fixed by the device is 2 Hz (with an amplitude of 0,15 V); the maximum amplitude of the input signal is 3 V. The device is powered by a Krona battery (it is possible to use an external source with a voltage of 7 ... 16 V), the current consumption is 10 ... 12 mA. It provides for changing the measurement time (0,1; 1 and 10 s), multiplying the readings by 1000 (when using an external frequency divider), holding readings, writing one frequency value to non-volatile memory and the possibility of subsequent reading.

Schematic diagram of the frequency meter is shown in fig. 1. Its basis is the PIC16F84A microcontroller, which counts the external signal pulses, processes the obtained values and displays the measurement results on the LCD. The functions of the microcontroller also include polling buttons (SB1-SB4) and power management of the device.

(click to enlarge)

Turn the frequency counter on and off with the SB1 button. After connecting the battery (or applying voltage from an external source), the device remains in the off state. When you press the button SB1, the supply voltage through the diode VD1 is supplied to the integral voltage regulator DA1, and from its output to the input stage (VT3), the microcontroller DD1 and the indicator HG1. Then, a high logic level appears at pin 1 (RA2) of the microcontroller, which leads to the opening of transistors VT1 and VT2. Next, the controller waits for the release of the SB1 button (controlling the signal at pin 6). After opening its contacts, the supply voltage is applied to the input of the stabilizer DA1 through the open transistor VT1 and the frequency measurement begins.

While holding SB1, the indicator screen (Fig. 2, a) displays the inscriptions "FREQUENCY" and "VERSION: 1.00" (controller firmware version). When you press SB1 again, the supply voltage is supplied to pin 6 (RB0) of the microcontroller, which then waits for its contacts to open, and when this happens, sets a low logic level at pin 1 (RA2). As a result, transistors VT1, VT2 close and the device is de-energized. If in the measurement mode the indicator shows zero readings for about 3 minutes, the microcontroller sets a low logic level at pin 1 (RA2), thereby disconnecting itself from the power source.

Economical multifunctional frequency meter

The measurement time selected by the SB2 button (0,1; 1 or 10 s) is displayed on the right side of the bottom line of the indicator (Fig. 2b). The price of the least significant bit is 10,1 or 0,1 Hz, respectively. With a measurement time of 0,1; 1 and 10 s as a maximum, the LCD can display seven, eight or nine digits, i.e. the maximum displayed value is 99,999.99, 99,999.999 or 99,999.999.9 MHz, respectively.

By pressing the SB3 button, the frequency readings are multiplied by 1000. This is done for the convenience of reading the readings when using an external divider by 1000 [1, 2]. The multiplication factor ("x1" or "x1000") is displayed in the middle of the bottom line.

To hold (fix) the readings, press the button SB4 ("Memory"). At the same time, the value of the frequency that was at the moment the button was pressed remains on the LCD. It can be stored in the non-volatile memory of the microcontroller using the SB2 button, the function of which in this case is "Remember" (Fig. 2, c). The previous value is then lost. If you need to read the frequency from the memory, click on SB3 (its new function is "Read"). To exit the memory mode, use the SB4 button (new function - "Exit"). In the memory mode, the frequency meter automatically turns off approximately 3 minutes after pressing any button, regardless of the indicator readings. After the power is turned off, the last measurement parameters (measurement time and multiplier) are stored in the non-volatile memory.

As VT1, VT2 in the device, you can use any transistors of the series indicated in the diagram. We will replace the KR1157EN502A stabilizer with 78L05, LM2931Z (when using the latter, the lower limit of the supply voltage will decrease to 5,5 V, and the current consumption will decrease by 2 mA).

The LCD must have a built-in controller with a command system compatible with the commands of the HD44780 controller, and Russian characters in the character generator table (almost all character-synthesizing LCDs satisfy this condition). Suitable, for example, indicators DV-16210, DV-16230, DV-16236, DV-16244, DV-16252 (DataVision), ITM-1602 (Intech), PC-1602 (PowerTip).

Microcontroller "firmware" codes in HEX format are shown in the table, program source code.

(click to enlarge)

The frequency meter is calibrated using a reference oscillator using a tuned capacitor C10. By selecting the resistor R5, the maximum voltage sensitivity of the device is achieved. The contrast of the displayed indicator readings is adjusted by selecting the resistor R11.

If the automatic power off and one-button power management functions are not needed, the device can be simplified by eliminating transistors VT1, VT2, diode VD1, resistors R1, R3, R4, R7, R8, R10 and button SB1. Pin 6 of the microcontroller in this case is connected to a common wire, and the supply voltage is applied directly to the input DA1.

Literature

Slinchenkov A. Improvement of the preliminary frequency divider. - Radio, 1999, No. 10, p. 29.
Beetle V. Frequency divider for the range of 1 ... 5 GHz. - Radio, 2001, No. 12, p. 28, 29.

Author: A. Sharypov, Vladimir

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