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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Frequency meter - digital scale with LCD display. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology The establishment of amateur radio structures is impossible without measuring equipment. A simple digital frequency counter can be assembled using a PIC controller. The version of the device proposed in this article allows you to use it as a digital scale in receivers and transceivers. The device was developed on the basis of the author's previous design, published in the January 2002 issue of the Radio magazine. The use of an LCD indicator in the new version of the device made it possible to reduce the current consumption, reduce the level of radiated interference, reduce the dimensions, and also simplify the circuit and design of the device. In addition to improving the purely electrical parameters in this design, the technical ones have also been improved. Having freed the PIC controller from the routine work of scanning the indicator, it was possible to expand the range of permissible frequencies of the reference crystal oscillator and significantly simplify the calibration process. The main parameters of the frequency meter, in comparison with the design on the LED indicator, are given in Table. 1.
Frequencies over 40 MHz can be measured using an external microwave divider with any division ratio (in the range of 2...255). When using the device as a digital scale, up to 15 intermediate frequencies in the range from 0 to 800 MHz can be recorded in its non-volatile memory. Their values are entered with an accuracy of up to 100 Hz and can be changed by the user at any time using three buttons located on the front panel of the device. In this case, the indicator readings will be determined by the formula Fin Kd ±Fp where Fin - input frequency; Kd - division factor of the external divider; Fp - intermediate frequency. When using the device as a digital scale, the measurement time can be 0,1 or 1 s. The 10 s limit is for accurate measurements of relatively low frequencies. For a digital scale, such accuracy is not needed, therefore, the readings at the limit of 10 s are determined by the formula [Fin·Kd]. The frequency meter provides the possibility of software calibration, which allows the use of any quartz resonators in the range of 1 ... 20 MHz. The values of all intermediate frequencies, the division factor of the used external divider, as well as the calibration constants can be changed by the user without the use of any additional devices. They are stored in the non-volatile memory of the controller. The principle of operation of the frequency meter is classical: measuring the number of pulses of the input signal for a certain time interval. The schematic diagram of the device is shown in fig. 1. The input shaper has a bandwidth of 10 Hz ... 100 MHz. However, the speed of the divider built into the DD2 controller limits the upper limit of the measured frequencies to 40...50 MHz. The lower limit for a sinusoidal signal is determined by the capacitance of capacitors C1 and C5. Diodes VD1, VD2 protect the field-effect transistor from failure when high voltage enters the input. High parameters of the input driver with a relatively simple circuit and power supply from only one 5 V source were obtained thanks to the use of the Schmitt trigger DD1.1. From its output, the generated pulses are sent to the PIC16CE625 controller.
The device is controlled by three buttons displayed on the front panel and five switches. Buttons SB1 - SB3 are used to switch the measurement time. When you press SB1, the limit is 0,1 s, and when you press SB2 or SB3 - 1 or 10 s, respectively. The new value will appear on the indicator after 0,1; 1 or 10 s after releasing SB1, SB2 or SB3. If you press and hold one of these buttons, the current frequency value will be fixed on the indicator. The frequency meter uses an LCD indicator of the KO-4V type from the PANA-PHONE telephone. It is made on the basis of the NT1613 "Holtek" controller and is produced by the Zelenograd company "Telesystems". Along with its advantages - 10 digits, efficiency, ease of management, it also has significant drawbacks - it can display only 16 characters and does not have decimal points. Therefore, to facilitate the perception of the displayed information, hundreds of hertz on the indicator are separated from units of kilohertz by an empty familiarity. Three LEDs HL1 - HL3 indicate the included measurement limit, and the HL4 LED is used as a 1,5 V zener diode. The closed state of the switch SA5 corresponds to the operation of the device with an external microwave divider, and the open state - without. When using a divider, the price of the least significant digit changes in accordance with Table. 2.
Switches SA1 - SA4 are used to select one of the 15 pre-programmed IF values. The corresponding IF number is dialed in code 1-2-4-8. If switches SA1 - SA4 are open, the IF is 0 (frequency counter mode). The SA5 leads are connected to the free contacts of the connector, which includes a microwave divider. A jumper is installed on the mating part of the connector between these contacts. Thus, the connection of the divider is automatically determined. If necessary, DIP switches can be installed on the board to select the inverter and divider. Transistor VT1 - field-effect with an insulated gate, an n-type channel and a gate-source voltage of 0.. .2 V at a drain current of 5 mA - KP305A - V; KP31 ZA.B; VT2, VT3 - KT316, KT368, etc. with a cutoff frequency of at least 600 MHz. DD1 - 74AC14 can be replaced by KR1554TL2 or TLZ. In the latter case, you will need to correct the drawing of the printed circuit board. Unused inputs of all DD1 elements should be connected to the +5 V bus. The use of TTL analogs in this circuit is undesirable, since this sharply reduces the upper limit of operating frequencies (up to 10 ... 15 MHz). A drawing of the printed circuit board of the frequency meter is shown in fig. 2. Indicator HG1, buttons SB1 - SB3 and limit indication LEDs HL1 - HL3 are placed on the side of the conductors. Switches SA1 - SA5 can be installed both on the side of the parts and on the side of the conductors. Despite the low level of interference emitted by the device, it is still desirable to shield it, especially if it will be used as a digital scale in conjunction with the receiver. As a power supply, you can use any unstabilized source with a voltage of 7,5 ... 14 V and a current of up to 50 mA. Switching or transformerless power supply is not recommended.
Establishing a frequency meter consists in setting the current of transistors VT1, VT2 to about 5 mA. It is set by selecting the resistor R2. The voltage at the VT2 collector should be approximately 3,6 V. Then, with a tuned resistor R7, the maximum sensitivity of the device at high frequencies is achieved. The voltage at the VT3 collector should be about 2,5 V. After manufacturing and checking the performance of the frequency meter, it is necessary to set all the necessary values of its parameters. They are set in the service mode with the buttons SB1 - SB3. To enter this mode, press these three buttons at the same time. In this case, the indicator will display the value of the measurement time, which will be selected by default when the device is turned on. By pressing the button SB1 or SB2, you can select one of three values - 0,1; 1 or 10 s. After that, press SB3. In this case, the selected value is stored in non-volatile memory, and the indicator shows the value of the division factor of the microwave divider, which will be used with the device. You can change its value by pressing SB1 or SB2, and then confirm the selection by pressing SB3. If one or more of the switches SA1 - SA4 are closed, the number of the enabled inverter and its sign (stylized + or -) appear on the indicator. The choice of the sign is made by SB1 or SB2, pressing SB3 confirms the choice, and the IF value is displayed on the indicator, which can be changed by pressing again SB1 or SB2. The rate of change will increase with the time the button is pressed, i.e. the longer the button is held down, the faster the reading will change. The price of the least significant bit is 100 Hz. Confirmation of the choice is similar to the previous modes - pressing SB3. After that, the symbols "------" appear on the indicator. If you do not press any of the buttons, after about 3 seconds the device will switch to the measurement mode with the newly selected parameters. To enter the calibration mode, press the SB3 button within these three seconds. The calibration process in this design is extremely simplified. To do this, simply enter the true frequency of the quartz generation by pressing the SB1 or SB2 buttons in the same way as entering the intermediate frequency values described above. Only the price of the least significant digit of the indicator in this mode is 1 Hz. Having set the desired value, you should press the SB3 button. The frequency meter can work with almost any quartz resonator, however, a value of about 4 MHz is optimal. At a lower frequency, the performance of the PIC controller decreases, and increasing the clock frequency increases the current consumption, without giving much advantage. It should be borne in mind that in this circuit, quartz is excited at a parallel resonance frequency, and on domestic resonators, a series resonance frequency is usually indicated, which may differ by several kilohertz. You can determine the true generation frequency of a quartz resonator by connecting an exemplary frequency meter to point XN1. In this case, the capacitor C8 should be in the middle position. The measured value is rounded off to the nearest multiple of 40 Hz, e.g. 4, 000, 000, etc. After calibration, this device and a reference frequency meter should be connected to a signal generator with a frequency of 20 ... 40 MHz and an amplitude of 0,2 ... 0,5 V. The exact correspondence of the frequency readings is finally achieved by adjusting the capacitor C8. If the range of its change is not enough, then the frequency of the quartz was entered incorrectly and it should be changed as described above. Author: Nikolai Khlyupin (RA4NAL), Kirov
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