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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Radio amateur frequency counter. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology Readers are offered a description of an amateur frequency meter on the AT89C52-24JC microcontroller and two attachments, with which, in addition to measuring the frequency and pulse duration, you can measure the capacitance and inductance of components. Over the past few years, several publications have appeared in the periodical literature devoted to the description of amateur radio frequency meters built on the basis of single-chip microcomputers. The advantages of such designs are obvious: the number of microcircuits used is reduced and, accordingly, the dimensions and power consumption are reduced, the ease of assembly and adjustment of the device, which is available for repetition even by novice radio amateurs. In addition, it becomes possible to modernize and increase service functions only by changing the control program. The frequency meter is designed for use in amateur radio practice. It allows you to measure:
The frequency meter can also be used as a digital scale for radio receivers. With the help of additional attachments, the frequency meter can measure the capacitance of capacitors and the inductance of chokes and coils. Main Specifications
The frequency meter (its diagram is shown in Fig. 1) consists of a signal comparator, an operating mode switch, a measurement cycle synchronizer, a pulse counter, a microcontroller, a keyboard, a liquid crystal indicator, and a power stabilizer.
The input stage is based on an Analog Devices AD8561AR (DA1) comparator. This comparator has a typical delay of about 7 ns. The input signal is fed to the XP1 connector and fed to the R1VD1VD2 protective circuit and the DA1 comparator. Resistors R4, R5 form the hysteresis of the comparator to eliminate the appearance of chatter with slowly changing signals. At the output of the comparator, the signal is represented by a pair of anti-phase logic levels, consistent with the levels of the logic circuits of the frequency meter. The operation mode switch is made on a DD2 digital multiplexer. The switch switches signals in accordance with the selected mode of operation of the frequency meter. The synchronizer (elements DD1.2, DD1.3, DD4) generates signals for the beginning and end of the measurement cycle. The pulse counter (DD3) counts the number of pulses in the input signal or duty pulses when measuring the pulse width. Microcontroller (DD5) from ATMEL AT89C52-24JC manages all elements of the device: operating mode switch, indicator, keyboard. The clock frequency of 10 MHz for the microcontroller is set by the quartz resonator BQ1. When setting up and checking the frequency meter, the clock frequency of the microcontroller is adjusted by capacitor C6 to a value of exactly 10 MHz using an industrial frequency meter. The signal from the microcontroller's quartz resonator (signal BF) is also used to measure the pulse duration. In this case, the repetition period of the filling pulses is 100 ns. Thus, the measurement error of the pulse duration also does not exceed this value. The microcontroller operates using the internal program memory (pin 35 DD5 is connected to the +5V bus). When the frequency meter is turned on, the microcontroller is reset to its initial state by a voltage drop transmitted by capacitor C5. The keyboard (buttons SB1, SB2) is used to select the operating modes and parameters of the frequency meter. The SB1 ("Mode") button selects the operating mode, the SB2 ("Parameter") button selects the mode parameter. For example, using the SB1 button set the "Frequency measurement" mode, and using the SB2 button select the value of the "Measurement time" parameter - 10 s. Approximately 1 s after selecting the operating mode or parameter, the frequency counter automatically switches to the measurement. An alphanumeric LCD module ITM1602ASR with two lines of 16 characters is used as an indicator. The first line shows the operating mode and parameters of the frequency meter, and the second line shows the measured value. Trimmer resistor R8 can be used to adjust the contrast of the indicator image. The indicator is connected to the XS3 connector and installed directly on the board. The indicator connected via an additional cable can be placed in a different place at the request of the user. An integral stabilizer DA2 is used in the supply voltage stabilization unit. The supply voltage from an external source is supplied to the XP2 connector. Capacitors C15, C16 - input filter; C13, C14 - stabilizer output filter. Capacitors C7 - C12 - blocking, they are installed near the microcircuits. The frequency meter uses domestic microcircuits of the KR1533 series (import analogue - 74ALS). The 74NS4040 chip with a maximum frequency of 50 MHz is used as a pulse counter, which limits the frequency measurement range. Consider the operation of the frequency meter in the mode of measuring the frequency of the input signal. The signal from the comparator (circuit F1) is fed to the operating mode switch (pin 4 DD2). The microcontroller sets the logic levels of the signals A = 0 and B = 1, and then issues a START signal (log. 1), which initiates the measurement process. Trigger DD4.1 switches and allows the signal to pass to the output of the switch (pin 7 DD2) and the input of the pulse counter (pin 10 DD3). The microcontroller generates a time interval, for example, with a duration of 1 s (signal TW). During this time, the input signal from the comparator output to the input pulse counter is allowed. Counter overflow pulses DD3 are counted by timer/counter 1 of the microcontroller. After the microcontroller waits for a given interval, the comparator latches its output (pin 5 DAI - LATCH) and the input signal pulse count stops. The microcontroller sets the logical levels of the signals A = 1, B = 1 and reads the accumulated number from the pulse counter (DD3) using "counting" pulses (signal CP). The microcontroller calculates the total number of pulses in the pulse counter for the selected time interval (and this is the signal frequency) using the formula X 1048576+ Y 4096 + Z, where X is the contents of the upper 8 bits of the timer/counter 1 of the microcontroller; Y is the contents of the lower 8 bits of the timer / counter 1 of the microcontroller; Z - the contents of the pulse counter (DD3). If the input frequency is very high, then the counter/timer 1 of the microcontroller may overflow. In this case, the microcontroller adds the number 268435456 to the result obtained by the previous formula. Consider the operation of a frequency meter using the example of measuring the duration of a pulse of positive polarity. The output signals of the comparator (signal F1 for a positive pulse or signal F2 for a negative pulse) are sent to the operating mode switch (DD2). The microcontroller sets the logical levels of signals A - 0, B - 0. Then a signal for setting the trigger DD4.1 to a single state (WR / CM signal) is issued. After that, the signal START (log. 1) corresponding to the beginning of the measurement is issued. The microcontroller is waiting for trigger switching DD4.2. The trigger DD4.1 allows filling pulses to pass from the element DD1.1 to the output of the switch (pin 7 DD2). With the onset of the input signal pulse, the filling pulses (signal BF) are fed to the input of the pulse counter (pin 10 DD3) through the DD1.1 element and the switch. Counter overflow pulses DD3 are counted by timer/counter 1 of the microcontroller. After the end of the input signal pulse, the trigger DD4.1 switches to the opposite state and the counting of the filling pulses stops. At the END signal, the microcontroller sets the signals A = 1, B = 1 and reads the accumulated value from the pulse counter (DD3) using counting pulses (signal CP). The microcontroller calculates the duration of the measured pulse by the formula (X 1048576 + Y 4096 + Z)x100, where X - contents of the senior 8 bits of the timer/counter of the 1st microcontroller; Y - contents of the lower 8 bits of the timer/counter of the 1st microcontroller; Z - the contents of the pulse counter DD3; 100 - the repetition period of the filling pulses, equal to 100 ns. Thus, when measuring pulse duration, the time gate is the pulse itself. To determine the duration of the negative pulse, the microcontroller will set the logic levels of the signals A = 1, B = 0. The software is written in "C" language for microcontrollers of the MCS-51 family. Structurally, the frequency meter is made on a double-sided printed circuit board (Fig. 2), on which all elements are mounted (Fig. 3), with the exception of the indicator.
On fig. 2 round pads, conditionally shown without holes, are connected to the corresponding pads on the reverse side of the board through metallized vias. In the amateur manufacture of a printed circuit board, metallization is replaced with thin conductors.
Detachable connectors - PLS-2, PBS-14, as well as a PLCC-44 socket for installing DD5. Setting the frequency counter After assembling the frequency meter, it is necessary to make three adjustment operations. 1. The indicator contrast is adjusted after power is supplied to the frequency meter by adjusting the tuning resistor R8. 2. To set the frequency of the crystal oscillator of the microcontroller, access to the frequency adjustment capacitor is required. Therefore, when the frequency meter is powered off, the indicator module is removed from the board and then, while holding the SB1 button pressed, the frequency meter is powered on. With a minimum capacitive coupling of the input of the exemplary frequency meter with point BF (Fig. 3), by adjusting the capacitor C6, the generator frequency is set to exactly 10 MHz. 3. The setting of the comparator in the input stage is performed without applying a signal to the frequency meter connector. After turning on the power of the device, you must first turn the slider of the resistor R6 to the extreme left position, and then slowly rotate the slider to the right until the message "NO SIGNAL" appears on the indicator. Below is a description of the operating modes of the frequency counter. Digital scale mode The "MODE" button sets the "DIGITAL SCALE" mode. The "PARAMETER" button selects the mode parameter - the frequency of the IF path. This frequency can be selected from the following values: +455 kHz; -455 kHz; +465 kHz; -465 kHz; +500 kHz; -500 kHz. The sign in front of the digital value Ff indicates the operation that the frequency counter performs. If the sign is "+", then the frequency Fpch is added to the measured frequency, if the sign is "-", then it is subtracted. The frequency measurement time in this mode is 0,1 s. View of the frequency meter indicator in the operating mode: Measuring the frequency of the input signal The "MODE" button sets the "FREQUENCY" mode, and the "PARAMETER" button selects the mode parameter - the measurement time. The parameter in seconds can take one of the following values: 0,1 s, 1 s; 10 s. Approximately 1 s after releasing the button, the frequency meter will automatically switch to the measurement mode. Selecting a new parameter interrupts the current measurement cycle and starts a new one with the new parameter value. Frequency units (Hz, kHz, MHz) are determined automatically, depending on the frequency of the input signal. View of the frequency meter indicator in the operating mode: with an input signal frequency of up to 1 kHz at input signal frequency up to 1 MHz at an input signal frequency equal to or higher than 1 MHz, The symbol ">" here and below means that the frequency meter is in the pulse counting mode. That is, the measurement result, which is currently present on the indicator, refers to the previous measurement cycle. Measuring the period of the input signal Use the "MODE" button to select the "SIGNAL PERIOD" mode. There are no parameters for this mode. Approximately 1 s after releasing the button, the frequency meter will automatically switch to the measurement mode. The period T of the input signal is the reciprocal of its frequency F. Therefore, the frequency meter first measures the frequency of the input signal at a measurement time of 1 s, and after the calculations, displays the result on the indicator. View of the frequency meter indicator in the operating mode: Frequency deviation measurement Use the "MODE" button to select the "DEVIATION" mode. There are no parameters for this mode. Approximately 1 s after releasing the button, the frequency meter will automatically switch to the measurement mode. Deviation (or departure) is defined as the difference between the current frequency and the frequency that was at the beginning of the measurement in this mode. In this case, the drift (deviation) of the frequency can be both positive and negative. Therefore, the deviation value is displayed on the indicator with a sign. To start a new drift tracking, you need to press the "PARAMETER" button. View of the frequency meter indicator in the operating mode: Measuring the pulse duration of positive polarity Use the "MODE" button to select the "IMPULSION" mode. Use the "PARAMETER" button to select the mode parameter - pulse polarity. For a positive pulse, its duration is indicated by "P", and the interval between pulses is indicated by "0". Approximately 1 s after releasing the button, the frequency meter will automatically switch to the measurement mode. View of the frequency meter indicator in the operating mode: Capacitance measurement If you have an attachment to the frequency meter that measures the period, you can measure the capacitance of any capacitor in the range from 10 pF to hundreds of microfarads. Its scheme is shown in Fig. 4.
The multivibrator assembled on the op amp DA1 generates pulses with a period proportional to the capacitance Cx. This is described by the expression Тх= 2CхRэ-lп[(R4+R4')/(R4-R4')]. Here the value of R4' corresponds to the resistance of a part of the tuning resistor between the engine and the lower output according to the circuit. If the slider of the resistor R4 is set so that ln[(R4 + R4 ') / (R4-R4 ')] - 0,5, then Tx \u1d CxRe, and at Re \u10d 10 MΩ, the capacitance value of 10 pF corresponds to the duration of the period of the generated pulses, equal to 1 μs, and at Re = 10000 kΩ, the value of XNUMX μF corresponds to a duration of XNUMX μs. The prefix contains a reference capacitor Ce (3000 ... 10000 pF), which allows you to calibrate the prefix, as well as measure capacitances less than 10 pF. It is desirable to select the accuracy of the reference capacitor with an error of no more than 0,5 ... 1%. Calibration of the prefix consists in setting the value of the reference capacitor on the frequency meter with a tuning resistor R2 (10 kOhm). Te in the frequency meter should be equal to 1 μs (Fe = 1 MHz). Due to pickups, the least significant digits can periodically change their value. But for most cases, the accuracy of capacitance measurement is quite satisfactory. To measure capacitance, use the "MODE" button to select the "capacity" mode. This mode has no options. Approximately 1 s after releasing the button, the frequency meter will automatically switch to the measurement mode. View of the frequency meter indicator in the operating mode: Measurement of inductance In the presence of a prefix (its diagram is shown in Fig. 5), it is possible to measure inductances in the range of 1 μH ... 2 H.
The principle of measurement based on the ratio of the period of harmonic oscillations with the inductance and capacitance of the oscillatory circuit of the generator in the attachment: T2 = LC/25330, where T is in seconds, L is in µH, C is in pF. Therefore, if we use the loop capacitance equal to 25330 pF, then the numerical value of the inductance is calculated from the following relationship: L \u2d T1 \u2d XNUMX / FXNUMX, where F is the oscillation frequency. To measure the inductance with a frequency meter with a prefix, the "MODE" button selects the "INDUCTION" mode. Approximately 1 s after releasing the button, the frequency meter will automatically switch to the measurement mode. The numerical values of the readings correspond to the inductance in µH. View of the frequency meter indicator in the operating mode: The prefix consists of a measuring generator (VT1-VT5), the frequency of which is determined by the capacitance of the capacitors C1, C2 (total capacitance is about 25330 pF) and the inductance connected to the input terminals of the coil. To generate a signal with a TTL level, a Schmitt trigger (VT6, VT7) is used. The oscillation amplitude is stabilized by a circuit based on diodes VD1, VD2 and transistors VT4, VT5, connected to the generator through an emitter follower on a transistor VT3. With the specified value of the capacitance C1, C2 and the measured inductance equal to 1 μH, the generation frequency will be 1 MHz. With an inductance of 2 H - 700 Hz. To cover this range, especially in the high-frequency region, it is necessary to select transistors VT1, VT2 with a base current transfer coefficient of at least 150. Capacitors C1, C2 - K73-17 or similar with a small TKE. In total, their capacity should not differ from the indicated one by more than 1 ... 2%. The width of the measurement range is also affected by the VT5 transistor, or rather its base current transfer coefficient. The best results were obtained when using GT311 transistors with a gain of 30...50. The prefix usually does not require configuration if the specified requirements are met. Software for the microcontroller Adapter for microcontroller AT89C52-24JC Authors: S. Zorin, N. Koroleva, Izhevsk
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