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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Digital scale of the AF generator. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology To set the frequency in measuring generators of sinusoidal signals, scale devices are most often used, which are mechanically connected to the control element of the device. Their shortcomings are known: this is the complexity of manufacturing, the need for calibration according to an exemplary generator or frequency meter, and in some cases the accuracy of setting the frequency is insufficient, depending not only on the design of the reading device, but also on the stability of the parameters of the radio elements of the frequency-setting circuits. The so-called electric scales are largely free from these shortcomings. In the simplest case, this is an analog frequency meter, the operation of which is based on measuring the average voltage of a sequence of pulses with a constant duration formed from a generated signal. However, such a scale also provides a relatively low frequency setting accuracy (at best, 1 ... 3%), and its calibration also requires an exemplary generator. The use of digital methods for measuring frequency allows you to get rid of all the shortcomings inherent in both mechanical and electrical scales. The frequency in this case is counted directly in digital form and with high accuracy, determined by the stability of the so-called measuring time interval. The digital scale simplifies the layout and manufacture of the generator, since it can be assembled as a separate functionally complete electronic unit and placed in any convenient place in the device. The simplest digital method for measuring frequency is the direct counting method, which consists in counting the number of periods of the generated signal for a known period of time - the measuring time interval. To determine the frequency with an accuracy of 1 Hz, it must be equal to 1 s. If a sequence of pulses is formed from a sinusoidal signal, the fronts of which coincide with the moments of the transition of the sinusoidal voltage through the zero level, and their number is counted, then with the same accuracy, the measuring time interval can be halved. The use of a doubling node in the digital scale reduces the time delay between the moment the frequency is changed by the control element and the beginning of the indication of the measurement result, which is of great importance when setting the frequency with an accuracy of 1 Hz. However, the time delay of 0,5 s with a rough setting of the generator is still large. Therefore, together with a digital scale that provides accurate frequency setting, an additional mechanical scale is sometimes used for coarse tuning. You can do it differently: reduce the time delay by another order, i.e. enter the second operating mode ("Rough") into the digital scale, in which the measuring time interval is 0,05 s, and the frequency measurement accuracy is ± 10 Hz. However, a simple decrease in the measuring time interval by 10 times leads to the fact that the value of the displayed frequency on the scale shifts to the right by one decimal place, making it difficult to read the information. To eliminate this shortcoming, a sequence of pulses of a double frequency sinusoidal signal in the "Rough" mode should be applied to the second decimal counter of the digital scale. In this case, each digit of the number that determines the measured frequency will always be displayed in the same place. The device provides frequency measurement in the range from 1 Hz to 1 MHz. The amplitude of the input signal is up to 15 V. The accuracy of measurement, the time of measurement and indication of frequency, depending on the operating mode, are ±10 Hz, 0,05 and 0,2 s (in the "Rough" mode) and 1 Hz, 0,5 and 2 with ("Exactly"). Current consumption - no more than 50 mA. The device consists of an input shaper, a frequency doubler, a sensor for measuring time intervals, a selector and a pulse counter, and an operating mode switching unit. The input shaper on the comparator DA1 is a Schmitt trigger. Its positive feedback circuit is formed by resistors R3 and R6. The sequence of pulses formed by him from a sinusoidal signal through the inverters DD1.1, DD1.2 comes to a frequency doubler made on the elements R5, C2 and DD3.1. Inverters DD1.1 and DD1.2 provide the necessary steepness of the fronts and slopes of the pulses, which determines the accuracy of the frequency doubler. From the output of the element DD3.1, a sequence of short positive pulses of doubled frequency is fed to one of the inputs (pin 9) of the selector, the functions of which are performed by the element DD1.3. The measuring time interval sensor contains a master oscillator, a frequency divider, an initial installation unit, and a zeroing pulse shaper. The master crystal oscillator, assembled on the elements DD2.1, DD2.2, generates pulses with a repetition rate of 100 kHz, which pass through the inverters DD2.3 and DD2.4 to the frequency divider on the DD4-DD9 microcircuits. The divider includes six counters, two of which (DD6, DD8) divide the frequency by five, and the rest by ten. The initial installation unit, made on the elements VD2, R10, C4, DD1.4, resets the divider counters when the device is powered on. The operating mode switching unit is assembled on a DD10 chip, DD11.1-DD11.3 elements, a VT1 transistor and an SB1 switch. In the "Precise" mode, the pulses from the output of the counter DD5 through the elements DD11.1, DD11.3 are fed to the input C of the counter DD6, and the entire divider is involved in the operation of the device. At the same time, a sequence of pulses with a duration of 9 s and a repetition rate of 0,5 Hz is formed at the output of the counter DD0,4. In the "Rough" mode, the DD5 counter is excluded from the divider, and the pulses from the output of the previous (DD4) through the elements DD11.2 and DD11.3 pass to the counter DD6, and a sequence of pulses with a duration of 0,05 s and a repetition rate of 4 Hz is formed at the output of the divider .
The pulses from the output of the counter DD9 are fed to the second input (pin 8) of the DD1.3 element and to the reset pulse generator assembled on the elements DD3.3, DD3.4, DD11.4. Short pulses appear at the output of the DD3.4 element, which periodically, before the start of each measurement cycle, set the pulse counter on the DD12-DD17 microcircuits to zero. Transistor key VT2 extinguishes the scale indicators for the duration of the frequency measurement. The pulses from the selector output are fed to the pulse counter through the DD3.2 element, which eliminates unnecessary operation of the counter on the edge of the pulse that sets the measuring time interval. The pulse counter includes six recalculation nodes of the same type. In the "Precise" mode, all nodes are connected in series through the elements DD10.2, DD10.4, and double-frequency pulses from the output of the selector come to the input of the low-order digit node (DD12, HG1). In the "Rough" mode, these pulses through the elements DD10.3, DD10.4, are fed to the second counting unit (DD13, HG2), and the transistor key VT1 turns off the indicator of the least significant decimal digit of the scale. The indicator point HG4 on the digital scale separates the digits indicating the frequency in kilohertz and hertz. If it is not necessary to measure the frequency with an accuracy of 1 Hz, the scale can be simplified by excluding the elements SB1, DD5, DD10, DD11.1-DD11.3, DD12, HG1, VT1, R11 and connecting the output of the DD4 counter to pin 4 of the DD6 chip, and the output of the element DD3.2 - with the input C of the counter DD13. With a decrease in the upper operating frequency from 1 MHz to 600 kHz, it is possible to further simplify the device and use the K176IE3 chip instead of K176IE4 in the high-order digit of the counter (DD17). In this case, the elements DD1.1, DD1.2, DD2.3, DD2.4 are additionally excluded, the output of the DD2.2 element is connected to the input C of the DD4 counter, and pin 7 of the DA1 chip is connected to pin 2 of the DD3.1 element and resistor R5 . The device uses a quartz resonator (ZQ1) from the "Quartz-21" set. Instead, you can use a quartz resonator at a frequency of 1 MHz by adding another K176IE4 counter to the frequency divider and including it between the DD2.4 element and the DD4 chip. Instead of those indicated in the diagram, the device can use both iconic LED indicators of other types, and cathodoluminescent ones. The wiring diagram for the IV3 cathodoluminescent indicator is shown in Fig. 2. In this case, the resistor R12 of the main circuit is connected not to a common wire, but to the emitter of the transistor VT2. In addition, an additional voltage source of 3 V is required to power the IV0,7 indicators.
The wiring diagram for LED indicators ALS324B or ALS321B is shown in Fig.3. As transistor switches VT1-VT7, you can use any silicon transistors with a permissible voltage collector - emitter and base - emitter of at least 10 V and a collector current of at least 10 mA (KT312B, KT3102B, KT315 with any letter index, K1NT251, etc.). In this case, the transistor VT2 of the device must be composite. The base of the additional transistor KT807B is connected to the emitter of the transistor VT2, the collector to its collector, and the emitter to the conversion nodes (pin 4). In addition, a more powerful power supply will be required, since the current drawn by the scale will increase to 300 mA.
Signals with an amplitude of up to 15 V can be applied to the digital scale input, since the permissible input voltage of the K521CA3 (DA1) comparator does not exceed 30 V. To measure the frequency of signals of a higher level, the scale must be supplemented with an overload protection unit or an input divider that lowers the voltage at the comparator inputs to allowed value. In the manufacture of the device, a 1000 pF capacitor is installed between the power leads of each microcircuit. To reduce the effect on the generator of impulse noise, the digital part of the scale is placed in a metal shield, which is connected to the common wire of the generator at one point. If the scale is designed to work with a sound generator that generates signals with a low level and harmonic coefficient, then the wires connecting the HG1-HG6 indicators with the meters are especially carefully shielded, since they can be sources of powerful impulse noise, especially when using the ALS324B or ALS321B. You can completely eliminate impulse noise by turning off the power to the scale after setting the generator frequency, for which you need to provide a separate switch. If you intend to use the digital scale of the generator to measure the frequency of signals from other sources, it is advisable to install an additional socket and a switch on its front panel that connects the input of the device either to the generator output or to this socket. When establishing, first, the presence of pulse sequences at the output of the sensor of measuring time intervals is checked with an oscilloscope. Then, a sinusoidal signal with an amplitude of about 0,5 V is fed to the input of the device. At the same time, pulses with an amplitude of at least 3 V should be observed at the output of the frequency doubler (pin 3.1 of the DD8 element). supply voltage of 8,1 and 9,9 V. In the event of a discrepancy between the readings of the scale and the frequency of the generator, it is necessary to select the capacitor C5, which affects the zeroing pulse divider. Author: V.Vlasenko
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