Frequency meter on microcircuits of the K176 series. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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This version of the frequency meter is five-digit, which makes it possible to measure the frequency of electrical oscillations from several tens to U9 U99 1ts (100 kHz) without any additional switching. The amplitude of the signal applied to the input of the device must be at least 0,5 V and not more than 30 V.

Schematic diagram of the frequency meter is shown in fig. 88.

The signal, the frequency of which must be measured, through the sockets XS1, XS2 "Input" and the capacitor C1 is fed to the input of the shaper formed by the field effect transistor VT1 and bipolar transistors VT2, VT3. The direct connection of bipolar transistors of different structures with the source and drain circuits of the field-effect transistor provides the driver with a trigger mode of operation. As a result, rectangular pulses are formed on the collector of the transistor VT3 of this node, the repetition rate of which exactly corresponds to the frequency of the input signal, the input impedance of the shaper is about 10 ohms, the frequency band is from a few hertz to 30 MHz, the gain is about 10.

From the output of the shaper, the signal is fed to the upper input of the element 2OR-NOT DD3 4, which performs the function of an electronic valve. And, if this valve is open (at a low level voltage at the lower input), then at its output, which means at the input of the five-digit counter formed by the DD4-DD8 microcircuits, pulses of the converted signal appear. The logical state of the pulse counter microcircuits is displayed by the corresponding seven-element luminescent indicators HG1-HG5. The lower input of the electronic valve is connected to the output of the shaper of the measuring time interval equal to 1 s. Therefore, digital indicators highlight the number of pulses that have passed through the valve to the counter during this time, that is, the input frequency in units of hertz.

The function of a pulse generator and a frequency divider up to a value of 1 Hz, necessary for the formation of time intervals and pulses to reset the counter at the end of the measurement result indication time, is performed by the familiar K176IE5 DD1 chip. The initial frequency of the generator (32 768 Hz) is determined by the natural frequency of the quartz resonator ZQ1 and capacitors C3, C4. The frequency of 1 Hz pulses generated at output 15 (pin 5) of this microcircuit serves as an exemplary one. The cyclic operation control unit of the frequency meter is formed by D-flip-flops DD2.1 and DD2.2 and logic elements 2OR-NOT DD3.1, DD3.2. These elements work in the indication time trigger pulse generator, the duration of which can be adjusted by a variable resistor R9. Element DD3.3 is used as a key in the counter reset circuit.

Recall the logic of the 2OR-NOT element: when the voltage is high at any of its inputs, the output will be a low voltage. The operation of the control device is illustrated by the timing diagrams shown in fig. 89. From the output 15 of the DD1 microcircuit, to the input C of the DD2.2 trigger, pulses of the reference frequency are continuously received (diagram a), and to the same input of the DD2.1 trigger, the pulses of the start generator assembled on the elements DD3.1 and DD3.2 (diagram b). For the initial take the moment when both triggers are in the zero state. At this time, a high-level voltage from the inverse output of the trigger DD2.2 is supplied to the lower input of the electronic valve DD3.4 and closes it. From this moment, the signal of the measured frequency passes through the valve to the input of the counter DD4-DD8.

With the appearance at the input C of the trigger DD2.1 of the trigger generator pulse, this trigger switches to a single state and, with a high-level voltage at the direct output, prepares the trigger DD2.2 for further work. At the same time, a low-level voltage appears at the top input of the DD3.3 element, connected to the inverted output of the DD2.1 trigger. The next pulse of the exemplary frequency generator switches the trigger DD2.2 to a single state. Now, at the inverse output of this trigger and at the lower input of the DD3.4 element, there will be a low-level voltage that opens the electronic valve and thereby allows the signal pulses of the measured frequency to pass through it.

But the direct output of the trigger DD2.2 is connected to the input R of the trigger DD2.1. Therefore, when the trigger DD2.2 is in a single state, it switches the trigger DD2.1 to the zero state with a high-level voltage at the direct output and keeps it in it until the measurement interval lasts. The next pulse of the reference frequency switches the trigger DD2.2 at the input C to the zero state, and the high-level voltage from the inverse output of the trigger closes the electronic valve. As a result, the passage of the signal pulses of the measured frequency to the counter is stopped and the digital indication of the measurement results begins (diagrams e, g).

Each interval of the measuring time is preceded by the appearance at the input R of the counters DD4-DD8 of a short-term high-level pulse (diagram d), which switches the counters to the zero state. It is from this moment that the counting cycle begins - an indication of the operation of the frequency meter. The zeroing pulse is generated at the output of the element DD3.3 at the moment of the coincidence of low-level signals at its inputs.

The duration of the indication of the measurement result within 2 ... 5 s can (optionally) be set by the variable resistor R9 of the start generator.

The counter-decoder DD4 and the indicator HG1 form the least significant bit, and the counter-decoder DD8 and the indicator HQ5 form the highest bit of the frequency meter. Therefore, in the digital display of the device, the HG5 indicator must be placed first on the left, and HG1 - the last on the right in a row of indicators.

The appearance of this version of the frequency meter and the placement of parts in its case are shown in Fig. 90.

Through a rectangular window in the front panel, covered from the inside with a plate of green transparent organic glass, luminous numbers of indicators are visible. On the right half of the front panel is the handle of the variable resistor R9 of the start pulse generator and the push-button power switch SB1. Input jacks XS1 and XS2 are located on the bottom left. All other parts of the device are mounted on two printed circuit boards with dimensions 115X60 mm made of foil fiberglass 1 mm thick. On one of them (Fig. 91) all parts related to the pulse voltage generator, the reference frequency source and the control device are mounted, on the other (Fig. 92) - DD4-DD8 counters and HG1-HG5 digital indicators. The wire leads of the indicators, the cylinders of which are placed vertically, are soldered to the contact pads at the outputs of the counters (in Fig. 92, the leads are indicated by arrows). On the first of these boards, the distance between the rows of holes in the DD3 chip is increased to 12 mm. In addition to the parts, five wire jumpers must be installed on this board (in Fig. 91 they are shown with dashed lines).

All fixed resistors - MLT, variable resistor R9 - SP1-1. Capacitors C2 and C6, blocking the power supply circuit of microcircuits, can be KLS or K73-17, C3 - ceramic KT-1 or KM, tuning C 4-KPK-MP. Non-polar capacitor C5 - K53-1A (it can be replaced with a set of capacitors K73-17 with a total capacity of 1 ... 1.5 μF). Power switch SB1-P2K with button return by repeated voltage.

The field-effect transistor (VT1) can be with letter indices D, E or F. It can be replaced with a KP306A transistor by connecting its second gate to the source terminal through a 100 kΩ resistor.

The K176IE5 (DD1) chip can be replaced with a K176IE12 similar to it - it was used in a stopwatch - for which you will have to adjust the pattern of printed conductors in accordance with its pinout.

To power the device, you can use a 7D-0,1 (GB1) battery or a Korund battery and one 373 (G1) cell. After assembling the device, first of all, it is necessary to carefully check the installation with the "Principal diagram", clean and rinse with alcohol or gasoline the sections of the boards between adjacent conductors, current-carrying pads of microcircuit outputs, transistors (especially field ones) of the pulse shaper. With error-free installation and correct interconnection of circuit boards during adjustment, it may only be necessary to adjust the frequency of the generator on the DD1 chip. Roughly, the frequency of the generator is adjusted by selecting the capacitor C3, and precisely by the tuning capacitor C4. The accuracy of the installation is controlled by an exemplary (industrial) frequency meter connected to pins 11 and 12 of the DDL chip. To control the logic levels at the outputs of the microcircuits of the control device, you can use the "Display" described above or similar indicator probes.

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