Input amplifier-shaper of the frequency meter. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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The widespread use of microcontrollers makes it possible to create frequency meters based on them. However, as a rule, such designs have a significant drawback, since they are designed to process signals with logic levels. Their capabilities are expanded by the described device.

The convenience of working with a universal frequency meter capable of measuring the frequency and duration of time intervals largely depends on the presence of an input shaping amplifier in its composition, as well as on the quality and capabilities of this amplifier. The requirements for this node, used in the frequency meter and the time interval meter, are different, and for some parameters they are completely contradictory.

Rectangular pulses, which are most convenient for processing by a frequency meter, have a finite rise and fall duration. For a time interval meter, the introduction of an additional time delay by the input amplifier is unacceptable. On the contrary, it should form the steepest edge and decay of pulses at a given level of the comparation threshold. This requirement is especially important for non-rectangular signals. For a frequency counter, an amplifier-shaper with hysteresis is more suitable.

Real signals have spikes, and noise is not excluded. The use of conventional filters is advisable for low-frequency frequency counters. For high-frequency frequency and time interval meters, a simple and practically reproducible amplifier-shaper circuit is proposed, capable of performing the following operations:

• process input signals of positive and negative polarity with voltage from 0,3 to 20 V;
• smoothly adjust the level of the input signal comparation threshold within 0,3. ..7,5 V;
• work reliably at input pulse frequencies from 0 to 2,5 MHz, maintaining performance almost up to frequency values ​​of 5 MHz
• measure the duration of pulses longer than 0,3 µs (when selecting the comparation threshold before changing the measurement limit, this threshold can be reduced to 0,1 µs).

The scheme is shown in fig. 1. The amplifier consists of an electronic switch-shaper assembled on the logic elements of the D1 microcircuit, and two channels connected in parallel and identical in construction, each of which is a voltage comparator on a two-channel high-speed operational amplifier A1, A2. The upper channel according to the scheme is intended for processing input signals of positive polarity, the lower one - of negative polarity. The input signal through the limiting resistor R1 and capacitor C1 is fed to input 3 of the operational amplifier A1. At the moment when the input voltage exceeds the comparation threshold, determined by the voltage at the input 2 of the OU A1, the voltage at the output 6 of the OU increases abruptly, and when it decreases below the comparation threshold, it abruptly decreases.

Rice. 1 Schematic diagram of the input amplifier

The comparation threshold is set by a variable resistor R10. The lower limit of the comparation threshold is determined by the voltage drop across the diode D3 and is approximately 0,3 V, and the upper limit is determined by the parameters of the elements of the voltage divider R9R10D3 and is equal to 7,5 V, respectively. The voltage at the output of the divider R3R4 determines the input signal limit threshold, when exceeded diode D1 opens. Excess voltage drops across resistor R1, preventing the uu from being overloaded at the input. The second channel of the device, assembled on the OU A2, works in the same way. From the outputs of the op-amp A1 and A2, the pulses through the diodes D5 and D6, which exclude the passage of signals of negative polarity, are fed to the electronic commutator-shaper. This node allows the passage of signals from the channel "positive" or "negative" polarity in accordance with the control signal that comes from the switch SA1. Switch SA1, the user manually sets the amplifier mode (channel selection) in accordance with the polarity of the measured signal. You can refuse channel switching by connecting the cathodes of diodes D6 and D5 and thereby ensuring automatic selection of the input signal polarity. However, when working with a bipolar signal, such as a sine wave, the frequency meter will show twice the frequency.

The amplifier is designed for a frequency counter assembled on CMOS chips.

When using it in devices with TTL microcircuits, it is necessary to provide a level converter at the output, assembled, for example, on a K561PU4 microcircuit.

The power supply voltage of the amplifier can be increased up to ±15 V. This improves its frequency response and expands the limits of the allowable input voltage.

The device can use any low-power silicon diodes of the KD521, KD503,1 N3207 series with a reverse voltage of at least 30 V; low-power germanium diodes of the D9, D311, 1N3206, 1N3203 series. Operational amplifiers KR544UD2B can be replaced by K544UD2 without changes in the board installation. It is allowed to use OU type K574UD1. When replacing the K561LA7 chip with the K561LE5, the SA1 switch function will change to the opposite. The use of K176LA7 or K176LE5 microcircuits will slightly worsen the frequency characteristics. When setting up the amplifier, it is necessary to set the voltage within 7,3 ... 7,5 V at the top terminal of the resistors R10, R 12, R4, R6 according to the circuit. If necessary, select resistors R9, R3, R11, R5.

Author: Vladimir Yepinin, vepin@i1.ru; Publication: radioradar.net

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