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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Prefix-frequency meter to the multimeter. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology Not all DMMs can measure frequency, and those inexpensive ones that can do so tend to have low sensitivity and limited frequency range. The proposed device is a frequency-voltage converter and, of course, does not replace a digital multi-digit frequency meter, but complements it. It has better parameters than those published in [1, 2]. With it, you can measure the frequency of an arbitrary waveform in the range of 5 Hz ... 2,5 MHz. In the range of 5 Hz ... 5 kHz, measurements can be carried out with a resolution of 1 Hz, if the digit capacity of the multimeter allows it (for multimeters with a 3,5-digit display - 5 Hz ... 1999 Hz). The error in measuring frequencies up to 50 kHz does not exceed 0,2% ± 1 unit. junior grade. At higher frequencies, the error increases slightly, but not more than 0,8%. Temperature instability of readings in the range of room temperatures - no more than 0,04% per 1°C. The device consumes current no more than 30 mA. The measurement period is 2...3 times per second, which corresponds to the measurement period of the multimeter. Frequency overload indicator is provided. The measured frequency range is divided into 4 intervals. For multimeters with a partial four-digit display (3999) these would be:
When measuring frequency, the type of operation switch on the multimeter is set to the position for measuring DC voltages. This allows you to use any multimeter with an input resistance of at least 1 MΩ with the attachment without the need to rebuild the attachment.
An arbitrary waveform input signal with an amplitude of 100 mV ... 50 V through a separating-protective circuit (Fig. 1) enters the gate of the field-effect transistor VT2. This stage has a high input impedance and a low input capacitance, so it practically does not shunt a signal with an amplitude of up to 3 V in the audio frequency range. The amplified input signal from drain VT2 is fed to a differential amplifier based on transistors VT3, VT4. A signal close to a rectangular shape is removed from the VT4 collector and fed to the Schmitt trigger DD1.1, DD1.2. The rectangular signal is taken from pin 11 of DD1.2 and fed for further processing to DD3...DD5 microcircuits, included as frequency dividers by 10. Depending on the frequency range selected by switch SA1, a signal is sent to the pulse shaper at DD1.3, DD1.4 from one of the counters DD3 ... DD5 or from the output of the inverter DD1.2. The differentiating circuit on C11-R16 sets the constant duration of the generated pulses, the duty cycle of which depends on the frequency of the signal under study. The generated pulses are fed to the power amplifier on parallel-connected inverters DD2.2...DD2.4. From the output of the amplifier, pulses stable in amplitude and duration are fed to a temperature-compensated stable current generator at VT5, VT6, R17, R18, VD9. When the voltage on the storage capacitor C9 exceeds the level of 600 mV (frequency 6 kHz at the output of DD1.4), the linearity of the frequency-voltage conversion deteriorates. To avoid errors, the device is equipped with an overload indicator on the transistor VT1, the inverter DD2.1 and the blinking LED HL1. A miniature incandescent lamp EL1, included in the discharge circuit of capacitor C9, compensates for a small negative temperature drift of the voltage at the output of the set-top box. A voltage regulator for 1 ... 2 V is assembled on the DA6 chip and the HL6,5 LED, which is necessary to ensure high accuracy of the set-top box. IC KR142EN17A is capable of operating with a low voltage drop between input and output and is the best suited for battery-powered devices. In its absence, the stabilizer can be assembled according to the scheme shown in Fig. 2. Detailed information about the KR142EN17 chip can be found in [3].
Details and design. Fixed resistors can be used type MTL-0,125, C1-4-0,125; trimmers - SPZ-38a, SPZ-386, RP1-63M. To facilitate tuning, it is better to take R15 multi-turn, types SP5-2, SPZ-39a, with a resistance of 470 ohms. Capacitor C11 - film, preferably with a minimum TKE, for example, K31-10, K31-11. Oxide capacitor C9 - niobium K53-4. In its place, you can put a capacitor of another type with low leakage (K52, K53). The remaining oxide capacitors are K50-24, K50-35 or their imported analogues. Non-polar blocking capacitors - KM-5, KM-6, K10-176. Diodes VD1 ... VD8, VD10 - KD503, KD510, KD522, 1N4148. Flashing LED HL1 - any type, preferably red. The HL2 LED must be of the AL307 series with indices A, B, K or L. The VD9 diode is necessarily germanium, for example, D20, D9. The VT2 field effect transistor can be replaced with any of the KP305 series. In the absence of field-effect transistors with an insulated gate and an n-channel, it is permissible to use transistors with a pn-junction, for example, KP307, KP303. VT1, VT3, VT4 - KT3102, KT3130, SS9018, 2SD734; VT5, VT6 - any of the KT3107, SS9015 series. Chips DD1, DD2 are interchangeable with similar series 564, KR1561. With a change in the switching circuit, the counters DD3 ... DD5 can be replaced with K561IE14, KR1561IE14. In place of DD4, DD5, you can also use K176IE4, K176IE2, also including them as frequency dividers by 10. The prefix is mounted on a board with dimensions of 110x60 mm (photo on the cover) by surface or printed wiring. Transistors VT5, VT6 and diode VD9 are placed close to each other. A small paper cylinder is approaching them, which is then filled with paraffin. Blocking capacitors C6, C7 are installed near the microcircuits DD1, DD2. Figure 1 shows the minimum required number of bypass capacitors. If the set-top box will be operated only in stationary conditions, then it is desirable to increase the supply voltage of the microcircuits to 9V. Having applied the supply voltage to the device, in the absence of a signal at the input, the voltage at the VT2 drain is measured, which should be about 2,4 V. If necessary, it is set by selecting R7. Next, VT5 and R18 are temporarily disconnected from the outputs DD2.2 ... DD2.4 and connected to the "+" terminal of the capacitor C8. By selecting R18, the collector current VT6 is set within 1,5 ... 2 mA. Having restored the previous connection, a sinusoidal signal with a frequency of 1000 Hz and an amplitude of 250 mV is applied to the input of the device from the generator. By controlling the signal on the VT4 collector with an oscilloscope, we achieve a meander by rotating the R11 engine. If this fails, R8 should be selected. The first stage of setup is completed. Further, a multimeter is connected to the output of the set-top box, switched on for measuring constant voltages (limits -1999,9 mV, 400 mV or 200 mV). A reference frequency meter is connected to the output of the signal generator. The frequency of 3800 Hz or 1800 Hz with an amplitude of 1 V is set on the generator. By selecting R19 and adjusting R15, the readings on the display are 380,0 mV (180,0 mV). Then the frequency of the generator is reduced by 10 times. If the readings on the digital frequency meter and the multimeter differ by more than ± 2 units. LSB, then you should check VT5, VT6, VD10, C9. In practice, there should not be any discrepancy in the testimony! Switching SA1, we make sure that the frequency dividers DD3 ... DD5 work. Thermal compensation of the entire device can be made by connecting a thermistor or incandescent lamp in series with R19. If the readings on the multimeter decrease with increasing ambient temperature, then a PTC thermistor or a small-sized 24 ... 60 V incandescent lamp should be connected. negative TCS. If overcompensation is obtained, then the temperature sensor must be shunted with a conventional resistor. The approximate resistance of the connected temperature sensor at a temperature of 25 ° C is 30 ... 300 Ohm. Thermal compensation can also be performed in a different way, for example, by connecting a ceramic capacitor in parallel with C11 for several tens of pico-farads with the required TKE. When mounting the VT2 transistor and microcircuits, the usual precautions for working with MOS devices should be observed. The terminals and the case of the field-effect transistor are temporarily wrapped with a soft wire jumper before removing the closing tube. If the set-top box needs to measure higher frequencies, then the microcircuits must be replaced with functional analogues from the KR1554 series, for example, KR1554IE6, the input amplifier must be redone and the IC supply voltage reduced to 5,5 V. Accordingly, the number of dividers will also need to be increased. When a higher sensitivity is required from the set-top box, you can add another stage on a field-effect transistor or build a differential amplifier (VT3, VT4) according to the current mirror circuit. If you encounter difficulties in acquiring a suitable small-sized switch, you can build its functional analogue on the K561TM2 chip, included as a two-digit binary counter, and the K561KTZ multiplexer. Range switching in this case is performed with one button (TD-06XEX SMD). It should be taken into account that after repeated overload, several seconds are required to restore high counting accuracy (due to local heating of the VT5, VT6 crystals). Literature
Author: A. Butov, village of Kurba, Yaroslavl region; Publication: radioradar.net
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