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Prefix-frequency to a multimeter


2015-12-23 00:00:00

Console-frequency to the multimeter Not all DMMs can measure the frequency, but those cheap, in which there is a possibility, usually have low sensitivity and limited frequency range.
The proposed device is a frequency-voltage converter, and, of course, does not replace the multi-bit digital frequency counter, but complements it. It has better parameters than published in [1, 2]. Use it to measure the frequency arbitrary waveform in the range of 5 Hz ... 2.5 MHz. In the range of 5 Hz to 5 kHz ... measurements can be made with a readability of 1 Hz, if it allows for a bit multimeter multimeters display sign by 3.5 -5 Hz ... 1999 Hz. Error of measurement frequencies up to 50 kHz shall not exceed 0,2% ± 1 unit. LSB. At higher frequencies, the error is increased somewhat, but not more than 0.8%. Temperature instability readings in the range of room temperature -no more than 0.04% per 1 ° C. The device consumes less than 30 mA. Measurement Period -2 ... 3 times a second, which corresponds to the multimeter measurement. Overload indicator provided by frequency.
Measured frequency range is divided into 4 intervals. Multimeter with incomplete four-digit display that 3999 will be:
First band -5 ... 3999 Hz, the second -50 ... 39990 Hz, the third -500 ... 399 900 Hz, <> Four -5 kHz ... 2.5 MHz.In frequency selector switch on the multimeter set to measure DC voltages. This allows to use any prefix multimeter impedance at least 1 Mohm consoles without the need for restructuring.

Fig.1. Schematic diagram of the frequency-prefix
Input freeform amplitude of 100 mV ... 50 V through the separation and protection circuit Figure 1 is applied to the gate of FET VT2. This stage has a high input impedance and low input capacitance, so virtually no shunts signal amplitude up to 3 in the range of sound frequencies. Amplified input signal Stock Photo VT2 enters the differential amplifier transistors VT3, VT4. With collector VT4 removed signal close to a rectangular shape and comes to the Schmitt trigger DD1.1, DD1.2. Square-wave signal is removed from the output 11 DD1.2 and served for further processing on the chip DD3 ... DD5, included as a frequency divider 10.
Depending on the switch SA1 range of frequencies at a pulse shaper to DD1.3, DD1.4 signal is applied to one of the counters DD3 ... DD5 or inverter output DD1.2. The differentiating circuit for C11-R16 sets a constant duration generated pulse duty factor of which depends on the frequency of the test signal. Generated pulses are applied to the power amplifier with parallel connected inverters DD2.2 ... DD2.4. The output of the amplifier stable in amplitude and duration of pulses are applied to temperature-compensated oscillator stable current VT5, VT6, R17, R18, VD9.
When the voltage on the storage capacitor C9 exceed the level of 600 mV 6 kHz frequency output DD1.4, the linearity of frequency-voltage conversion deteriorates. That there were no errors, the device is equipped with overload indicator on the transistor VT1, inverter and LED is blinking DD2.1 HL1.
Miniature incandescent EL1, included in the discharge circuit capacitor C9, a small negative offset voltage temperature drift output consoles.
On the chip and LED DA1 HL2 assembled voltage regulator 6 ... 6.5 V, which is necessary to ensure high accuracy of consoles. IC KR142EN17A able to operate at low voltage drop between input and output and is perfectly suited for battery powered devices. In its absence, the stabilizer can be assembled in the circuit shown in Figure 2. For details on the chip KR142EN17 can be found in [3].

Fig.2. voltage regulator 6 ... 6.5 V
Details and design. Fixed resistors can be used type MTL-0, 125, C1-4-0, 125; Trimmers -SDR-38a, SDR-386, RP1-63M. For ease of configuration, R15 is better to take multiturn, types SP5-2, SDR-39a, 470 ohms resistance. Capacitor C11 -film, preferably with a minimum TCC, e.g., 10-K31, K31-11. Oxide capacitor C9 -niobium K53-4.
In its place, you can put a different type of capacitor with low leakage K52, K53. The remaining oxide capacitors -K50-24, K50-35 or their imported counterparts. Nonpolar bypass capacitors -the CM-5, CM-6, K10-176. Diodes VD1 ... VD8, VD10 - KD503, KD510, KD522 , 1N4148 . Flashing LED HL1 -any type, preferably red glow. LED HL2 must be series AL307 with indices A, B, K, or L. Diode VD9 -certainly germanium, for example, D20, D9.
FET VT2 can be replaced by any of a series KP305. If no field-effect transistors and insulated gate n-channel transistors are applied with acceptable pn-junction, e.g., KP307, KPZOZ . VT1, VT3, VT4 - KT3102, KT3130 , SS9018, 2SD734 ; VT5, VT6 -any of a series of KT3107, SS9015 .
Microchips DD1, DD2 interchangeable same series 564, KR1561. With the change of switching circuits counters DD3 ... DD5 can replace K561IE14, KR1561IE14 . On-site DD4, DD5 can be used K176IE4, K176IE2 , also including them as frequency dividers 10.
Console mounted on board size 110x60 mm cover photo attachments or printed circuit board. Transistors VT5, VT6 VD9 and diode placed close to each other. They are approaching a small paper cylinder, which is then filled with paraffin. Bypass capacitors C6, C7 chips installed near DD1, DD2. Figure 1 shows the minimum number of bypass capacitors. If the console is operated in steady-state conditions, the voltage supply circuits, it is desirable to increase the 9V.
Applied to the device supply voltage, in the absence of the input signal is measured drain voltage of the VT2, which should be about 2.4 V. If necessary, the selection is set by R7. Next VT5 and R18 is temporarily disconnected from the output DD2.2 ... DD2.4 and connects to the "+" capacitor C8. R18 selection set collector current VT6 within 1.5 ... 2 mA. Restoring the former connection, the input device with a sine wave oscillator is a frequency of 1000 Hz and an amplitude of 250 mV. Controlling the oscilloscope signal at the collector VT4, engine rotation R11 achieve meander. If this fails, you should choose the R8. The first configuration step is completed.
Next, set-top box is connected to the output of the meter included on the measurement mode DC voltage limits -1999.9 mV, 400 mV and 200 mV. To the output of the signal generator is connected reference frequency. On the generator set frequency 3800 Hz or 1800 Hz amplitude of 1 V. By selecting R19 and R15 are making adjustment display readings 380.0 mV 180.0 mV. Then, the frequency of the generator is reduced by 10 times. If the reading on the digital multimeter, frequency counter and separated by more than ± 2 units. LSB, check VT5, VT6, VD10, C9. In practice, no discrepancies in the testimony should not be! Switching SA1, we see in the frequency dividers DD3 ... DD5.
Temperature compensation of the device can be made by connecting in series with the thermistor or R19 bulb. If the reading on the multimeter decrease with increasing ambient temperature, it is necessary to connect a PTC thermistor or a small bulb at 24 ... 60 V. If the meter readings increase with increasing temperature naturally, at a constant frequency of the input signal, connect NTC . If turned overcompensation, the sensor should shunt resistor usual.
Approximate resistance temperature sensor connected at 25 ° C -30 ... 300 ohms. Temperature compensation can be made in another way, for example, connecting in parallel with ceramic capacitor C11 to a few tens of pico-farad with the required TKE.
When mounting the transistor VT2 and chips should observe the usual precautions when working with MOS devices. Conclusions and FET body before removing the tube closing temporarily wrapped soft jumper wire.
If the prefix required to measure higher frequencies, the chip must be replaced functional analogs of KR1554 series, for example, KR1554IE6, alter the input amplifier and reducing the supply voltage to 5.5 V. IC Accordingly, the need to increase the number of divisors. When the prefix require higher sensitivity, you can add another FET stage or build a differential amplifier VT3, VT4 on the current mirror circuit.
If you have difficulties with the acquisition of a suitable compact switch, you can build a functional analog on-chip K561TM2 included as a two-bit binary counter and multiplexer K561KTZ. Range switch in this case is made one-button TD-06XEX SMD. Note that after repeated overload takes a few seconds to restore the high counting accuracy due to local heating of the crystal VT5, VT6.
Sources
Nechayev I. Combined frequency. -Radio, 1993, N9, P.22-24.Griev Yu Analog frequency with automatic selection of the measuring range. To help the radio amateur. -Moscow: The Patriot, 1990, N108, P.40-51.Nefedov A. IC series KR142EN17 -voltage. -Radio, 1998, N6, P.65.



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