Multifunctional DMM design

According to the f / V conversion principle, the DMM converts the frequency of the measured signal into a DC voltage for measurement. The block diagram is shown in Figure 3. The specific circuit is shown in Figure 4.

. T is fixed, the higher the trigger frequency, the more pulses generated per unit time, The bigger. This indicates It is strictly proportional to f. In order to make the measured frequency value equal to the DC voltage value, the value of R5 can be adjusted. If a 200mV DC gear is used as the frequency gear, the maximum value of the frequency is 200kHz. In order to expand the measurement range, three frequency dividers of 10, 100 and 1000 are used in the front stage, so that the frequency measurement range can reach 200 MHz. Because the effective value range of the 555's 2-pin input signal is 100mV ～ 10V, in order to improve the measurement range, a voltage amplifier and attenuator are added in the front stage; since the frequency of the measurement signal may be very high, both the amplifier and the attenuator should adopt frequency compensation Circuit to achieve the best compensation. The shaping and buffering circuit converts the amplified or attenuated input signal into a steep rectangular pulse for the f / V conversion.

Figure 5 is a circuit diagram for measuring the effective values ​​of various standard AC signals. In the figure, C1 is the peak hold capacitor, D is the detection diode, R1 (R2) is the voltage divider resistor, which is connected in series with the input impedance R IN of the DMM, as the load of the detector, C2 is the high frequency carrier filter capacitor. During the positive half cycle of the signal, D is turned on, and Vi charges C1 through D. If the on-resistance of the detection diode is RD and the internal resistance of the signal source is Ri, then the time constant of charging is τ1 = (Ri + RD) C1. Since the values ​​of RD and Ri are very small, the value of τ1 is extremely small, that is, the voltage on C1 reaches the peak of the signal in a very short time. In the negative half of the signal, D is turned off and the voltage on C1 is discharged to the load. The discharge time constant is τ2 = (RIN

During the period T, it can be considered that the voltage on C1 remains unchanged within a signal period. Therefore, the output voltage on the detector is the peak value of the measured signal. Since R1 (R2) and RIN form a voltage divider, according to the crest factor KP of the standard signal, the peak value of the standard signal can be converted into the effective value of the signal. The KP of standard sine wave, triangle wave and square wave are respectively and 1 . The input impedance RIN of the DMM is constant. If RIN is 10MΩ, R1 is respectively taken, R2 is 4.14MΩ, 7.32MΩ, and the switch K is set to 1, 2, and 3 when the DCV of the DMM corresponds to the effective values ​​of the standard sine wave, square wave, and triangle wave, respectively. This subtly converts the peak voltage into an effective value. In order to increase the measurement upper limit frequency, these devices should be made into a high-frequency probe and placed in a well-insulated electrical bakelite tube. C1 should be welded to the probe to reduce the influence of distributed capacitance. In order to improve the measurement accuracy, the accuracy of R1 (R2) should be above 0.2, and the insulation resistance of C2 should be greater than 10GΩ. The highest signal frequency of DMM processed in this way can reach about 50MHz, which can meet most needs. ?

The extended design of DMM can not only perform online measurement and automatic range conversion, but also can be used as transistor millivoltmeter and frequency meter. The circuit is also relatively simple, and it has high practical value for workers engaged in electronic technology.