Precautions in the debugging of board debugging techniques

Whether the debugging result is correct or not is largely affected by the correctness of the measurement and the accuracy of the measurement. In order to ensure the effect of debugging, it is necessary to reduce the measurement error and improve the measurement accuracy. To do this, pay attention to the following points:

(1) Proper use of the grounding end of the measuring instrument

When measuring with an electronic device that terminates the chassis, the ground terminal of the instrument should be connected to the ground of the amplifier. Otherwise, the interference introduced by the instrument case will not only change the working state of the amplifier, but also make the measurement result. An error has occurred. According to this principle, when debugging the emitter bias circuit, if you need to measure VCE, you should not connect the two ends of the instrument directly to the collector and emitter, but should measure VC and VE separately, then both Subtracted to VCE. If a dry-battery-powered multimeter is used for measurement, since the two inputs of the meter are floating, it is allowed to be directly connected between the measurement points.

(2) The input impedance of the instrument used to measure the voltage must be much larger than the equivalent impedance of the measured part. If the input impedance of the measuring instrument is small, it will cause shunting during the measurement, which will bring a large error to the measurement result.

(3) The bandwidth of the measuring instrument must be greater than the bandwidth of the circuit under test

For example, the operating frequency of the MF-20 multimeter is 20 to 20000 Hz. If the amplifier's fh = 100 kHz, we can't use the MF-20 to test the amplitude-frequency characteristics of the amplifier. Otherwise, the test results will not reflect the true condition of the amplifier.

(4) To correctly select the measurement point

When measuring with the same measuring instrument, the measuring points are different, and the error introduced by the internal resistance of the instrument will be different. For example, for the circuit shown in Figure 1, when measuring C1 point voltage VC1, if e2 is selected as the measurement point, VE2 is measured, and the result obtained by VCl=VE2+VBE2 may be smaller than the error of VC1 obtained by directly measuring the Cl point. many. Therefore, this situation occurs because Re2 is small, and the measurement error introduced by the internal resistance of the instrument is small.

(5) Measurement methods should be convenient and feasible

When it is necessary to measure the current of a certain circuit, it is generally possible to measure the voltage as much as possible without measuring the current, because the measured voltage does not have to change the circuit to be tested, and the measurement is convenient. If you need to know the current value of a branch, you can obtain the voltage across the resistor on the branch and convert it.

(6) During the debugging process, not only must we carefully observe and measure, but also be good at recording

The recorded contents include experimental conditions, observed phenomena, measured data, waveforms, and phase relationships. Only when there are a large number of reliable experimental records and compared with the theoretical results can we find problems in circuit design and improve the design.