PCB News - PCBA test instrument selection and configuration

PCBA test instrument selection and configuration
The choice of PCBA test instrument is related to the accuracy of PCBA test and the reliability of test data. The choice of instrument should refer to the following aspects.
PCBA test instrument selection

How to choose PCBA test equipment
PCBA test instrument error range

The working error of the measuring instrument should be much smaller than the error required by the tested parameter

In PCBA test work, it is usually required that the error generated in the test is negligible for the error of the measured parameter. In addition to the working error of the test instrument, the error generated in the PCBA test must also be considered the error of the test method and the test system, but the latter has been considered when the test plan is formulated and measures have been taken to eliminate it. The error can be ignored. For the working error of the test instrument, it is generally required to be less than one-tenth of the error of the measured parameter. Take the measurement of voltage and current as an example. If the test accuracy is high, a high-precision analog meter can be used, and the accuracy level is above 0.5. If a digital electric meter is selected, the measurement accuracy can be higher. For example, the measurement accuracy of a five-digit DC digital voltmeter can reach ±0.01%~0.03%.

PCBA test instrument measuring range

The measuring range and sensitivity of the instrument should conform to the numerical range of the measured electricity

If the working frequency is low, a low-frequency signal generator can be used. Its frequency range is 20Hz~200kHz (up to 1MHz), and the output signal amplitude is tens of millivolts to several volts;
If the working frequency is higher, a high-frequency signal generator can be used, the frequency range is about 100kHz~30MHz, and the signal output amplitude is about 1μV~1V.
Of course, when selecting a signal source, the signal output mode, output impedance and other indicators must also meet the requirements.

PCBA test instrument accuracy

The selection of the range of the PCBA test instrument should meet the requirements of measurement accuracy

For example, a pointer instrument is expressed by the measurement accuracy at full scale. The closer the measured value is to the full scale value, the smaller the error. Therefore, when selecting the range, the measured value should be more than two-thirds of the full scale value. If a digital instrument is selected, the measurement error generally occurs on the last digit. Therefore, the measurement range should be selected so that the effective number of digits of the measured value should be equal to the indicated number of digits as much as possible. For example, a five-digit digital voltmeter measures DC voltage, and its measurement accuracy is ±0.01%~0.03%. If it is used to measure the 12V voltage, it should be placed in the 20V range, and five significant digits can be measured. If it is used to measure the voltage of 24V, it should be placed in the 200V range as well to ensure four significant digits.

Test instrument input impedance

It is required that after connecting the circuit under test, the working state of the circuit under test should not be changed, or the measurement error generated after connecting the circuit should be within the allowable range. For example, when measuring voltage, the meter is connected to both ends of the circuit under test in parallel, and a voltmeter with high input impedance or high voltage sensitivity should be selected. This is especially important for small current and high impedance circuits. When measuring current, the meter is connected in series in the circuit, so an electric meter with a small internal resistance should be selected, which is particularly important for low-voltage and high-current circuits.
In the UHF measurement, attention should be paid to the matching of the input impedance of the test instrument and the impedance of the circuit under test, so as to avoid terminal reflection at the connection and cause waveform distortion.

Measuring frequency of test equipment

It should conform to the frequency range (or frequency response) of the power being measured. Otherwise, measurement errors will occur due to waveform distortion. For example: to measure AC voltage, use a multimeter to measure (sine wave), the frequency range is generally only 45~1000Hz, a few multimeters can only extend to about 100kHz, and a video transistor millivoltmeter can measure AC voltages of tens of megahertz.
The frequency range of the oscilloscope test signal is described by the frequency response of the Y axis. Generally, the frequency of a general-purpose oscilloscope can reach tens of MHz, the frequency of a broadband oscilloscope can reach about 100MHz, and the sampling oscilloscope can reach more than 1000MHz, while an ultra-low frequency oscilloscope can be used A voltage waveform below 0.1 Hz is observed.

How to configure PCB test instrument

Generally, general-purpose electronic measuring instruments have only one or several functions. Usually, PCBA testing usually requires multiple test instruments, auxiliary equipment, accessories, etc. to form a test booth or test system. For example, to test the nonlinear distortion of an amplifier, a test booth should be composed of a low-frequency signal generator, amplifier to be tested, distortion meter, DC stabilized power supply and other instruments.
What types of instruments and equipment should a test consist of is determined by the test plan. After the test plan is set, in order to ensure the normal operation and certain accuracy of the instrument, the following issues need to be paid attention to how to configure the PCBA test instrument.

The arrangement of various instruments should be convenient for observation

Observe the waveform or read the test results (data), the parallax should be small, not easy to fatigue (for example, pointer instruments should not be placed too high or too biased).
The arrangement of the instrument should be easy to operate

That is, the position of the adjustable knobs should be arranged according to the arrangement of the adjustable knobs on the panels of different instruments, so that the adjustment is convenient and comfortable. For example: For instruments that need to adjust the resonance or state, it should be convenient for the operator to use the right elbow to support the desktop, in order to achieve the purpose of flexible adjustment.

When stacking the instrument, pay attention to safety and stability
Put a small and light weight on it. In some instruments, high-power transistors are installed outside the casing, and care should be taken not to cause a short circuit when overlapping. For instruments with high power and high heat generation, pay attention to the heat dissipation of the instrument and the influence on the surrounding instruments.

The arrangement of the instrument should strive for the shortest wiring

For high gain, weak signal or high frequency measurement, special care should be taken not to close or cross the input and output wiring of the DUT to avoid signal crosstalk and parasitic oscillation.