Precautions for the use of commonly used electronic instruments - Solutions - Huaqiang

Electronic scale crystal oscillator 3.2*2.5mm 3225 16M (16.000MHZ) 12PF 10PPM 20PPM 30PPM

Brand AVX TPSE226M035R0125 Low impedance tantalum capacitor AVX 22

Crystal oscillator

To use the instrument correctly, you must understand the general rules and common sense in the instrument. If you do not follow these rules, it will not necessarily lead to errors, but only in certain occasions or under certain circumstances will you get obvious wrong results. This often leads people to think that the rules or common sense in these measurements do not seem to be so strict or so useful, especially for students with insufficient practical experience. The following is a description and explanation of some common sense and precautions that should be understood in general instrument use.
1. Regarding the impedance of the instrument as a signal source, the output impedance is very low. The communication series instruments (such as high-frequency signal generators, etc.) are typically 50Ω, and the TV series instruments are typically 75Ω. For example, the sweep output of the sweeper or the RF output of the TV signal generator). Although some low-frequency signal generators also have output terminals of several hundred ohms output impedance, the output impedance of the terminal as a voltage output generally does not exceed 1KΩ (except for the power output terminal of the low-frequency signal generator). The reason why the output impedance of the signal source is generally low is because the signal source generates the signal. In the measurement process, it is necessary to couple its own signal to the circuit under test. If the impedance of the signal source is made low, it is easy to couple the signal generated by the signal source to the circuit under test with high input impedance. . In addition, for high-frequency measurement, since the impedance of the RF input terminal of the communication device and the television device is 50 Ω and 75 Ω, the output impedance of the instrument is set to 50 Ω and 75 Ω, and the required impedance matching can be satisfied during the measurement.
Generally, in low frequency measurement, it is not necessary to match impedance. In most cases, the input impedance of the circuit under test is much larger than the output impedance of the signal source. For the signal source, it is often equivalent to an open output (ie, no load). In the case of high frequency, it is generally necessary to match the impedance. Otherwise, due to the influence of the reflected wave, the amplitude of the signal coupled to the circuit under test is related to the length of the feeder, which may cause the amplitude of the signal coupled to the input of the circuit under test. This is different from the indicated value on the source, which can result in incorrect measurement results. This effect becomes significant when the measurement frequency rises to tens of megahertz or even hundreds of megahertz.
For example, for the sweeper, when the “Zero Decibel Correction” is performed, if the impedance does not match, in the lower frequency band, the scan line on the screen is straight (not the baseline), but at the higher frequency band. The scan line will become undulating. This is especially true for wideband measurements, which introduces large errors.
In addition, the signal amplitude of the signal source coupled to the circuit under test is different in the matched and unmatched states. The output amplitude indicated on the instrument panel is generally either the amplitude of the no-load output or the amplitude of the matched output. Determined by instrument use instructions or by actual measurement. If the input impedance of the circuit under test is not much larger than the output impedance of the source and does not match the output impedance of the source, then the panel indication of the source may not be used to determine the amplitude of the signal coupled to the circuit under test. To be determined by actual measurement.
As a voltmeter (such as a transistor millivoltmeter) or an oscilloscope, the instrument that takes signals from the circuit under test to measure, the general input impedance is high, typically 1MΩ, and some (such as oscilloscope) are also marked with input Capacitance (eg 25pF). The reason why their impedance is to be higher is because it can make them less affected by the circuit under test. However, when the output impedance of the circuit under test is large compared to their input impedance, the influence of the input impedance of the instrument on the circuit under test becomes significant, and the measurement results are often inaccurate (whenever encountered In this case, this is often easily overlooked by beginners).
For the input capacitance of the instrument, there is no significant impact on the measurement at low frequencies. But at high frequencies, sometimes you have to be careful. For example, if an oscilloscope is used to directly measure an unbuffered oscillator, since the capacitance at the input of the oscilloscope is directly connected in parallel to the oscillator under test, it will affect the operation of the oscillator, and the obtained measurement result will be inaccurate.
2. Avoid damage to the instrument During the operation of the instrument, incorrect operation may cause damage to the instrument. Moreover, this situation sometimes seems to be inexplicable. For instruments such as signal sources, you cannot short-circuit their outputs. Although short-circuiting the output of the voltage output terminal of the signal source generally does not damage the instrument, it should also develop the habit of not short-circuiting the output.
For the DC regulated power supply used in the laboratory, there is generally a protection circuit, and short-circuit short-circuit usually does not damage the instrument. However, even if there is no damage, the internal power supply of the regulated power supply is in a high-power state due to a short circuit, and it may not be able to stand for a long time, especially when the heat dissipation is poor. For the power output signal source or the power output terminal of the signal source, the output terminal cannot be short-circuited, otherwise it means the instrument is damaged. In use, not only can it not short-circuit its output, but it should not be overloaded (ie the impedance of the circuit under test is too low).
For instruments such as millivoltmeters or oscilloscopes, be aware that the voltage coupled to its input must not exceed its maximum allowable value. Such instruments are generally not damaged as a result of the fact that the maximum allowable value of their inputs tends to be large, with very few voltages coupled to their inputs exceeding the maximum allowable for their inputs. However, the frequency meter is different. Many frequency meters can work at a frequency of 1000MHz. To achieve such a wide frequency range, the tube used in the preamplifier must be a high frequency small power tube and its withstand voltage. The value is not large, and for some reason to work at such a high frequency, it is not easy to set a protection circuit at its input (this will cause its operating frequency to drop), so just feed a slightly larger voltage at its input. (for example, ten volts or less), it is very easy to cause damage to the tube in the front-end circuit, resulting in damage to the instrument.
3. There are many instruments in the grounding of the instrument case. The metal case itself is a conductor, and since it is often large, it is itself a special shape antenna, which is easy to receive electromagnetic interference from the space. The electromagnetic interference received by it is coupled to the instrument's circuitry through various channels, causing the instrument's output to be impure (ie, causing clutter output mixed with the wanted signal). In order to avoid such interference, instruments with a metal casing generally do not connect the casing to the ground wire inside the instrument, and the ground wire of the internal circuit of the instrument passes through the feeder connected to the circuit under test, and the ground of the circuit under test. Connected so that the interference is shorted to ground. However, some instruments have their housings not connected to the ground of their internal circuits, such as DC regulated power supplies, because when their output voltage is used as a positive power supply, then the negative terminal should be connected to the ground of the circuit under test; When the output voltage is output as a negative power supply, then the positive terminal should be connected to the ground of the circuit under test. At this time, its outer casing should not be connected to the positive terminal of the output terminal or to the negative terminal, so it is often A ground terminal is arranged on the instrument panel, and the ground terminal is neither connected to the positive terminal of the output terminal nor connected to the negative pole, and it is only connected to the outer casing. When in use, it should be connected to the ground of the circuit under test.

When measuring the whole machine, it is often necessary to use many instruments at the same time. In engineering, the outer casing of all the instruments is often connected by wires to prevent the interference introduced by the outer casing of the metal instrument. After the outer casings of the instrument are connected, the instrument is connected to the ground of the circuit under test through the feeder connected to the circuit under test, so as to achieve the shielding effect.
However, if the outer casing of the instrument is not connected to the ground of the circuit under test, it will not necessarily have a significant effect on the measurement results. This depends on whether it is a large signal measurement or a small signal measurement. Because the interference amplitude of the space electromagnetic radiation received by the instrument casing as an antenna is small, when the signal amplitude at the input end of the circuit under test is large (for example, tens or hundreds of millivolts or more), it is introduced by the instrument casing. The interference is negligible and there is no impact on the measurement results. However, when the signal amplitude at the input of the circuit under test is small, the influence of the interference becomes significant, and the measurement result will be inaccurate.
4. Probes and Feeders Each instrument has its own probe or feeder. Some instruments have some kind of circuit (such as attenuator, detector, etc.) in the probe. Such instrument probes are generally not interchangeable with the probes of other instruments. In low frequency measurements, the use of probes or feeders is less stringent, but in high frequency measurements, the use of probes or feeders is much more stringent. The first is the matching problem. For example, there are two kinds of feeders at the sweep output of the sweeper: one is that there is no matching resistor, and the other is that there is a matching resistor. When using, determine the feeder to be used according to the input impedance of the circuit under test. For any instrument, no two wires can be used in the high frequency measurement instead of the matching cable. In addition, some feeders or probe pins are short. This is because the probe of the probe cannot be made too long in high-frequency measurement, otherwise the measurement result will be affected, so the probe should not be lengthened at will. However, in low-frequency measurements (for example, within 1MHz), the probe lengthening has little effect on the measurement results.
In the use of a regulated power supply, the feeder is a general conductor. However, if a high-voltage circuit is used to supply power to the high-frequency circuit, the longer the conductor exhibits a large inductive reactance at the high frequency, which causes the internal resistance of the power supply to increase (the high-frequency internal resistance of the regulated power supply is inherently higher than The low-frequency internal resistance is much larger, and the internal resistance index refers to the low-frequency internal resistance. In order to reduce the influence of the feeder on the actual internal resistance of the power supply, it is often necessary to connect the small-capacitance capacitor decoupled in parallel with the power supply end of the circuit under test. This is necessary for circuits that require a slightly higher level, such as oscillators with higher frequency stability.