Summary of basic application knowledge of test instruments (I)

    　Electronic engineers cannot do without the instruments used for testing and measurement in their daily electronic design. How to use these test instruments accurately to improve design efficiency and shorten product design cycles has become a necessary skill for qualified electronic engineers. In order to provide engineers with more comprehensive application knowledge related to measuring instruments, for learning, reference, or reviewing, Electronics Enthusiasts will continue to integrate and launch a series of chapters on "Summary of Basic Application Knowledge of Test Instruments". Please pay attention.

　　1. How to debug a newly designed circuit board

　　For a newly designed circuit board, debugging often encounters some difficulties, especially when the board is large and has many components, it is often difficult to start. But if you master a set of reasonable debugging methods, debugging will be twice the result with half the effort. For the new PCB board just brought back, we must first roughly observe whether there are any problems on the board, such as whether there are obvious cracks, short circuits, open circuits, etc. If necessary, you can check whether the resistance between the power supply and the ground wire is large enough.

　　Then it is time to install the components. If you are not sure that the independent modules are working properly, it is best not to install them all at once, but install them one by one (for smaller circuits, you can install them all at once). This makes it easier to determine the scope of the fault, so as to avoid being at a loss when encountering problems. Generally speaking, you can install the power supply first, and then power on to test whether the power output voltage is normal. If you are not very sure when powering on (even if you are very sure, it is recommended that you add a fuse just in case), you can consider using an adjustable regulated power supply with current limiting function. First preset the overcurrent protection current, then slowly increase the voltage value of the regulated power supply, and monitor the input current, input voltage and output voltage. If there are no problems such as overcurrent protection during the upward adjustment, and the output voltage has reached normal, it means that the power supply is OK. Otherwise, disconnect the power supply, find the fault point, and repeat the above steps until the power supply is normal.

　　Then gradually install other modules, and power on each module for testing. Follow the above steps when powering on to avoid overcurrent and component burnout due to design errors and/or installation errors.

　　There are generally several ways to find the fault:

　　① Voltage measurement method. First, check whether the voltage of each chip power pin is normal, then check whether various reference voltages are normal, and whether the working voltage of each point is normal. For example, when a general silicon transistor is turned on, the BE junction voltage is about 0.7V, and the CE junction voltage is about 0.3V or less. If the BE junction voltage of a transistor is greater than 0.7V (except for special transistors, such as Darlington transistors, etc.), it is possible that the BE junction is open.

　　②Signal injection method. Add the signal source to the input terminal, and then measure the waveform of each point in turn to see if it is normal, so as to find the fault point. Sometimes we also use a simpler method, such as holding a pair of tweezers in hand to touch the input terminal of each level to see if there is a response at the output terminal. This is often used in audio, video and other amplification circuits (but be aware that this method cannot be used in circuits with hot base plates or high voltages, otherwise it may cause electric shock). If there is no response when touching the previous level, but there is a response when touching the next level, it means that the problem lies in the previous level and should be checked in detail.

　　③ Of course, there are many other ways to find the fault point, such as looking, listening, smelling, touching, etc. "Looking" means to see if the component has obvious mechanical damage, such as cracking, burning, deformation, etc.; "Listening" means to listen to whether the working sound is normal, such as some things that should not be ringing are ringing, the places that should be ringing are not ringing or the sound is abnormal, etc.; "Smelling" means to check whether there is any odor, such as the smell of burning, the smell of capacitor electrolyte, etc. For an experienced electronic maintenance personnel, these odors are very sensitive; "Touching" means to use your hands to test whether the temperature of the device is normal, such as too hot or too cold. Some power devices will heat up when working. If they feel cool to the touch, it can basically be judged that it is not working. But if the place that should not be hot is hot or the place that should be hot is too hot, it is not okay. For general power transistors, voltage regulator chips, etc., it is completely fine to work below 70 degrees. What is the concept of 70 degrees? If you can hold your hand for more than three seconds, it means the temperature is probably below 70 degrees (note to touch it tentatively first to avoid burning your hand).

　　2. Several important indicators for selecting electronic testing instruments

　　Take digital oscilloscopes as an example. Many users may know some traditional indicators of oscilloscopes, such as bandwidth, sampling rate, memory depth, etc. Some even compare the indicators when selecting models, thinking that larger indicators are better than smaller ones. This is not the case! To truly understand digital oscilloscopes, we must deeply understand the true performance and quality of the products hidden behind the nominal indicators, just like many consumers often care about the number of pixels when buying digital cameras. In fact, in addition to this "number", there are many (more) important indicators and even materials that need to be considered.

　　These parameters are very important in terms of scalability, the number of communication standards supported, test accuracy, dynamic range and demodulation bandwidth. In the future, base stations may evolve towards dual-mode and multi-mode. Many mobile phones already have multi-mode functions, such as GSM and WCDMA dual-mode mobile phones. If the instrument supports more communication standards, the variety and quantity of instruments to be purchased will be greatly reduced. In addition, with the emergence of technologies such as 3G and LTE, higher requirements are placed on instruments. Instruments with high test accuracy, large dynamic range and large demodulation bandwidth are very popular. Mobile communication technology is developing rapidly. At present, there is no large-scale commercial use of 3G in China, and LTE, as a follow-up technology of WCDMA and TD-SCDMA, is about to launch a prototype. Network operators may accelerate the introduction of new technologies, which is indeed a challenge for base station and terminal manufacturers: the test instruments they purchase now must have good scalability and can be easily upgraded to future technologies, so as to protect the manufacturer's investment to the greatest extent.

　　In addition, the bandwidth and sampling rate of the oscilloscope are common parameters of the oscilloscope. Due to the development of manufacturing and R&D technology, the bandwidth of the oscilloscope can be corrected and compensated. However, these corrections and compensations are not always good things. Some customers do not want to bring these technologies into the test. They need more original test data, such as radar experiments. At present, Tektronix provides pure hardware oscilloscopes in the full range of 1G~2G oscilloscopes. The oscilloscope bandwidth is the most real. Tektronix's front-end technology can ensure that the hardware preamplifier of the oscilloscope is good enough. The sampling rate is an indicator of ADC. The capture rate parameter reflects a concept of memory management (whether the required signal can be found in the saved signal). Tektronix uses segmented management to save information when the signal jumps. Including Inspector and other methods.
 

　　3. Two methods to test battery power

　　There are usually two ways to detect whether the ordinary zinc-manganese dry battery has sufficient power. The first method is to estimate the internal resistance of the battery by measuring the instantaneous short-circuit current of the battery, and then determine whether the battery has sufficient power; the second method is to use an ammeter in series with a resistor of appropriate resistance, and calculate the internal resistance of the battery by measuring the discharge current of the battery, so as to determine whether the battery has sufficient power.

　　The biggest advantage of the first method is that it is simple. You can directly determine the battery power by using the high current range of the multimeter. The disadvantage is that the test current is very large, far exceeding the limit of the allowable discharge current of the dry battery, which affects the service life of the dry battery to a certain extent. The advantage of the second method is that the test current is small, the safety is good, and it generally does not have an adverse effect on the service life of the dry battery. The disadvantage is that it is more troublesome.

　　The author used the MF47 multimeter to test a new No. 2 dry cell and an old No. 2 dry cell using the above two methods. Assuming ro is the internal resistance of the dry cell, RO is the internal resistance of the ammeter, when using the second test method, RF is the additional series resistance, with a resistance of 3Ω and a power of 2W.

　　The measured results are as follows. The new No. 2 battery E=1.58V (measured with 2.5V DC voltage range), the internal resistance of the voltmeter is 50kΩ, which is much larger than ro, so it can be approximately considered that 1.58V is the electromotive force of the battery, or the open circuit voltage. When using the first method, the multimeter is set to the 5A DC current range, the internal resistance of the meter RO=0.06Ω, and the measured current is 3.3A. Therefore, ro+RO=1.58V÷3.3A≈0.48Ω, ro=0.48-0.06=0.42Ω. When using the second method, the measured current is 0.395A, RF+ro+RO=1.58V÷0.395A=4Ω, the internal resistance of the current 500mA range is 0.6Ω, so ro=4-3-0.6=0.4Ω.

　　When the old No. 2 battery is measured by the first method, the open circuit voltage E=1.2V is measured first, the internal resistance of the meter RO=6Ω, the reading is 6.5mA, the multimeter is set to 50mA DC current range, ro+RO=1.2V÷0.0065A≈184.6Ω, ro=184.6-6=178.6Ω. Using the second method, the current is measured to be 6.3mA, ro+RO+RF=1.2V÷0.0063A=190.5Ω, ro=190.5-6-3=181.5Ω.

　　Obviously, the results of the two test methods are basically the same. The slight difference in the final calculation results is caused by many factors such as reading error, resistance RF error and contact resistance. This slight error will not affect the judgment of the battery power. If the capacity of the battery being tested is small and the voltage is high (such as 15V, 9V laminated battery), the resistance value of RF should be increased accordingly.

　　4. Test instrument selection: How to choose the appropriate oscilloscope bandwidth

　　Bandwidth is the first parameter most engineers consider when choosing an oscilloscope. This article will provide you with some useful tips on how to choose the right oscilloscope bandwidth for your digital and analog applications. But first, let’s look at the definition of oscilloscope bandwidth.

　　Definition of Oscilloscope Bandwidth

　　All oscilloscopes exhibit a low-pass frequency response that rolls off at higher frequencies as shown in Figure 1. Most oscilloscopes with bandwidth specifications of 1 GHz and below typically exhibit a Gaussian response, with a slow roll-off starting at about one-third of the -3 dB frequency. Oscilloscopes with bandwidth specifications exceeding 1 GHz typically have a maximally flat frequency response, as shown in Figure 2. This frequency response typically exhibits a relatively flat response in-band, with a steeper roll-off at about the -3 dB frequency.

　　

　　Figure 1: Low-pass frequency response

　　

　　Figure 2: Maximally flat frequency response

　　Each of these two frequency responses of an oscilloscope has its own advantages and disadvantages. An oscilloscope with a maximally flat frequency response will attenuate in-band signals less than an oscilloscope with a Gaussian frequency response, which means that the former will measure in-band signals more accurately. However, an oscilloscope with a Gaussian frequency response will attenuate out-of-band signals less than an oscilloscope with a maximally flat frequency response, which means that under the same bandwidth specification, an oscilloscope with a Gaussian frequency response will generally have a faster rise time. However, sometimes the attenuation of out-of-band signals can help eliminate high-frequency components that may cause aliasing according to the Nyquist criterion (fMAX < fS). For a more in-depth discussion of Nyquist sampling theory, see Agilent Application Note 1587.

　　Whether your oscilloscope has a Gaussian frequency response, a maximally flat frequency response, or something in between, we consider the lowest frequency at which the input signal is attenuated by 3 dB after passing through the oscilloscope to be the bandwidth of the oscilloscope. The bandwidth and frequency response of an oscilloscope can be measured using a sine wave signal generator sweep. The attenuation of the signal at the -3dB frequency of the oscilloscope can be converted to an amplitude error of approximately -30%. Therefore, we cannot expect to accurately measure signals whose main frequency components are close to the bandwidth of the oscilloscope.

　　Closely related to the oscilloscope bandwidth specification is its rise time parameter. An oscilloscope with a Gaussian frequency response, measured from the 10% to 90% standard, has a rise time of about 0.35/fBW. Oscilloscopes with a maximally flat frequency response typically have a rise time specification in the 0.4/fBW range, depending on the steepness of the oscilloscope's frequency roll-off characteristics. But we must remember that the oscilloscope's rise time is not the fastest edge speed that the oscilloscope can accurately measure, but the fastest edge speed the oscilloscope can achieve when the input signal has a theoretically infinitely fast rise time (0 ps). Although this theoretical parameter is impossible to measure in practice because a pulse generator cannot output a pulse with an infinitely fast edge, we can measure the oscilloscope's rise time by inputting a pulse with an edge speed 3 to 5 times the oscilloscope's rise time specification.
 

　　Oscilloscope bandwidth required for digital applications

　　A rule of thumb is that the bandwidth of an oscilloscope should be at least 5 times greater than the fastest digital clock rate of the system being measured. If the oscilloscope we choose meets this criteria, then the oscilloscope will be able to capture the 5th harmonic of the signal being measured with minimal signal attenuation. The 5th harmonic of a signal is very important in determining the overall shape of a digital signal. However, if high-speed edges need to be accurately measured, this simple formula does not take into account the actual high-frequency content contained in fast rising and falling edges.

　　Formula: fBW ≥ 5 x fclk

　　A more accurate way to determine the bandwidth of an oscilloscope is to base it on the highest frequency present in the digital signal, rather than the maximum clock rate. The highest frequency of a digital signal is determined by the fastest edge speed in the design. Therefore, we first need to determine the rise and fall times of the fastest signal in the design. This information can usually be obtained from the public data sheets of the devices used in the design.

　　Step 1: Determine the fastest edge speed

　　Then you can use a simple formula to calculate the maximum "real" frequency component of the signal. Dr. Howard W. Johnson wrote a book on this subject, High Speed ​​Digital Design. In it, he calls this frequency component the "knee" frequency (fknee). All fast edges have an infinite number of frequency components in their spectrum, but there is a point of inflection (or "knee") above which the frequency components are insignificant in determining the shape of the signal.

　　Step 2: Calculate fknee

　　fknee = 0.5/RT （10% - 90%）

　　fknee = 0.4/RT （20% - 80%）

　　For signals with a rise time characteristic defined by a threshold of 10% to 90%, the knee frequency fknee is equal to 0.5 divided by the rise time of the signal. For signals with a rise time characteristic defined by a threshold of 20% to 80% (as is often done in device specifications today), fknee is equal to 0.4 divided by the rise time of the signal. But be careful not to confuse the signal rise time here with the rise time specification of the oscilloscope; we are talking about the actual signal edge speed here.

　　The third step is to determine the oscilloscope bandwidth required to measure the signal based on the accuracy required to measure the rise and fall times. Table 1 shows the relationship between the oscilloscope bandwidth required and fknee for various accuracy requirements for oscilloscopes with Gaussian or maximally flat frequency response. However, it is important to remember that most oscilloscopes with bandwidth specifications of 1 GHz and below are usually Gaussian frequency response types, while those with bandwidths above 1 GHz are usually maximally flat frequency response types.

　　Table 1: Factors for calculating the required bandwidth of an oscilloscope based on the accuracy required and the type of oscilloscope frequency response

　　

　　Step 3: Calculate the Oscilloscope Bandwidth

　　Let's explain this with a simple example:

　　Determine the minimum bandwidth required for an oscilloscope to have a correct Gaussian frequency response when measuring a 500ps rise time (10-90%)

　　If the rise/fall time of a signal is approximately 500 ps (defined by the 10% to 90% standard), then the maximum actual frequency component of the signal (fknee) is approximately 1 GHz.

　　fknee = （0.5/500ps） = 1 GHz

　　If a 20% timing error is allowed when measuring rise time and fall time parameters, then an oscilloscope with a bandwidth of 1 GHz will meet the requirements of this digital measurement application. However, if the timing accuracy is required to be within 3%, then an oscilloscope with a bandwidth of 2 GHz will be better.

　　20% timing accuracy:

　　Oscilloscope bandwidth = 1.0 x 1 GHz = 1.0 GHz

　　3% timing accuracy:

　　Oscilloscope bandwidth = 1.9 x 1 GHz = 1.9 GHz

　　Next we will use several oscilloscopes with different bandwidths to measure a digital clock signal with similar characteristics to the signal in this example.

　　Comparison of measurements of the same digital clock signal using oscilloscopes with different bandwidths

　　FIG3 shows the waveform result of measuring a 100MHz digital clock signal with an edge speed of 500ps (from 10% to 90%) using Agilent's MSO6014A oscilloscope with a bandwidth of 100MHz.

　　

　　image 3

　　As can be seen from the figure, the oscilloscope mainly passes only the 100MHz basic frequency component of the clock signal, so the clock signal appears to be in the shape of a sine wave. An oscilloscope with a bandwidth of 100MHz may be very suitable for many MCU-based 8-bit designs with clock rates ranging from 10MHz to 20MHz, but it is obviously not enough for the 100MHz clock signal measured here. Figure 4 shows the results of measuring the same signal using Agilent's 500MHz bandwidth oscilloscope MSO6054A.

　　

　　Figure 4

　　As can be seen from the figure, the oscilloscope can capture up to the fifth harmonic of the signal, which just meets the first empirical suggestion we gave above. However, when we measured the rise time, we found that the rise time measured by this oscilloscope was about 750ps. In this case, the oscilloscope's measurement of the signal rise time is not very accurate. The measurement result it obtains is actually very close to its own rise time (700ps), rather than the rise time of the input signal (close to 500ps). This shows that if timing measurement is more important, then we need to use an oscilloscope with a higher bandwidth to meet the requirements of this digital measurement application.

　　After switching to Agilent's 1-GHz bandwidth oscilloscope MSO6104A, we get a more accurate signal image (see Figure 5).

　　

　　Figure 5

　　After selecting the rise time measurement in the oscilloscope, we get a measurement result of about 550ps. This measurement result has an accuracy of about 10%, which is very satisfactory, especially when considering the cost of the oscilloscope. However, sometimes even this measurement result obtained by a 1GHz bandwidth oscilloscope may be considered inaccurate. If we require an edge speed measurement accuracy of 3% for this signal with an edge speed of 500ps, then we need an oscilloscope with a bandwidth of 2GHz or higher, which we have already mentioned in the previous example.

　　After switching to an oscilloscope with a 2 GHz bandwidth, what we see now (see Figure 6) is a more accurate clock signal, with a rise time measurement of approximately 495 ps.

　　

　　Figure 6

　　Agilent Infiniium series high bandwidth oscilloscope has an advantage, that is, the bandwidth can be upgraded. If 2 GHz bandwidth is enough for today's application, you can just buy an entry-level 2-GHz oscilloscope at first, and then gradually upgrade it to 13 GHz when you need higher bandwidth.