1. Use a multimeter (R×10k block) to measure the resistance value at both ends of the crystal oscillator. If it is infinite, it means that the crystal oscillator has no short circuit or leakage; then insert the test pen into the electrical jack, pinch any pin of the crystal oscillator with your fingers, and touch the other pin to the metal part at the top of the test pen. If the neon bulb of the test pen turns red, it means that the crystal oscillator is good; if the neon bulb does not light up, it means that the crystal oscillator is damaged.

2. Use a digital capacitance meter (or the capacitance range of a digital multimeter) to measure its capacitance. Generally, the capacity of a damaged crystal oscillator is significantly reduced (different crystal oscillators have a certain range of normal capacity that can be measured), and the good ones are generally tens to hundreds of PF.

3. Shake it close to your ear. If there is a sound, it must be bad (the internal crystal has been broken, and if it can still be used, the frequency has also changed).

4. Test the output pin voltage. Under normal circumstances, it is about half of the power supply voltage. Because the output is a sine wave (the value is close to the source voltage), when tested with a multimeter, it is almost half.

5. Use the substitution method or oscilloscope to measure.

Quartz crystal resonators can be divided into HC-49U, HC-49U/S, HC-49U/S·SMD, UM-1, UM-5 and columnar crystals according to their appearance and structure.

HC-49U is suitable for electronic products with wide space such as communication equipment, televisions, telephones, and electronic toys.

HC-49U/S is suitable for various thin and small electronic devices and products with limited space height. HC-49U/S·SMD is a quasi-surface mount product, suitable for various ultra-thin and small computers and electronic devices.

Columnar quartz crystal resonators are suitable for stable frequency timing electronic products with narrow space such as timers, electronic clocks, calculators, etc. UM series products are mainly used in mobile communication products such as BP machines, mobile phones, etc.

In our daily life, we often use two types of crystal oscillators, constant temperature and temperature compensation. But under what circumstances will constant temperature be used and when will temperature compensation be used?

Constant temperature crystal oscillator and temperature compensation crystal oscillator are both crystal oscillators, and both are active crystal oscillators, so the composed oscillation circuits need power supply to work. The following will briefly introduce the difference between the two.

1. From the definition of the two:

Constant temperature crystal oscillator is referred to as constant temperature crystal oscillator, abbreviated as OCXO in English. It uses a constant temperature tank to keep the temperature of the quartz crystal resonator in the crystal oscillator constant, and reduces the oscillator output frequency change caused by ambient temperature changes to the minimum. OCXO is composed of a constant temperature tank control circuit and an oscillator circuit. Usually people use a differential series amplifier composed of a thermistor "bridge" to achieve temperature control. Temperature compensated crystal oscillator is referred to as temperature compensated crystal oscillator, abbreviated as TCXO in English, which is a quartz crystal oscillator that reduces the oscillation frequency change caused by ambient temperature changes through an additional temperature compensation circuit. The term temperature compensated crystal oscillator comes from a compensation method of quartz crystal oscillator that has achieved the accuracy requirements of product applications. The definition of temperature-compensated crystal oscillator is a quartz crystal oscillator made by compensating the original physical properties of piezoelectric quartz crystal (frequency changes with temperature in a cubic curve under piezoelectric effect) through the reverse change of the peripheral circuit, so that the original frequency of the quartz crystal changes with temperature and becomes as small as possible.

2. From the working principle of the two:

For constant temperature crystal oscillator, since the oscillation frequency of the crystal oscillator changes with the change of temperature, in order to maintain the stability of the frequency, the crystal oscillator is controlled to work at a constant temperature to improve the phase-frequency characteristics of the crystal oscillator. For temperature-compensated crystal oscillator, since the oscillation frequency of the crystal oscillator changes with the change of temperature, in order to offset the influence of temperature on the crystal oscillator frequency, the resonant capacitance of the crystal oscillator is controlled to change with the change of temperature, offsetting the influence of temperature crystal and improving the frequency stability.

3. From the measurement accuracy of the two:

General constant temperature crystal oscillator has a frequency stability more than two orders of magnitude higher than that of temperature-compensated crystal oscillator. For example, temperature-compensated crystal oscillator can generally reach the order of -7, while constant temperature crystal oscillator can reach the order of -9. Therefore, constant temperature crystal oscillators are generally used in high-end measuring instruments, such as frequency meters, signal generators, network analyzers, etc. The startup characteristics of temperature compensated crystal oscillators are better. Even if the constant temperature crystal oscillator uses the best heating element now, it still needs a heating process. To reach the -7 level, it takes at least 5 minutes, and to reach the -9 level or above, it may even take a day. Therefore, it is not suitable for equipment that needs to work as soon as it is turned on. General constant temperature crystal oscillators can be made more accurate and better than temperature compensated crystal oscillators. Whether it is a constant temperature crystal oscillator or a temperature compensated crystal oscillator, it is nothing more than a signal source, providing a time base for your device. As long as you understand its performance indicators, you can use it interchangeably.

Quartz is a device that is used to carry out piezoelectric properties. The piezoelectric properties of quartz crystal are as follows: If the electrodes of a piezoelectric quartz crystal are placed in opposite directions and a voltage is applied between the electrodes, a strong pressure acts on the electric charges inside the crystal. If the crystal is mounted correctly but the inside of the crystal is deformed, an electromechanical system is formed, which, if it can be properly excited, will form an oscillating frequency.

The crystal oscillator that people often talk about is generally called a crystal resonator. It is an electromechanical device. It is made of quartz crystal with very low power loss, which is precisely cut and ground, plated with electrodes and welded with leads. This crystal has a very important characteristic. If it is powered, it will produce mechanical oscillations. Conversely, if it is given mechanical force, it will generate electricity. This characteristic is called electromechanical effect. They have a very important characteristic that their oscillation frequency is closely related to their shape, material, cutting direction, etc. Since the chemical properties of quartz crystal are very stable and the thermal expansion coefficient is very small, its oscillation frequency is also very stable. Since the control of geometric dimensions can be very precise, its resonant frequency is also very accurate.

According to the electromechanical effect of quartz crystal, we can equate it to an electromagnetic oscillation circuit, that is, a resonant circuit. Their electromechanical effect is the continuous conversion of machine-electric-machine-electric..., and the resonant circuit composed of inductance and capacitance is the continuous conversion of electric field-magnetic field. The application in the circuit is actually to treat it as a high-Q electromagnetic resonant circuit. Since the loss of quartz crystal is very small, that is, the Q value is very high, when used as an oscillator, it can produce very stable oscillation, and when used as a filter, it can obtain a very stable and steep bandpass or bandstop curve. Knowledge about crystal resonators can be found in some basic circuit theory books.

The difference between ceramic resonators and quartz resonators lies in their accuracy and temperature stability. Quartz crystal oscillators have higher accuracy and better temperature stability than ceramic crystal oscillators. The accuracy of quartz crystal oscillators can reach six decimal places, and the unit is ppm (parts per million). For example, the error of the 4M, 11.0592M quartz crystal oscillator you use is generally less than +/-30ppm, that is, its accuracy is between 30 parts per million. The accuracy of ceramic resonators can only meet three decimal places, expressed in khz, such as a 4MHz ceramic resonator. Its accuracy is generally +/-750kHz.

In terms of technical parameters, quartz resonators can replace ceramic resonators, but ceramic resonators may not replace quartz resonators. Ceramic resonators are mostly used in TV remote controls, toys and other products that do not require high accuracy, while quartz resonators are needed in consumer electronic products such as instruments, communications, etc. where accuracy is required. At the same frequency point, their prices are closely related to the package (plug-in or SMD). When they are both DIP or SMD packages, the price of ceramic resonators is much lower than that of quartz resonators.

Nominal frequency: the center frequency or nominal value of the frequency output by the oscillator.

Frequency accuracy: the deviation of the oscillator output frequency from the nominal frequency at room temperature (25℃±2℃).

Adjusted frequency difference: the maximum allowable frequency deviation of the oscillator output frequency from the measured value at 25℃ within the specified temperature range.

Load resonant frequency (fL): one of the two frequencies when the crystal is connected in series or in parallel with a load capacitor under specified conditions, and its combined impedance is resistive (resistance occurs). When the load capacitor is connected in series, the load resonant frequency is the lower of the two frequencies, and when the load capacitor is connected in parallel, it is the higher of the two frequencies.

Static capacitance: the capacitor connected in parallel with the series arm in the equivalent circuit, also called parallel capacitance, usually represented by C0.

Operating temperature range: the temperature range that can ensure that the oscillator output frequency and its various characteristics meet the specifications.

Frequency temperature stability: the maximum allowable frequency deviation without implicit reference temperature or with implicit reference temperature under nominal power supply and load, working within the specified temperature range.

ft=±(fmax-fmin)/(fmax+fmin)

ftref＝±MAX[｜(fmax-fref)/fref｜，｜(fmin-fref)/fref｜]

ft: Frequency temperature stability (without implicit reference)

ftref: Frequency temperature stability (with implicit reference temperature)

fmax: Maximum frequency measured within the specified temperature range

fmin: Minimum frequency measured within the specified temperature range

fref: Frequency measured at the specified reference temperature

Note: The production difficulty of the crystal oscillator using the ftref index is higher than that of the crystal oscillator using the ft index, so the price of the crystal oscillator using the ftref index is higher.

Load capacitance: The effective external capacitance that determines the load resonant frequency FL together with the crystal, represented by CL.

Load capacitance series: 8PF 12PF 15PF 20PF 30PF 50PF 100PF

Excitation level: The characteristic value of the power consumed by the crystal when it is working. The optional values ​​of the excitation level are: 2mW, 1mW, 0.5mW, 0.2mW, 0.1mW, 50μW, 20μW, 10μW, 1μW, 0.1μW, etc.

Aging rate: the relative change of the output frequency within a certain time.

Fundamental frequency: the vibration frequency of the lowest order in the vibration mode.

Overtone: the mechanical harmonic of crystal vibration. The ratio of the overtone frequency to the fundamental frequency is close to an integer multiple but not an integer multiple, which is the main difference between it and electrical harmonics. Overtone vibrations include 3rd overtone, 5th overtone, 7th overtone, 9th overtone, etc.

Recently, I changed the crystal oscillator of the circuit to 4M, which of course also involved many problems. I used a ceramic crystal oscillator before, and now I need to update it to a better metal crystal oscillator. Of course, I also know that there are better crystal oscillators, but I didn't use them because of the cost. After changing the crystal oscillator [when there was no problem in the experiment, production was carried out, but problems still occurred one after another] two more important fault phenomena appeared: ① The crystal oscillator has a 3% probability of not oscillating ② The crystal oscillator is unstable, oscillating for a while and not oscillating for a while, and the circuit freezes.

The retrieved circuit board was analyzed and tested with an oscilloscope. It is indeed not oscillating, so I went online to read a lot of people's opinions on this aspect. Now our problem has been solved. The problem is that the crystal oscillator does not match the crystal oscillator connected in series with the ground. The power of the microcontroller determines the power P1 of the crystal oscillator, and the size of the capacitor is derived from the power of the microcontroller. It is appropriate to choose a suitable one that matches the crystal oscillator. Please note that the matching of P1 and capacitor complies with p1=c1*c2/c1+c2+p2 [P2 here is generally a value of 3~5p]. The larger the value of the selected capacitor, the stronger the power consumption of the microcontroller. And it is easy to cause the situation of non-oscillation.

The following is a summary of some discussions on the forum: it is worth referring to.

* This problem has troubled many technicians. I have also done a detailed analysis. The main considerations are: 1 The dynamic impedance problem of the two ends of the crystal oscillator at work. This impedance has a certain range, so a resistor of several hundred K will be connected in parallel during design to stabilize the dynamic impedance; 2 The matching of the resonant capacitor; 3 The temperature of the soldering iron is too high during welding

* The main function of the matching capacitor of the crystal oscillator is to match the crystal oscillator and the oscillation circuit, so that the circuit is easy to start and is in a reasonable excitation state, and it also has a certain "fine-tuning" effect on the frequency. For MCU, the key to correctly select the matching capacitor of the crystal oscillator is to fine-tune the excitation state of the crystal to avoid over-excitation or under-excitation. The former makes the crystal easy to age and affects the service life and causes the EMC characteristics of the oscillation circuit to deteriorate, while the latter is not easy to start and the operation is unstable, so it is very important to correctly select the crystal matching capacitor.

*Quartz crystal oscillators are divided into several types, including non-temperature compensated crystal oscillators, temperature compensated crystal oscillators (TCXO), voltage controlled crystal oscillators (VCXO), constant temperature controlled crystal oscillators (OCXO) and digital/μp compensated crystal oscillators (DCXO/MCXO). Among them, the non-temperature compensated crystal oscillator is the simplest one. In the Japanese Industrial Standard (JIS), it is called a standard package crystal oscillator (SPXO).

Yours may be a TCXO type. How about adding a thermistor and a capacitor in series between the crystal oscillators?

*The difference, application range and usage of passive crystals and active crystal oscillators:

1. Passive crystal-passive crystals need to use the oscillator in the DSP chip, and there is a recommended connection method on the datasheet. Passive crystals do not have voltage problems, and the signal level is variable, that is, it is determined by the oscillator circuit. The same crystal can be applied to a variety of voltages and can be used for a variety of DSPs with different clock signal voltage requirements. The price is usually low, so for general applications, if conditions permit, it is recommended to use crystals, which is especially suitable for producers with rich product lines and large batches. The disadvantage of passive crystals compared to crystal oscillators is that the signal quality is poor, and usually requires precise matching of peripheral circuits (capacitors, inductors, resistors, etc. for signal matching). When replacing crystals of different frequencies, the peripheral configuration circuit needs to be adjusted accordingly. It is recommended to use quartz crystals with higher precision, and try not to use ceramic crystals with low precision.

2. Active crystal oscillator - Active crystal oscillator does not require the internal oscillator of DSP, has good signal quality, is relatively stable, and has a relatively simple connection method (mainly to do a good job of power supply filtering, usually using a PI-type filter network composed of capacitors and inductors, and a small resistance resistor is used at the output end to filter the signal), and does not require a complex configuration circuit. The usual usage of active crystal oscillators: one foot is suspended, two feet are grounded, three feet are connected to the output, and four feet are connected to the voltage. Compared with passive crystals, the disadvantage of active crystal oscillators is that their signal level is fixed, and it is necessary to select the appropriate output level, which is less flexible and more expensive. For applications with sensitive timing requirements, I personally think that active crystal oscillators are better because you can choose more precise crystal oscillators, or even high-end temperature compensated crystal oscillators. Some DSPs do not have an internal oscillator circuit and can only use active crystal oscillators, such as TI's 6000 series. Active crystal oscillators are usually larger than passive crystals, but now many active crystal oscillators are surface-mounted, with a size comparable to that of crystals, and some are even smaller than many crystals.