1. Analysis of Passive Crystal Oscillator Characteristics
Passive crystal oscillators are passive devices, generally referring to quartz crystals. They are electronic components that use the piezoelectric effect of quartz crystals to generate high-precision oscillation frequencies.
The key parameters of the passive crystal oscillator are shown in the figure below.
Among the above parameters, those that have a direct impact on LoRa communication are as follows.
Load Capacitance (Load Capacitance) directly affects the oscillation frequency of the crystal oscillator. When developing and designing the LoRa module, it is necessary to base the 0ppm crystal oscillator sample (you can also use a crystal oscillator with a known certain frequency deviation), and adjust the frequency by welding and debugging capacitors with different values. Bias calibration, and finally determine a reasonable load capacitance value. In particular, Semtech's new LoRa chip has an internal adjustable load capacitor. The external load capacitor can be used by default, but it is recommended to reserve it.
Frequency Tolerance at room temperature refers to the maximum range of differences that may exist between all factory-made crystal oscillators at 25°C. This parameter is limited by the cutting angle of the crystal oscillator raw material wafer. The current processing level in the industry is around ±10ppm. If there is higher demand, you can ask the original crystal oscillator manufacturer to supply products within ±3ppm@25℃, but the purchase cost will increase.
Temperature drift frequency deviation (Frequency Stability) refers to the frequency deviation range of a single crystal oscillator in the operating temperature range of -40-85°C relative to 25°C. In particular, for the same model of passive crystal oscillator, within the range of -40-85°C, the frequency offset change trend of all factory crystal oscillators is the same, but the amplitude of change is different. It is generally believed that for a specific temperature, the maximum difference between the frequency offset variation amplitudes of all crystal oscillators (the range indicated by the arrow below) is 7-10ppm.
Operating Temperature: Under normal circumstances, the stable operating temperature required by the LoRa module is -40~85°C. Special note that this temperature refers to the ambient temperature. In actual use, it is also necessary to consider whether the PCB board level temperature is higher than the ambient temperature. If the temperature of the PCB board where the passive crystal oscillator is located is higher than the ambient temperature, heat dissipation measures need to be reserved or the PCB crystal oscillator part needs to be trenched and insulated, as shown in the figure below.
It is especially important to reserve measures for heat dissipation and trench insulation, especially for LoRa products that generate large amounts of heat such as external PA.
Aging rate (Aging), the aging rate refers to the annual frequency deviation change amplitude of the passive crystal oscillator. The frequency deviation change amplitude of the passive crystal oscillator in the first year is generally about ±3ppm. This parameter especially has a significant impact on LoRa module low-bandwidth (BW=62.5KHz and below) communication. If LoRa products must operate under low-bandwidth conditions, it is recommended to use an active temperature-compensated crystal oscillator to eliminate the impact of aging rate.
Static capacitance (Shunt Capacitance) refers to the parasitic capacitance inside the passive crystal oscillator. The typical value can generally be within 1pF. This capacitor is combined with the two load capacitances welded externally to the crystal oscillator on the PCB, and the parasitic capacitance of the crystal oscillator circuit on the PCB. Together, they determine the load capacitance of the crystal oscillator.
As shown in the circuit diagram below, it is clear that the calculation formula between C1, C2, CP, C0, and CL is: CL=(C1*C2)/(C1+C2)+CP+C0. Special instructions: 1.C1 and C2 should be as consistent as
possible
. , if not, the difference is generally required to be no more than about 2pF.
2.CP represents the parasitic capacitance of the crystal oscillator circuit on the PCB
. This formula also further explains why the frequency offset needs to be calibrated during the R&D and testing phase of module products, and the R&D samples must be ensured. It is processed in the same board factory as the mass-produced modules and uses the same processing technology. The main purpose is normalization processing, and at the same time, it is necessary to ensure production consistency and avoid variable factors causing abnormal module usage under extreme conditions.
2. Problems encountered when using passive crystal oscillators in LoRa products
At present, because the LoRa module uses a passive crystal oscillator, the following two points need to be paid attention to:
1. In the development stage of the LoRa module, the load capacitance values of the two actually welded crystal oscillators need to be debugged. This process is called frequency offset. calibration process.
Imagine that we mark each passive crystal oscillator to be welded with its nominal frequency offset value at 25°C. Then we use the debugged load capacitor to solder it to the PCB board and actually test it with a spectrum analyzer. The offset of the LoRa signal center frequency point should be the same as the nominal frequency offset of the crystal oscillator.
In addition, the frequency offset calibration process also has the purpose of ensuring that different models of LoRa modules can communicate normally under low bandwidth (62.5KHz and below). According to the requirements of LoRa communication, to ensure stable communication between two modules, at least the limit condition that must be met is to ensure that the frequency offset difference between the two communicating parties is within BW/4 (under 62.5KHz bandwidth, that is, it is required to be less than 15KHz, and it is recommended to leave a margin Generally 10-12KHz).
Then, the frequency deviation of the passive crystal oscillator at room temperature is ±10ppm, which is converted into frequency of ±5KHz@470MHz. In other words, we have selected a passive crystal oscillator to be used on a certain model of LoRa module. At room temperature of 25 degrees Celsius When used under 62.5KHz bandwidth, it has reached the limit of communication under the 62.5KHz bandwidth, not to mention the temperature drift difference in high and low temperature environments, the impact of aging rate, and different LoRa modules may use different types of passive crystal oscillators, etc.
Based on the above, the LoRa module is required to use a passive crystal oscillator and needs to perform frequency offset calibration. It is also recommended to use an active temperature-compensated crystal oscillator as much as possible, unless the LoRa module is used for communication, which requires the use of high-bandwidth configuration.
2. The frequency offset difference of the passive crystal oscillator cannot be very small, and there is also an obvious temperature drift phenomenon, which leads to high heat generation and poor heat dissipation in high-temperature environments (generally around 70°C) and in modules. Under low-bandwidth configuration (62.5KHz and below) communication abnormalities are prone to occur.
The following analysis process for high-temperature communication problems of different modules is used as an example.
The following figure shows the test results at rate 1 at a high temperature of 85°C. It can be found that there is also a situation where data cannot be received after working for a period of time (high temperature, frequent uninterrupted, large data volume communication).
The picture below shows a module from the manufacturer sending and receiving data at rate 1 at a high temperature of 75°C. The packet loss rate is about 20.8%.
In the figure below, when manufacturer 2 sends and receives data at a high temperature of 75°C at rate 1, the packet loss rate is about 7.3%.
The manufacturer shows the temperature rise of the crystal oscillator position of the transmitter module at a high temperature of 75°C.
Manufacturer 2 shows the temperature rise at the crystal oscillator of the transmitting module and the crystal oscillator of the receiving module at a high temperature of 75°C.
3. Things to note when using passive crystal oscillators in LoRa product design
Based on the above analysis, the LoRa module uses a passive crystal oscillator. The following points need to be noted:
1. During the research and development stage, the two external load capacitors used by the passive crystal oscillator must be debugged and calibrated.
2. Modules with high heat generation must be in Dig as large an empty slot as possible for the crystal oscillator part on the PCB for heat insulation treatment.
3. It is recommended to ask the original crystal oscillator manufacturer to provide passive crystal oscillator products within ±3ppm@25℃.
4. Reserve sufficient heat dissipation measures on LoRa products to avoid temperature rise. Too large (preferably controlled within 10°C)
5. Use a passive crystal oscillator. It is not recommended to use a low-bandwidth configuration (62.5KHz and below) for communication
6. If a low-bandwidth (62.5KHz and below) configuration must be used To communicate, it is necessary to reduce the amount of data sent in a single packet and reserve a long enough sending time interval to reserve sufficient cooling time. Through the analysis of this article, it can be found that due to the production process of the
passive crystal oscillator itself, each crystal oscillator The frequency offset difference is large and the drift is obvious with the ambient temperature. If the LoRa module itself generates a lot of heat and the heat dissipation is not good, it can easily lead to abnormal phenomena in the transmitting module and the receiving module during low-bandwidth communication.
The solution to the above problems is also very clear, which is to optimize the selection of passive crystal oscillators, try to use active temperature-compensated crystal oscillators, or reserve sufficient heat dissipation and insulation measures during PCB design to minimize the impact.