In the world of electronics, precise clock pulse generators are at the heart of many applications – from automotive engineering to industrial automation. While integrated circuits (ICs) generally work with a wide range of timing components, crystal units are often the best choice due to their high frequency stability and reliability. However, the full potential of these components can only be realized by carefully matching the peripheral circuit.
What is a crystal unit?
A crystal unit is a passive component based on the piezoelectric effect: When voltage is applied to a quartz crystal, it begins to oscillate. These mechanical oscillations are converted into electrical signals and provide a precise frequency. However, a suitable oscillator circuit is required to produce these oscillations. The actual crystal determines the clock frequency of the entire system.
Challenges of circuit matching
Correct functioning of a crystal unit depends largely on the design of the surrounding circuit (Fig. 1). The key parameters are:
- Load capacitances (C1, C2) influence the resonance frequency and the stability.
- Feedback resistance (RF) ensures the necessary amplification.
- Damping resistance (RD) prevents unwanted oscillations.
- Layout-related parasitic capacitances may impact the total load capacitance.
Incorrect scaling of these components can lead to malfunctions, such as start-up problems and frequency deviations, or result in unstable operation.
The principle of negative resistance
A key criterion for reliable oscillation is the ratio between the negative resistance of the circuit and the resonance resistance of the crystal (RL). The circuit can only compensate for the losses of the crystal and generate stable oscillations when the negative resistance is large enough:
Start condition of oscillation: RL < |-R|
Operating state of oscillation RL = |-R|
with RL = resonance resistance of the crystal unit under load
Figure 2 shows the load resistance characteristics depending on the frequency
If this condition is not met, oscillation will not start, or unstable clock signals will occur. Typical errors due to non-optimized crystal circuits are:
- No oscillation or sporadic starting behavior
- Frequency deviation beyond tolerance limits
- Instable communication with peripheral ICs
- Impairment of EMC properties
Influence of the peripheral circuit
The choice and matching of the circuit components impact both oscillation frequency and stability. This is particularly critical in modern, highly integrated ICs that often place very specific demands on the clock pulse generator. Even small deviations in the capacitance values can lead to significant frequency shifts or even failure of the circuit.
Matching service and design support
A remedy is provided by exact calculations and simulations of the circuit, including a matching service. In doing so, realistic IC parameters, layout conditions and environmental influences are considered. The matching reports contain specific recommendations for the C1/C2 values, oscillator amplification and layout adaptations. Kyocera AVX offers a comprehensive portfolio of crystals for a wide range of applications. An overview is provided in Table 1.
Table 1: Comparison of crystal series
| Series | Package (mm) | Frequency (MHz) | Temperature (°C) | Tolerance (ppm) | Applications |
| CX2016SA | 2.0 x 1.6 | 16 - 60 | -40 to +125 | ±15 / ±50 | Automotive: ADAS (camera, radar, LiDAR), ECU, BMS, Infotainment, EPS, inverter, Eps, V2X |
| CX3225GA | 3.2 x 2.5 | 12 - 30 | -40 to +150 | ±50 / ±150 | |
| CX2016DB | 2.0 x 1.6 | 12 - 50 | -40 to +85 | ±10 to ±30 | Industry: Factory automation, measuring devices, safety, energy Consumers: Wearables, household appliances |
| CX2016FB | 2.0 x 1.6 | 12 - 50 | -40 to +85 | ±20 to ±50 | |
| CX3225FB | 3.2 x 2.5 | 8 - 54 | -40 to +85 | ±20 to ±50 |
Summary
The choice and correct integration of a crystal unit is crucial for the reliability and precision of electronic circuits. Matching services, a sophisticated circuit design and the selection of the appropriate crystal series offer developers in the automotive and industrial sectors real-world practical solutions.
Global support centers provide technical advice on circuit matching and IC compatibility testing in collaboration with semiconductor suppliers. Approvals for crystal products are specifically obtained, such as for use in Bluetooth ICs, from a leading Nordic supplier. These services help to shorten design cycles and to ensure system stability from an early stage.
For more information and a direct ordering option, please visit our e-commerce platform at www.rutronik24.com.
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