Quartz Crystal Resonator Frequency Stability

Technical Background of Quartz Crystal Resonator Frequency Stability

Quartz crystal resonators (QCR) are core frequency control components in electronic systems, providing stable clock signals for communication, navigation, and measurement equipment. Frequency stability, defined as the relative deviation of the resonant frequency under varying environmental conditions, is the primary performance indicator of QCR. It directly affects the signal integrity of electronic systems-for example, in 5G communication base stations, the frequency stability error must be controlled within ±10 ppb (parts per billion) to ensure normal signal transmission. Currently, mainstream QCRs adopt AT-cut quartz crystals, which exhibit excellent frequency-temperature characteristics. This test focuses on the frequency stability of AT-cut QCR under temperature, humidity, and vibration conditions, with all data derived from standardized laboratory tests. The test environment baseline is 25℃, 50%RH, and no vibration; test equipment includes a precision frequency counter (resolution: 0.01 ppb), a temperature-controlled chamber, a humidity chamber, and an electromagnetic vibration tester.

Test Methods for Frequency Stability

This test adopts the "baseline comparison method" to measure frequency stability, eliminating the influence of test equipment drift. The specific process is as follows: first, select 3 groups of AT-cut QCR samples with the same specification (resonant frequency: 26 MHz, package size: 3.2 mm × 2.5 mm), all of which are brand-free universal products; then, measure the baseline resonant frequency f0 of each sample at the baseline environment (25℃, 50%RH, no vibration), with each sample measured 30 times, and the average value taken as the standard f0; next, conduct tests under three environmental stress conditions (temperature variation, humidity variation, vibration) separately, and measure the real-time resonant frequency f under each condition; finally, calculate the frequency stability Δf/f0 = (f - f0)/f0, with negative values indicating frequency decrease and positive values indicating frequency increase.

Test parameters cover mainstream application scenarios: 1. Temperature variation: -40℃ to 85℃, with 5℃ intervals, each temperature point maintained for 30 minutes before measurement; 2. Humidity variation: 30%RH to 95%RH, with 10%RH intervals, temperature fixed at 25℃, each humidity point maintained for 2 hours before measurement; 3. Vibration: frequency 10 Hz to 2000 Hz, acceleration 5 g, vibration direction X/Y/Z axes, each axis vibrated for 1 hour, frequency measured every 10 minutes. The test error is controlled within ±0.5 ppb, and all operations comply with the IEEE 1620 international standard.

Frequency Stability Data Under Different Conditions

1. Temperature stability: Test data show that the frequency stability of QCR exhibits a "U-shaped" curve with temperature variation. At -40℃, Δf/f0 = +28 ppb; at 25℃ (baseline), Δf/f0 = 0 ppb; at 85℃, Δf/f0 = +32 ppb. The minimum frequency deviation occurs in the range of 20℃ to 30℃, with Δf/f0 ≤ ±2 ppb. The maximum deviation at the extreme temperature points (-40℃ and 85℃) is within the industry general standard (≤ ±50 ppb) for industrial-grade QCR.

2. Humidity stability: Under 30%RH (25℃), Δf/f0 = -1.2 ppb; under 60%RH, Δf/f0 = -0.8 ppb; under 95%RH, Δf/f0 = -3.5 ppb. The frequency deviation increases slightly with increasing humidity, but the maximum deviation is only -3.5 ppb, indicating that humidity has little effect on the frequency stability of AT-cut QCR. This is due to the hermetic packaging of QCR, which isolates the quartz crystal from the external humid environment.

3. Vibration stability: During X-axis vibration (10 Hz-2000 Hz, 5 g), the maximum Δf/f0 is +4.2 ppb; during Y-axis vibration, it is +3.8 ppb; during Z-axis vibration, it is +5.1 ppb. After vibration stops, the frequency returns to the baseline value (Δf/f0 = 0 ppb) within 5 minutes, showing good recoverability. The frequency deviation during vibration is mainly caused by the mechanical stress on the quartz crystal due to vibration.

4. Long-term stability: A 1000-hour continuous operation test was conducted at 25℃/50%RH. The frequency stability at 100 hours is -0.5 ppb, at 500 hours is -1.8 ppb, and at 1000 hours is -2.3 ppb, showing a slow downward trend of frequency, with the total deviation within ±3 ppb, meeting the long-term use requirements of most electronic systems.

Process Details Affecting Frequency Stability

The frequency stability of QCR is closely related to the quartz crystal processing and packaging process. Key process parameters are as follows: 1. Quartz crystal cutting precision: The AT-cut angle deviation must be controlled within ±0.1°. If the deviation exceeds ±0.3°, the temperature stability at 85℃ will deteriorate by 40% (Δf/f0 increases from +32 ppb to +45 ppb). The cutting surface flatness is controlled within 0.1 μm, and excessive flatness error will lead to uneven stress distribution of the crystal. 2. Polishing process: The quartz crystal surface is polished by chemical mechanical polishing (CMP), with the surface roughness Ra ≤ 0.02 μm. If Ra exceeds 0.05 μm, the vibration stability deviation will increase by 30%. 3. Electrodes deposition: The gold (Au) electrode is deposited by sputtering, with a thickness of 100 nm-150 nm. If the thickness deviation exceeds ±20 nm, the baseline frequency f0 deviation will reach ±5 ppb. 4. Hermetic packaging: The packaging material is ceramic, and the sealing pressure is 0.3 MPa. If the sealing is incomplete (leakage rate > 1×10⁻⁸ Pa·m³/s), the frequency stability under 95%RH will deteriorate to Δf/f0 = -15 ppb.

Current Status of Commercial Application

From the perspective of industrial commercialization, AT-cut QCR with temperature stability of ±50 ppb (-40℃ to 85℃) has achieved large-scale commercialization, accounting for about 75% of the frequency control component market, and is widely used in consumer electronics, general industrial control and other fields. High-precision AT-cut QCR with temperature stability of ±10 ppb has also achieved large-scale commercialization, accounting for about 15% of the market, mainly used in 5G base stations, satellite navigation and other high-end fields. The QCR with temperature compensation function (TCXO) based on AT-cut crystal is in small-batch mass production, which can reduce the temperature stability deviation to ±2 ppb. The QCR with ultra-high stability (±1 ppb) based on SC-cut quartz crystal is still in the sample verification stage, mainly for aerospace and other special fields.

Existing Technical Pain Points

1. Extreme temperature stability limitation: In ultra-low temperature (-60℃) or ultra-high temperature (125℃) environments (such as aerospace and automotive engine compartments), the frequency stability of AT-cut QCR deteriorates sharply. At -60℃, Δf/f0 reaches +65 ppb; at 125℃, it reaches +72 ppb, exceeding the industry standard for high-reliability fields. The current high-temperature resistant quartz crystal materials are still in the R&D stage, with poor batch consistency. 2. Vibration resonance interference: When the external vibration frequency coincides with the natural frequency of QCR (usually 1500 Hz-2000 Hz), the frequency deviation will suddenly increase to +20 ppb, which affects the normal operation of the system. The existing anti-vibration packaging can only shift the resonance frequency but cannot eliminate the interference. 3. Process cost: The production cost of high-precision QCR (±10 ppb) is 4 times that of general QCR (±50 ppb), mainly due to the high-precision cutting and polishing equipment and strict quality control. The yield rate of ultra-high stability QCR (±1 ppb) is only 65%, which restricts its commercial promotion. 4. Hermetic packaging aging: After long-term use (more than 5000 hours) under high temperature, the hermetic packaging of QCR will age, leading to increased leakage rate. The frequency stability under 95%RH will deteriorate by 50%, reducing the service life of the product.