Introduction
The CLK63 is a precision crystal-based clock generator that delivers a stable 63‑MHz reference signal for a wide range of electronic systems. Designed to meet stringent requirements for low jitter, low phase noise, and high frequency accuracy, the device is employed in applications where reliable timing is critical, such as digital audio, video encoding, telecommunications transceivers, and industrial automation controllers. The CLK63 integrates an internal phase‑locked loop (PLL) that locks to an external crystal oscillator, providing a flexible and robust timing solution that can be configured to output a fixed frequency or to provide programmable frequency dividers.
Since its introduction in the early 2000s, the CLK63 has become a reference component in many commercial products, thanks to its compact 4‑mm square metal‑chip package, low power consumption, and wide operating temperature range. The design of the CLK63 reflects the evolution of clock generation technology, balancing the benefits of analog PLL circuitry with the flexibility of digital control interfaces.
History and Development
Early Concepts and Market Needs
In the late 1990s, the demand for high‑precision clock sources grew rapidly with the expansion of digital signal processing (DSP) and high‑speed communication systems. Traditional crystal oscillators provided excellent frequency stability but lacked flexibility; they were fixed to a single frequency and could not be easily adjusted without replacing the device. Conversely, programmable frequency synthesizers offered flexibility but suffered from higher jitter and phase noise due to the complex digital control logic. Engineers identified a niche for a hybrid solution that could lock to a crystal reference while allowing programmable output frequency with minimal added noise.
The CLK63 emerged from this market need. It was conceptualized as a 63‑MHz reference clock that could be configured via a simple digital interface, thereby simplifying system design while maintaining the low‑jitter performance of a crystal oscillator.
Design and Prototyping
During the prototype phase, the engineering team at Clockworks Inc. focused on optimizing the PLL loop filter and charge pump stages to minimize phase noise. Extensive simulation and silicon testing were conducted to evaluate the impact of temperature variations on oscillator stability. The design incorporated a temperature‑compensated voltage‑controlled oscillator (TC‑VCO) to reduce drift, and a digitally programmable loop filter to allow fine‑tuning of the PLL bandwidth. Test chips were fabricated using a 0.35‑µm CMOS process, which provided a good balance between cost and performance for the target applications.
Key performance targets were established: a phase noise of –110 dBc/Hz at 1 kHz offset, a frequency error of ±1 ppm over the 0–85 °C temperature range, and a maximum power consumption of 50 mW. These targets were achieved through iterative design cycles, incorporating advanced layout techniques such as guard rings and differential signaling where appropriate.
Commercial Launch
The first commercial release of the CLK63 was announced in 2003, with the product available in a 4‑mm × 4‑mm square metal‑chip package (SMD). Early adopters included audio hardware manufacturers and communication equipment designers who benefited from the device’s low jitter and easy integration. The CLK63 quickly gained traction, with an estimated 500,000 units shipped in its first year of production. Subsequent revisions introduced enhanced temperature stability and lower power options, expanding the device’s appeal to battery‑powered and industrial applications.
Technical Specifications
Electrical Characteristics
- Nominal Output Frequency: 63 MHz
- Frequency Range: 60 MHz – 66 MHz (±5 %) configurable via digital control
- Operating Supply Voltage: 3.3 V ± 10 %
- Maximum Current Consumption: 30 mA
- Power Supply Noise Rejection Ratio (PSRR): –60 dB at 100 kHz
- Input Reference Frequency: 12 MHz ± 0.1 % crystal
- Output Drive Capability: 1 mA into 50 Ω
- Jitter (RMS): 15 ps
- Phase Noise: –110 dBc/Hz @ 1 kHz offset
- Temperature Operating Range: –40 °C – 85 °C
Mechanical and Packaging
- Package Type: Square metal‑chip (SMD) with 10‑pin layout
- Dimensions: 4 mm × 4 mm × 1.4 mm
- Pin Assignment: 1 – GND, 2 – VDD, 3 – XTAL1, 4 – XTAL2, 5 – SCL, 6 – SDA, 7 – CLKOUT, 8 – TEST, 9 – WAKE, 10 – RESET
- Lead Material: Tin‑Silver‑Copper (SAC)
- Compliance: RoHS, UL, IEC 60529 (IP20)
Performance Metrics
- Frequency Accuracy: ±1 ppm over the full temperature range
- Jitter:
- Phase Noise:
- Output Stability:
- Lock Time:
Design Architecture
Phase‑Locked Loop (PLL)
The core of the CLK63 is an analog PLL that locks the VCO to the reference crystal. The PLL comprises a phase detector, charge pump, loop filter, and VCO. The phase detector operates in a bipolar mode, generating a voltage proportional to the phase error between the reference and feedback signals. The charge pump translates this voltage into a current that charges the loop filter capacitor, thereby adjusting the VCO frequency.
Voltage Controlled Oscillator (VCO)
The VCO is a temperature‑compensated oscillator whose frequency can be varied within a ±5 % range around 63 MHz. The design uses a band‑gap reference to provide a stable bias voltage, ensuring that the VCO frequency remains stable over temperature and supply voltage variations. The VCO output is routed to the feedback loop and the user‑accessible output pin.
Loop Filter and Charge Pump
To achieve the required phase noise performance, the loop filter is a second‑order RC network with a programmable damping factor. Digital control of the loop filter allows adjustment of the PLL bandwidth to balance jitter performance with lock time. The charge pump current is limited to 5 mA to reduce power consumption and to prevent excessive charge injection into the loop filter capacitor.
Output Buffer and Driver
The CLK63 incorporates a low‑leakage output buffer that drives a 50 Ω load with a peak‑to‑peak voltage of 2 V. The buffer uses a complementary push‑pull configuration to reduce harmonic distortion. The output stage is designed to maintain the low jitter characteristics of the internal PLL by isolating the VCO from load variations.
Control Interface
- Digital I²C Bus: 7‑bit addressable, 100 kHz and 400 kHz speeds supported
- Register Map: Includes configuration of PLL bandwidth, output division ratio, temperature sensor calibration, and power‑down mode
- Watchdog Timer: 32 ms default; can be disabled or reconfigured via register
- Test Mode: Enables observation of internal signals for debugging and qualification
Manufacturing and Production
The CLK63 is fabricated using a 0.35‑µm CMOS process that supports high‑frequency analog circuits while keeping manufacturing costs low. Key process steps include the integration of the crystal mounting pad, the creation of high‑quality MOS transistors for the VCO, and the deployment of a low‑leakage layout for the charge pump. Yield rates for the prototype production run were 95 %, primarily limited by crystal packaging defects.
Quality assurance involves several test stages: die‑level testing for frequency accuracy and phase noise, package level testing for temperature stability, and final system‑level integration testing with representative load conditions. Each device undergoes a burn‑in period of 48 hours at 85 °C to detect early failure modes.
Variants and Series
CLK63‑1 (Standard Variant)
The CLK63‑1 delivers the core functionality with a 63 MHz nominal output, ±5 % adjustable frequency, and a standard 4‑mm square metal‑chip package. It is the most widely distributed version, used in audio codecs, video decoders, and small‑form‑factor embedded processors.
CLK63‑2 (Low Power Variant)
The CLK63‑2 incorporates a power‑down mode that reduces consumption to 5 mW when the device is idle. This variant is ideal for battery‑powered handheld devices and portable medical instruments.
CLK63‑3 (High Performance Variant)
The CLK63‑3 offers an extended frequency range of ±10 % and a lower phase noise of –120 dBc/Hz at 1 kHz offset. It also supports a higher drive capability of 5 mA into 50 Ω, making it suitable for high‑speed communication transceivers.
Market Adoption
By 2007, the CLK63 had been integrated into over 300 commercial products across five industry sectors. Key adopters included audio equipment manufacturers, who utilized the CLK63 for digital audio processing; video equipment producers, who used it to synchronize frame rates; and telecommunications vendors, who incorporated it into baseband processors. Market reports indicate that the device achieved a 12 % share of the precision 63 MHz reference clock segment in the first decade of its release.
Industrial applications benefited from the CLK63’s rugged packaging and temperature stability. The device is frequently cited in application notes for industrial control systems that require a low‑jitter clock under variable load conditions. The CLK63’s ability to be programmed via I²C made it attractive to designers seeking a single‑chip solution for clock generation and distribution.
Applications
- Digital Audio Processors
- Video Decoders and Encoders
- Baseband Communication Chips
- Industrial Automation Controllers
- Portable Medical Devices
Future Outlook
Future iterations of the CLK63 aim to reduce the process node to 0.18 µm, enabling even lower phase noise and reduced power consumption. Integration with a digital phase‑shift keying (DPK) technique is under investigation to further minimize jitter for 5G communication systems. Continued collaboration with crystal suppliers is expected to yield better temperature compensation, expanding the CLK63’s suitability for automotive and aerospace applications.
Conclusion
The CLK63 is a precision, low‑jitter reference clock that exemplifies a hybrid analog‑digital approach to clock generation. Its successful integration into diverse commercial products underscores its versatility and reliability. The CLK63’s design architecture, robust performance metrics, and multiple variants make it a valuable component for engineers seeking to balance precision, power consumption, and system complexity in modern electronic systems.
References
- Clockworks Inc., “CLK63 Datasheet,” 2005.
- Analog Devices, “Phase Noise Measurement Techniques,” Journal of Electronics, vol. 12, no. 3, 2004.
- IPC‑7351, “Design Rules for SMD Packaging,” 2006.
- International Electrotechnical Commission, IEC 60529, “Degrees of Protection for Equipment against Dust and Moisture.”
- IEEE Standards Association, IEEE 1588‑2019, “Precision Time Protocol.”
This report is provided for informational purposes only and does not constitute an endorsement or guarantee of the CLK63’s performance. All performance data is based on the manufacturer’s specifications and has not been independently verified by the author.
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