Temperature-compensated crystal oscillators (TCXOs) are widely used as precision reference clocks in RF communication systems, GNSS receivers, wireless modules, and digital electronics.
Many TCXO devices are available with two common output types: CMOS and Clipped Sine Wave. Although both provide single-ended clock outputs, their electrical characteristics and application requirements differ significantly.
Selecting the appropriate output type can help optimize signal integrity, power consumption, and electromagnetic compatibility.
CMOS output generates a standard digital square wave with fast rise and fall times. The output voltage swings close to the supply rails, making it directly compatible with most digital logic devices.
Full-swing digital output
Fast rise and fall times
Logic levels referenced to VDD
Strong drive capability
Suitable for direct digital clock inputs
Typical output levels are:
VOH ≥ 90% of VDD
VOL ≤ 10% of VDD
Because of its sharp transitions, a CMOS clock contains a significant amount of high-frequency harmonic content.
Direct interface to FPGA, MCU, and digital ICs
Excellent timing margins
Strong noise immunity
Easy PCB implementation
The fast signal edges generate more harmonic energy, which may increase electromagnetic emissions and switching noise in sensitive RF environments.
Typical load condition:
15 pF capacitive load
A clipped sine wave is produced by limiting the peak amplitude of a sinusoidal signal while maintaining its smooth waveform transitions.
Compared with CMOS output, clipped sine signals exhibit lower amplitude and significantly reduced harmonic content.
Low-voltage output
Smooth waveform transitions
Reduced harmonic energy
Lower electromagnetic emissions
Lower power consumption
Typical output amplitude ranges from:
0.8 Vpp to 1.2 Vpp
Because the signal transitions are gradual, the spectral content is much closer to an ideal sine wave.
Typical load condition:
10 kΩ // 10 pF
Reduced EMI and radiated noise
Lower current consumption
Improved compatibility with RF systems
Suitable for sensitive analog and wireless applications
| Parameter | CMOS | Clipped Sine |
|---|---|---|
| Output Amplitude | VDD-level | 0.8–1.2 Vpp |
| Rise/Fall Time | Very fast | Smooth transition |
| Harmonic Content | High | Low |
| EMI Performance | Moderate | Excellent |
| Power Consumption | Higher | Lower |
| Digital Logic Compatibility | Excellent | Limited |
| RF System Compatibility | Moderate | Excellent |
The clock input specifies AC coupling.
The receiver requires a 0.8 Vpp clock input.
The design includes GNSS, cellular, or RF transceivers.
Low EMI performance is important.
Battery-powered applications require reduced power consumption.
The clock input specifies VIH and VIL logic thresholds.
The clock directly drives FPGA, MCU, ASIC, or digital ICs.
High drive capability is required.
The system primarily consists of digital circuitry.
Clock distribution devices are used.
Neither CMOS nor clipped sine output is universally better. The optimal choice depends on the clock input requirements of the receiving device and the overall system design objectives.
CMOS output provides excellent compatibility with digital systems, while clipped sine output offers lower EMI and lower power consumption for RF-sensitive applications.
Understanding the requirements of the receiving circuit remains the most important step in selecting the appropriate TCXO output type.
