What frequency generating device is suitable for my application?

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What frequency generating device is suitable for my application?

Posted Date: 2024-01-23

Understanding the performance characteristics of frequency-generating devices is critical to determining the correct solution for the target use scenario. This is a quick guide designed to help RF system engineers become familiar with the entire selection process.

▶ Main performance criteria

We first define the criteria commonly used to characterize the performance of frequency-generating devices. The selection process generally starts with the most basic criterion, which is output frequency range. To generate frequencies across the spectrum, a wide variety of devices have been designed to support frequencies ranging from single tones to frequencies spanning multiple octaves. However, when selecting devices based on output frequency, it is important to note that broadband and high-frequency capabilities are often traded for other fundamental characteristics, including frequency stability, output spectral purity, and switching speed.

Frequency stability represents short-term and long-term changes in the output signal. Short-term stability is related to changes that are much smaller than a complete signal period. These changes are represented by phase jitter and phase noise. Phase jitter defines small fluctuations in the phase of a signal in the time domain, and phase noise is its spectral representation, described by the relative noise power level contained in a 1 Hz bandwidth at different offset frequencies relative to the carrier frequency. If the frequency change occurs over a longer period of time, we usually use long-term stability to describe it, which refers to the output frequency drift (usually expressed in ppm) due to temperature, load conditions, aging, etc.

Spectral purity is another important characteristic to consider in the device selection process. It is described by the presence of spurious components in the device output spectrum, usually quantified in terms of harmonic levels and feedthrough components expressed relative to the fundamental frequency level.

In addition to the stability and spectral purity of the output signal, switching speed (also known as settling time or locking time) is another typical trade-off parameter that needs to be considered when selecting the optimal frequency generator solution. It describes the time required for the device to switch from one frequency to another, this requirement can vary significantly depending on the end application.

▶ Main types of devices

Having defined above the main performance criteria used to characterize frequency-generating devices, we now briefly describe its main types, which are designed to provide different combinations of characteristics related to these criteria. This overview should ultimately serve as a guide for selecting the right type of device to meet the needs of your target application.

Crystal (XTAL) oscillators (XO) use a piezoelectric resonator (usually quartz) to produce a fixed output frequency from a few kilohertz to several hundred megahertz. There is a special type of XO called a voltage-controlled crystal oscillator (VCXO) that allows the frequency to be changed, but only by a small amount to support fine-tuning. XOs are electromechanical transducers with extremely high Q factors (can exceed 100,000), producing very stable output frequencies with very low phase noise. The XO has limited maximum output frequency and tuning capabilities, but it is an excellent choice when a single accurate reference for other types of devices is required for higher frequencies.

A voltage controlled oscillator (VCO) is a different type of frequency generating device that relies on an LC resonant circuit. The Q-factor of an electrical circuit element is much lower (usually 1000 times lower) than a crystal, but it allows for much higher output frequencies and a wide tuning range. The frequency of the output signal generated by the VCO is controlled by the external input voltage. The core of the VCO can use different resonant circuits. Single-core VCOs using high-Q resonators provide low phase noise performance over a limited frequency range, while lower-Q oscillators target broadband operation and have mediocre noise characteristics. A multi-band VCO using multiple switched high-Q resonator circuits is a compromise solution that supports wideband operation while providing low phase noise performance, but at the cost of slower tuning due to the time required to switch different cores . A VCO is an excellent all-round solution, but it generally does not provide a stable output signal, which is why a VCO is often used with a phase-locked loop (PLL) to improve output frequency stability.

A phase-locked loop (PLL) or PLL frequency synthesizer ensures the VCO output frequency stability required for many frequency synthesis and clock recovery applications. As shown in Figure 1a, the PLL contains a phase detector that compares N divided by the VCO frequency to a reference frequency and uses this difference output signal to adjust the DC control voltage applied to the VCO tuning line. This allows any frequency drift to be corrected instantly so the oscillator remains stable. A typical PLL IC contains an error detector (phase frequency detector or PFD with a charge pump) and a feedback divider (see the dashed area in Figure 1a), in addition to an external loop filter, a precision reference frequency, and a VCO. To form a complete feedback system to produce stable frequency. Implementation of this system can be greatly simplified using a frequency synthesizer IC with an integrated VCO.

Frequency synthesizers with integrated VCOs combine the PLL and VCO in a single package, requiring only an external reference and loop filter to achieve the required functionality. The integrated PLL frequency synthesizer is a versatile solution with a wide range of digital control settings to support precise frequency generation. It often includes integrated components, multipliers, dividers and tracking filters, with frequency coverage extending beyond the VCO's fundamental frequency range to several octaves. The intrinsic parameters of all these components determine the output frequency range, phase noise, jitter, lock time, and other characteristics that characterize the overall performance of the frequency synthesis circuit.

Conversion loops are another type of frequency synthesizer based on the PLL concept, but implemented using a different approach. As shown in Figure 1b, an integrated downconversion mixing stage is used in the feedback loop instead of an N divider. The loop gain is set to 1 and the in-band phase noise is minimal. The conversion loop IC (see the dashed area in Figure 1b) is designed for applications that are highly sensitive to jitter and is used in combination with an external PFD and LO to achieve a complete frequency synthesis solution in a compact size, delivering instrument-grade performance.

Direct Digital Synthesizers (DDS) are an alternative to integrated PLL frequency synthesizers and are implemented using a different principle. The schematic diagram of the basic DDS architecture is shown in Figure 1c. It is a digital control system that includes a high-precision reference frequency that represents the clock signal, a digitally controlled oscillator (NCO) that creates a digital version of the target waveform, and a digital-to-analog converter (DAC) that provides the final analog output. DDS ICs offer very fast switching speeds, fine frequency and phase resolution, and low output distortion, making them particularly suitable for applications where excellent noise performance and high frequency agility are critical.

▶ Conclusion

Frequency generation devices are used in a wide range of applications and can perform a variety of functions, including frequency conversion, waveform synthesis, signal modulation, and clock signal generation. Different types of frequency generating devices need to be designed for the different requirements imposed by the end application, and this article briefly explains the main types of these devices. For example, communication systems require low in-band noise to maintain low error vector magnitude (EVM), spectrum analyzers rely on local oscillators with fast lock times to enable fast frequency sweeps, and high-speed converters require low jitter clocks to ensure high SNR performance.

Figure 1. Simplified block diagram of (a) PLL, (b) conversion loop, (c) DDS

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