Supporting compact LO solutions, or relying on the best combination of "PLL + VCO"

Radio designers for cellular/4G, microwave radio, test equipment, and defense subsystem applications rely on high-quality local oscillators (LOs) to achieve system-level goals of low BER (bit error rate), low spurious outputs, and low phase noise. All RF and microwave communications and sensor systems, whether based on analog or digital modulation, require a clean LO signal source; the higher the capacity of the radio, the higher the requirements for the LO signal.

There are many different architectures available, but one of the most common methods of generating a stable LO source is to combine a low phase noise voltage controlled oscillator (VCO) with a stable reference voltage and a phase locked loop (PLL) to form a frequency synthesizer. However, designers seeking the best LO performance are faced with many challenges associated with the interaction between the PLL/synthesizer, VCO, charge pump, and loop filter, not to mention issues due to board layout and undesirable power supply noise.

As shown in the functional block diagram of Figure 1, these products implement advanced fractional-N frequency synthesizers and ultra-low noise VCOs in standard 5 mm × 5 mm and 6 mm × 6 mm QFN plastic packages; this high level of integration minimizes external component count. Designed for ultra-low phase noise commercial and defense applications, it includes a very low noise phase frequency detector (PFD), a precision controlled charge pump, and an advanced modulator design that provides ultra-fine frequency steps.

Figure 1. Functional block diagram and typical application circuit of ADI's PLL product with integrated VCO

The architecture features ultra-low close-in phase noise and low spurious behavior, enabling wider loop bandwidths, faster frequency hopping, and low microphonic noise; spurious outputs are low enough that expensive direct digital synthesis (DDS) reference sources are no longer required in many applications.

The HMC830LP6GE is one of eight wideband PLLs with integrated VCOs for cellular/4G, microwave backhaul IF, and test and measurement applications. Each member of the family combines a high-performance fractional-N PLL/frequency synthesizer with a low-noise VCO. The PLL architecture with integrated VCO for RF applications enables high-performance VCOs to be tuned below five volts (see Figure 2).

Figure 2. Tuning voltage vs. frequency for the HMC830LP6GE PLL with integrated VCO

Eliminating the need for an op amp in the loop filter saves cost and board space while improving performance. The PLL with integrated VCO can lock at one temperature extreme and then operate over the full temperature range without relocking or recalibration; this feature is needed in high reliability applications but is not available in some competing products.

As shown in Figure 3, these devices offer excellent phase noise performance, typically 10 dB better than competitors both in-band and at the far end, all without having to choose between low spurious or low noise modes. Integrated noise from 100 Hz to 1 MHz is typically –55 ​​dBc, equivalent to 0.1° of rms jitter, or 278 fs rms at FOUT = 1 GHz.

Figure 3. SSB phase noise vs. offset frequency for the HMC830LP6GE PLL with integrated VCO

As shown in Figure 4, the HMC830LP6GE provides a clear improvement over alternative integrated solutions.

Figure 4. Worst spurious, fixed 50 MHz reference, output frequency = 2 GHz

The HMC830LP6GE has stable performance across temperature at the band edges, ensuring that there is no “dropout”.

For example, the HMC830LP6GE produces approximately 5 dB close-in phase noise and 7 dB phase noise floor at offset frequencies greater than 20 MHz, both of which are lower than the TI LMX2581. In addition, the HMC830LP6GE also provides superior spurious performance, with much lower fractional spurs across the entire frequency band, resulting in a cleaner overall spectrum output.

The ADF5355 PLL with integrated VCO covers the widest frequency spectrum in its class, from 55 MHz to 13.6 GHz, while the ADF4355-2 covers a frequency range of 55 MHz to 4.4 GHz. Both devices integrate ultralow phase noise VCOs, producing –138 dBc/Hz (1 MHz offset) phase noise at 3.4 GHz for the ADF4355-2 and –132 dBc/Hz (1 MHz offset) phase noise at 6.8 GHz for the ADF5355-2.

ADI's cost-effective, ultra-wide bandwidth PLL frequency synthesizer IC also features up to 125 MHz phase comparator frequency and 38-bit resolution, which reduces jitter and allows very fine step sizes, while the integrated PLL and VCO using advanced BiCMOS technology can significantly reduce package size and power consumption compared to discrete GaAs implementations. In addition, since the operating frequency of a single PLL frequency synthesizer can be configured from 55 MHz to 13.6 GHz, designers can reconfigure their system designs more quickly and reduce component inventory while still supporting multiple frequency bands.

The HMC764LP6CE PLL with integrated VCO is optimized for narrowband, high performance microwave communications applications. Analog Devices is the first in the industry to offer integrated PLLs and VCOs above 6 GHz. These products provide ADI's outstanding microwave VCO performance with the added functionality of an integrated advanced fractional frequency synthesizer. Typical applications include microwave and millimeter wave radios, industrial/medical test equipment, defense communications, and electronic countermeasures (ECM) subsystems.

As shown in Figure 5, the HMC764LP6CE has stable tuning sensitivity and up to 16 dBm output power across its bandwidth, making it ideal for directly driving the LO section of many ADI high linearity, double balanced and I/Q mixers, as well as up- and down-conversion products.

Figure 5. Tuning sensitivity and RF output power vs. output frequency for the HMC764LP6CE PLL with integrated VCO

Figure 6 shows the excellent single sideband (SSB) phase noise performance of the HMC764LP6CE vs. offset frequency for the low, mid, and high frequency ranges. This data was measured with a reference frequency of 50 MHz, a loop bandwidth of 100 kHz, and a PFD comparison frequency of 50 MHz. Due to the monolithic construction, the phase noise performance is also stable over temperature and under mechanical shock conditions. In addition, the built-in FSK mode allows the device to be used as a simple, low-cost direct FM transmit source.

Figure 6. SSB phase noise vs. offset frequency for the HMC764LP6CE PLL with integrated VCO

Even with such advanced features and high integration, developing a high performance programmable local oscillator still requires a lot of design time. Therefore, ADI has developed a PLL reference designer's kit with integrated VCO so that the design at hand can be measured immediately.

The typical evaluation PCB shown in Figure 7 is part of a common, easy-to-use evaluation kit that minimizes design time and facilitates rapid prototyping. The reference designer’s kit includes an on-board reference oscillator and voltage regulators, and supports common loop filter configurations.

Figure 7. PLL evaluation PCB with integrated VCO, included in each designer's kit

The included software allows the user to program the PLL and access its advanced features. A complete operating guide provides step-by-step instructions to quickly power up and initialize the evaluation board. This guide provides a comprehensive discussion of the components used within the evaluation board, covering topics such as reconfiguring the evaluation board for an external reference voltage and implementing the on-board selectable sequential passive or active loop filters.

Each reference designer's kit includes ADI's proprietary ADIsimPLL design tool , which allows users to customize standard evaluation PCB loop filters for their specific applications. Comprehensive PC-based PLL control software is provided, and the PLL can be programmed using a PC-compatible register file via a USB interface. Users can evaluate a fully locked local oscillator in a very short time using only a PC, a signal analyzer, and some DC power supplies. ADI's team of skilled application engineers can also help customers quickly become familiar with this unique product.

Hybrid frequency synthesizers typically use a fiberglass substrate material with a discrete VCO, a large resonator, and a stamped metal cover. This assembly technique can cause problems with RF grounding in the user's system and can also produce objectionable electrical and microphonic coupling effects.

These PLLs with integrated VCOs are housed in RoHS-compliant QFN leadless packages, making them ideal for high-speed, high-volume SMT assembly lines.

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