Radio designers working in cellular/4G, microwave radio, test equipment and military subsystem applications rely on high quality local oscillators (LO) in order to meet system level goals for low bit error rate (BER), low spurious output and low phase noise. RF and microwave communication systems require a clean LO signal source; the higher the capacity of the radio, the more demanding the requirements on the LO signal. There are different architectures in use, 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 and a phase-locked loop (PLL) to form a frequency synthesizer. However, designers seeking the very best LO performance must contend with numerous challenges related to the interaction between the PLL/synthesizer, VCO, charge pump, and loop filter, not to mention issues that arise due to board layout and unwanted power supply noise.
Analog Devices has developed a core expertise in frequency generation components such as MMIC VCOs, phase-locked oscillators (PLO), low noise prescalers, phase-frequency detectors (PFD), and a line of dual mode (fractional/integer) PLL/synthesizer ICs with RF input frequencies to 13.6 GHz. Their new PLL with integrated VCO products for communications systems are summarized in Tables 1 and 2.
These products implement an advanced fractional-N synthesizer and an ultra low noise VCO in a standard 5 or 6 mm QFN plastic package; this high level of integration minimizes the number of external components. They were designed for ultra low phase noise commercial and military applications and include a very low noise phase frequency detector (PFD), a precision controlled charge pump, and an advanced modulator design that allows ultrafine frequency steps. Ultra low phase noise and low spurious also permit architectures with wider loop bandwidths for faster frequency hopping and low microphonics; spurious outputs are low enough to eliminate the need for costly direct digital synthesis (DDS) references in many applications.
PLLswith Integrated VCOsfor RF Market Applications
The HMC830LP6GE is one of eight wideband PLL with integrated VCO products which are targeted to cellular/4G, IF of microwave backhaul, and test and measurement applications. Each product within the family combines the functions of a high performance fractional-N PLL/synthesizer with a fully integrated low noise VCO. The architecture of the PLL with integrated VCO for RF applications enables high performance VCOs with sub-five volt tuning (see Figure 1). No op amp is required in the loop filter, saving both cost and board space, while improving performance. The PLL with integrated VCO can be locked at one temperature extreme and then operated over the full temperature range without the need for relocking or recalibration; this capability is required in high reliability applications, but not fully supported by some competing solutions.
Figure 1 Tuning voltage vs. frequency for the HMC830LP6GE PLL with integrated VCO.
As shown in Figure 2, these devices offer excellent phase noise performance that is typically 10 dB better than competing devices both in-band, and at the far out noise floor, all without the need to choose between low spurious or low noise modes. The typical -55 dBc integrated noise from 100 Hz to 1 MHz is equivalent to 0.1° of rms jitter, or 278 fs rms at fOUT = 1 GHz.
As shown in Figure 3, the HMC830LP6GE represents a significant improvement over alternative integrated solutions. For example, it offers approximately 5 dB lower close-in phase noise, and a 7 dB lower phase noise floor at offset frequencies greater than 20 MHz, compared to another competing part. The HMC830LP6GE offers better spurious performance, with much lower fractional spurs across the band and a cleaner overall spectral output and provides consistent over temperature performance at the band edges, to ensure no “dropouts” are experienced.
The ADF5355 PLL with integrated VCO covers 55 MHz to 13.6 GHz, the widest in its class, while the ADF4355-2 covers 55 MHz to 4.4 GHz. Both parts integrate low phase noise VCOs delivering phase noise of -138 dBc/Hz at 1 MHz offset, at 3.4 GHz for the ADF4355-2 while the ADF5355 delivers phase noise of -132 dBc/Hz at 1 MHz offset at 6.8 GHz.
The VCO phase noise performance of the ADF5355 and ADF4355-2 is achieved through unique VCO topologies and architectures that were developed in conjunction with ADI’s new advanced SiGe-BiCMOS processes. In ultra-wideband RF and microwave communications applications, this low phase noise has the benefit of improving overall system bit error rates and increasing data throughput while enabling better noise immunity and wider dynamic range.
Figure 2 SSB phase noise vs. offset frequency for the HMC830LP6GE PLL with integrated VCO.
ADI’s new, cost-effective, ultra-wideband PLL synthesizer ICs also feature up to 125 MHz phase comparator frequency and 38-bit resolution, which lowers jitter and allows for very fine step sizes, while the integrated PLL and VCO are fabricated on an advanced BiCMOS process significantly reducing package size and power consumption compared to discrete GaAs implementations. Furthermore because a single PLL synthesizer can be configured to operate from 55 MHz to 13.6 GHz, designers can more quickly reconfigure their system designs and reduce their parts inventory while still supporting multiple frequency bands.
Microwave Market Applications
The HMC764LP6CE PLL is optimized for narrow-band but high performance microwave communication applications. These products offer the same exceptional microwave VCO performance which Analog Devices is recognized for, with the added functionality of an advanced, integrated fractional synthesizer. Typical applications include microwave and millimeter-wave radios, industrial/medical test equipment, military communications, electronic warfare (EW), and electronic countermeasure (ECM) subsystems.
As shown in Figure 4, the HMC764LP6CE exhibits consistent tuning sensitivity and high output power of up to 16 dBm across its bandwidth, making it ideal for directly driving the LO port of many of Analog Devices high linearity, double balanced and I/Q mixer and up and down-conversion products. The single sideband (SSB) phase noise performance vs. offset frequency for the low, mid and high frequency range of the HMC764LP6CE is shown in Figure 5. This data was measured with a reference frequency of 50 MHz, a loop bandwidth of 100 kHz, and in the presence of a comparison frequency of 50 MHz at the PFD. The phase noise performance is consistent over temperature and mechanical shock, due to the monolithic construction. A built-in FSK mode allows the device to be used as a simple, low cost direct FM transmitter source.
Figure 3 Worst spur, fixed 50 MHz reference, output frequency = 2 GHz.
Figure 4 Tuning sensitivity and RF output power vs. output frequency for the HMC764LP6CE PLL with integrated VCO.
Hybrid-based synthesizers typically employ a fiberglass-based substrate material with a discrete VCO, a large resonator, and a stamped metal cover that can present problems in users’ systems related to RF grounding as well as unwanted electrical and microphonic coupling effects. Analog Devices PLLs with integrated VCOs have much smaller monolithic resonators and hence offer better microphonic performance compared to larger hybrid designs.
Figure 5 SSB phase noise vs. offset frequency for the HMC764LP6CE PLL with integrated VCO.
Developing a high performance programmable local oscillator can still require considerable design time. Therefore Analog Devices has developed a PLL with integrated VCO reference designer’s kit that enables immediate measurement of the design at hand. The typical evaluation PCB is part of an easy to use, universal evaluation kit which can minimize design time and facilitate rapid prototyping. The reference designer’s kit includes an on-board reference oscillator and voltage regulators, and supports universal loop filter configurations. Included software allows the user to program the PLL and access its advanced features. The guide includes a comprehensive discussion of the components used within the evaluation board, and covers advanced topics such as reconfiguring the evaluation board for an external reference, and implementing on-board selectable order passive or active loop filters.
Each reference designer’s kit contains the Analog Devices ADIsimPLL design tool allowing users to tailor the standard evaluation PCB loop filter to their specific application. Comprehensive PC-based PLL control software is provided, with PC-compatible register files to program the PLL via the USB interface. ADI’s group of applications engineers is also available to help customers quickly become familiar with this unique product.
Analog Devices PLL with integrated VCO products uniquely combine the attributes of low phase noise, advanced features, and small size, making them ideal for numerous small form factor applications including microwave/millimeter wave radios, test equipment, microwave sensors, fiber optic communications, and military communications and sensors.
Analog Devices Inc.
Norwood, Mass.
www.analog.com