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PLL Fractional-N Synthesizers to 14 GHz

Integration, performance and functionality are three of the most common requests made by RF and microwave designers striving for leading-edge systems with components that can help them deliver that often elusive market edge. In the world of signal generation, Hittite Microwave Corp. is meeting these challenges with the introduction of broadband frequency synthesizers and phase locked loop (PLL) integrated circuits that are designed to deliver superior performance and innovative features, all within a miniature surface-mount package.

Consumer demand for mobile bandwidth and data capacity has pushed mobile backhaul systems to higher levels of digital modulation with more bits/Hertz. These radios require integrated synthesizers that deliver high accuracy and can support signals with exceptional signal-to-noise-ratio (SNR). The phase noise and distortion of the synthesizer can limit the maximum achievable SNR of a modulated signal, while increases in the data throughput of the radio place an even greater demand on the synthesizer phase noise. In addition, synthesizer spurious products and far out noise must be kept to a minimum so as not to impair radios operating in adjacent bands.

Low Noise Synthesizer IC Includes Integrated Sweeper Function

Hittite Microwave has developed a core expertise in frequency generation products including MMIC and Silicon VCOs, low noise prescalers, multipliers and now a new line of dual-mode integrated synthesizer solutions. It is important to note that all Hittite synthesizers are designed to work with Hittite’s wide selection of high performance VCOs and low noise dividers. This gives Hittite a unique capability to guarantee performance of the PLL and the VCO components together, while offering the flexibility of standalone VCOs.

Hittite synthesizers feature very high reference frequency operation and low noise delta sigma modulator design, two critical factors in achieving ultra-low phase noise.

Figure 1 Fractional-N frequency synthesizer IC block diagram.

The HMC702LP6CE is a SiGe BiCMOS fractional-N frequency synthesizer IC that operates in fractional or integer mode over a frequency range of 100 kHz to 14 GHz providing the ability to develop high performance phase locked loops for cellular base stations and point-to-point radio links using the same device. Primarily designed for ultra low phase noise applications, its internal architecture includes a 14 GHz 16-bit RF N-counter, 24-bit delta-sigma modulator, a very low noise digital phase frequency detector (PFD) and a precision controlled charge pump (see Figure 1).

Figure 2 Integer and fractional mode phase noise performance of the HMC702LP6CE.

Figure 3 HMC702LP6CE phase noise at 30 kHz when combined with various Hittite VCOs (fractional mode).

Figure 2 shows the exceptional integer and fractional mode phase noise performance of the HMC702LP6CE. Ultra low in-close phase noise allows wider loop bandwidths for faster frequency hopping. A Cycle Slip Prevention (CSP) feature can reduce the time to arrive at the new frequency by 50 percent versus conventional PFDs. The HMC702LP6CE also contains a built-in linear sweeper function, which allows it to perform frequency chirps with a wide variety of sweep times, polarities and dwells, all with an external or automatic sweep trigger. Other features include a General Purpose Output (GPO) and square wave and 50 Ohm sinusoidal crystal source buffers in the reference path. The HMC702LP6CE can be combined with many of Hittite’s MMIC VCOs to create compact, state-of-the-art PLL synthesizer/VCOs. Figure 3 shows the typical phase noise performance when operating with some of Hittite’s most popular VCOs. Performance is shown at 30 kHz offset from the carrier when operating in fractional mode with a wide loop bandwidth. For lower frequency applications, the HMC701LP6CE accepts VCO input frequencies from 100 kHz to 8 GHz (7 GHz fractional) and offers similar performance and features as the HMC702LP6CE.

The HMC700LP4E is a SiGe BiCMOS 8 GHz fractional-N frequency synthesizer IC in a 44 mm SMT package. The synthesizer includes a very low noise digital phase PFD and a precision controlled charge pump. The fractional synthesizer features an advanced delta-sigma modulator design that allows both ultra-fine step sizes and low spurious products. The HMC700LP4E PFD features CSP technology that allows faster frequency hopping times. Ultra low in-close phase noise and low spurious also permit architectures with wider loop bandwidths for faster frequency hopping and low micro-phonics. FSK mode allows the synthesizer to be used as a simple low cost direct FM transmitter source.

Table 1 summarizes Hittite’s fractional-N PLL synthesizer IC products. Hittite Microwave is committed to providing superior component solutions and product support that enables customers to be the best in the RF and microwave industry. The company’s product strategy is to continue to provide levels of integration that deliver incremental performance and value while maintaining the features that permit user flexibility.

All of Hittite’s fractional synthesizers feature built in 5 V charge pumps which can work directly with low voltage VCOs, or drive external op amps for high tuning voltage VCOs. Hittite offers advanced software for modeling both active and passive loop filters. Models are available for all of Hittite’s VCOs, and accurately predict phase noise and loop bandwidths for a wide variety of PLL-VCO combinations. Hittite’s fractional PLLs are supported with PC control software and simple USB port control.

Samples and evaluation PC boards for all SMT packaged products can be ordered via the company’s e-commerce site or direct from the company. Detailed datasheets may be found on the Hittite web site. For design assistance, please contact

Hittite Microwave Corp.
Chelmsford, MA
(978) 250-3343

RS No. 303

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