Single Vendor for Antenna-to-Bits Signal Chain Offers Tangible Benefits to Designer Engineers

RF/wireless designers have traditionally been served, and served well, by specialized vendors offering critical pieces of the signal chain between the analog antenna and the digitized bits. The vendor universe was crowded – and still is – with independent suppliers of PLLs, LNAs, ADCs, and DACs, among the functional blocks and components of the receive and transmit channels.

This multisource model is evolving, however, as an increasing number of RF/wireless functions are implemented as ICs and modules, as a result of advances in the technology. Improved IC processes are enabling IC vendors to move into ever-higher frequencies ranges while also providing functions which were previously served by multiple devices from different vendors. Instead of piecing together the signal chain from disparate sources, the design engineer can now look to a single qualified source for all or nearly all of the components needed.

The result is that designer can take a new approach to planning, analyzing, and fulfilling the project's signal-chain requirements. Instead of juggling multiple web sites and vendor data-sheet styles and definitions, the designer can focus on finding the components with the proper balance of must-have and nice-to-have attributes for their requirements. This saves time, reduces frustration, and simplifies proper matching of components in today's design environment, where every dB in performance is critical, losses accumulate, and second-and third-order specifications are the difference between a barely "good-enough" design and one which fully meets requirements, Figure 1.

Figure 1: Today's design engineer must incorporate more functions at higher frequencies, along with stringent performance requirements, into complex designs.

There are tangible benefits to designers who look to vendors with a full signal-chain product line, as demonstrated by the 2014 acquisition of Hittite Microwave Corp. by Analog Devices, Inc (ADI). At the time, each vendor was a respected leader primarily in separate functional and frequency regions of RF signal chain, with only modest overlap. While the combination yielded internal benefits in terms of production efficiency, test, and supply-chain management, the real beneficiaries are the designers who must decide on components and vendors. Designers now have one-stop access to best-in-class technology and deliverable, supported products

There are several reasons for these advantages to engineers. First, designers now more easily see a much-broader set of possibilities for the component their signal chain, in terms of performance tradeoffs and range, as well as price/performance increments. It also reduces design-in headaches, time, and risk since the components of the signal chain are better matched to each other in parameters and performance. There's less design-in spent choosing suitable high-end A/D converter, for example, and then searching for a matching buffer amplifier, when instead a single vendor can provide both — and can suggest the pairing to the designer, as well via online selection guides. 

Consider a representative RF design of a low-end spectrum analyzer, Figure 2. For all the active functions, Analog Devices now offers a choice in components with different performance specifications among the multiple parameters which fully characterize each device.

Figure 2: Analog Devices offer component choices for each active-component function in this block diagram of a typical low-end spectrum analyzer, rather than "one size fits all."

The availability of a broad and deep product line affects the designer in many positive ways. First, there are more fully characterized solutions, with assurance of electrical and specification compatibility between stages. The  consistency in parameter definitions and terminology across these means designers no longer have to try to figure out if the definition and test conditions used by one vendor are the same as those used another vendor for a given function.

Factors such as SFDR, dynamic range, bandwidth, ENOB, IP3, and noise figure are more easily compared in a meaningful part-to-part comparison. At the same time, the designer has assurance that the interfaces between functional blocks will have consistent definitions and compatibility, thus easing the process of selecting the end-to-end string of components.

There are also the implications with respect to reference designs, which today's design teams rely on heavily to establish their project framework and baseline. When a single vendor offers all or most of the active devices in the signal chain, it's feasible to develop complete, tested, and fully characterized reference designs and documentation, rather than just piece-meal designs for a single component or sub-function, such as the digitizer. This speeds user decisions and evaluations. In addition, the vendor can offer critical support software including GUIs and test suites, and even formal qualification to industry standards.

For example, Analog Devices' CN0320 is a flexible, frequency-agile, direct-conversion IF-to-baseband receiver with variable baseband gain and programmable baseband filtering, and a dual ADC, Figure 3. The core of the circuit is an integrated I/Q demodulator with fractional-N PLL and VCO. With just one reference frequency, the PLL/VCO can provide a local oscillator between 750 MHz and 1150 MHz. Precise quadrature balance and low output dc offsets ensure that there is minimal degradation of the error vector magnitude (EVM).

The interfaces between all of the components in this circuit are fully differential, and the bias levels of the adjacent stages are compatible with each other where dc coupling is required between stages. The complete reference-design package includes schematic, BOM, Gerber files, Allegro files, and even the assembly drawing.

Figure 3: Vendor-supplied reference designs are an increasingly important part of the design decision; this simplified schematic of a direct-conversion IF-to-baseband receiver uses a trio of Analog Devices' RF components as the core of the antenna-to-bits signal chain.

As a result, time-pressed designers of systems spanning moderate-performance, cost-sensitive products to leading-edge, high-performance systems will spend less time spent trying to decide on "best" part for each stage. Instead, they can focus on the most critical stage(s) in their applications, with more time for detailed system design and test. As a further benefit, their BOM challenges will be reduced since they can deal with a single qualified, reputable supplier. They will even have an easier option of being able to re-use a known and understood component in upgraded designs or product extensions, thus minimizing potential surprises that often accompany using any new part for the first time.

On the vendor side, there are tangible benefits to proving full signal-chain support, and these soon extend to the user as well. The vendor's applications group, an important factor in time to market, is in a better position to help users, due to deeper technical insight into full antenna-to-bits path and the specific application. Further, the vendor is like to recognize and fill any potential component gaps in needed device or performance, such as a converter buffer preamp optimized for specific application.

The vendor can offer a broad array of parts, based on access to multiple design techniques and packaging options (such as monolithic, multichip, flip chip), rather than limiting the user to a restricted menu. Users can choose between parts with best-in-class performance along different parameters.

For example, the HMC703 fractional synthesizer for DC-to-8 GHz RF inputs (Figure 4) has the industry's best phase-noise and spurious performance (-112 dBc/Hz @ 8 GHz fractional, 50 kHz offset) – thus enabling higher-order modulation schemes. It also has the best figure of merit (FOM) of -230 dBc/Hz (fractional mode) and -233 dBc/Hz (integral mode). Further, the HMC703 offers frequency sweep and modulation features, external triggering, double-buffering, exact frequency control, phase modulation, among other attributes.

Figure 4: The HMC703 fractional synthesizer for DC-to-8 GHz RF inputs features industry's best phase-noise and spurious performance and figure of merit (FOM).

For users who prefer higher levels of functional integration, the ADF4355 6.8 GHz wideband synthesizer (Figure 5) has an integral VCO with a fundamental output frequency ranging between 3400 MHz and 6800 MHz. The VCO frequency is connected to divide-by-1, 2, 4, 8, 16, 32, or 64 circuits that allow the user to generate frequencies as low as 54 MHz. The fractional-N synthesizer and integer-N synthesizer feature a high-resolution 38-bit modulus counter; the VCO offers low phase noise between -113 dBc/Hz (100 kHz offset from 5.0 GHz carrier) and -155 dBc/Hz (10 MHz offset from 3.4 GHz carrier) along with normalized in-band phase-noise floor of -221/-223 dBc/Hz (Fractional Channel/Integer Channel) and Integrated RMS jitter of 150 fsec.

Figure 5: the ADF4355 6.8 GHz wideband synthesizer with internal VCO also includes a 38-bit modulus counter, along with impressively low phase noise and in-band phase-noise floor.

A broad-line supplier brings other advantages to the project. The vendor has deeper insight into the entire signal chain and application match-up, as so can develop better, broader modeling and simulation tools. Setting up simulations will be easier, with few or no gaps in the signal chain.

For example, ADIsimRF is an easy-to-use RF signal chain calculator which calculates cascaded gain, noise figure, IP3, P1dB, and power consumption, Figure 6. It contains embedded device models for most of ADI's RF components (including those offered by Hittite, now part of ADI's portfolio) and supports up to 20 stages; they can be easily inserted, removed or temporarily muted. The calculator can be switched between Transmit Mode and Receive Mode with data presented as output-referred and input-referred, respectively.

Figure 6: An essential up-front step in RF channel design is circuit modeling and simulation; ADIsimRF tool calculates critical performance results for a signal chain of up to 20 components. 

There are other tools, as well. ADIsimPLL is an easy-to-use PLL synthesizer design/simulation tool which supports simulation of the key non-linear effects that impact PLL performance including phase noise, Fractional-N spurs, and anti-backlash pulse; as with ADIsimRF, it includes the Hittite components. For designers who are selecting or evaluating high speed ADCs, VisualAnalog™ combines a powerful set of simulation and data analysis tools with a user-friendly graphical interface, and allows designers to customize their input signal and data analysis. Its customizable calculations allow designers to predict adjacent channel power ratio (ACPR), and display I/Q constellation plots on simulated ADC outputs or data collected from evaluation boards.

Conclusion

Looking to a supplier with a broad portfolio from antenna to bits can simplify the RF/wireless design across many dimensions. It eases not only basic component consideration and selection, but also modeling, simulation, and software-driver issues. Further, it provides a better design-support path, including reference designs, application engineering, and applications notes.