A building-block strategy for faster RF designs
We live in a world that is increasingly connected, driven by the growing phenomenon that is the Internet of Things (IoT). The need to provide wireless communications is omnipresent in electronics design. From wearables to smart home appliances the range of applications using RF is growing rapidly.
By Nigel Wilson, CTO, CML Microcircuits
As a consequence, the number of standards for such radio links (air interfaces) is also increasing quickly. Further, there are the traditional professional RF markets of wireless data, satellite communications, avionics and marine, along with a host of other scientific, medical and industrial applications.
Even with the ubiquity of 2.4 GHz wireless applications such as Wi-Fi and Bluetooth, the lower radio spectrum is still very much alive with the diversity of HF, VHF and UHF systems, some of which are occupying bands previously used by terrestrial broadcasting. Faced with implementing RF features into a new product design and mindful of the constant pressures of time-to-market, many engineers are finding the task increasingly daunting. The traditional approach of embarking on a discrete design is quickly dismissed, yet the need to deliver connected designs remains.
To help solve this conundrum, RF designers need silicon radio suppliers that are expert in the design, development and supply of low-power analogue, digital and mixed-signal semiconductors for global telecommunications systems. Further, there is a need to provide a selection of modular building blocks for implementation as part or all of the ‘wireless front end’.
In summary, such devices are designed to provide the flexible, high-performance ICs required for HF/VHF/UHF designs that enable engineers to adopt a building-block method for RF design; an approach that uses versatile, low-power and well-supported RF IC devices to speed the design cycle.
The challenge of RF design and applications
Designing RF circuits can be extremely challenging. Some engineers prefer the approach of using discrete devices for creating designs. This methodology, however, is at odds with the fast-moving commercial pressures to get products to market before the competition. These factors are driving significant change in wireless applications.
Evolution of wireless networks towards higher data rate and higher capacities, typified in applications for Wi-Fi and Bluetooth, is one of the dominant drivers for the semiconductor industry. It also introduces many challenges to current design techniques, particularly the demanding requirement for (ultra-)low power consumption. The rapid growth of wireless services increases the need for highly integrated and low-cost solutions. The fast-growing market and heated competition require very short system development cycles. To cope, new design practices for wireless systems have become imperative.
Built on the paradigm of platform-based design, a new, better-adapted approach than the traditional method of deploying discrete RF devices has emerged. It features the adoption of higher levels of abstraction, i.e. building blocks, better reusability and early consideration of system performance.
A good example is designing a wireless receiver – a complicated system consisting of RF, analogue and mixed-signal components. Traditionally, system design and discrete circuit design are conducted separately. However, under the building-block design approach, several challenges in the wireless receiver design are reconciled.
UK-headquartered CML Microcircuits is a world leader in delivering RF building-block solutions that are ideally suited to diverse range of systems. The diagrams below (figure 2) show examples of typical system architectures for high-integration implementations for: digital/analogue two-way radio (TWR), wireless data (WD) telemetry, and software-defined radio (SDR).
The building blocks of such designs typically include the following elements:
- Receiver (Rx) – Design considerations include distributing the gain across the RF, intermediate frequency (IF) and baseband (BB) electronics.
- Transmitter (Tx) – An RF transmitter performs modulation, up-conversion and power amplification.
- Transceiver (XCVR) – An integrated combination of the above two.
- Power amplifier (PA) – The RF PA is a key factor in achieving performance, reliability and acceptable cost.
- Mixer – A frequency translation device with two prime functions:
- Convert RF frequency to an intermediate frequency (IF) or baseband.
- Convert BB signal or IF to a higher IF or RF for transmission.
- Local oscillator (LO) – Oscillation occurs when an amplifier is provided with a feedback path that satisfies amplitude and phase conditions. A voltage-controlled oscillator (VCO) can be used as part of a programmable phase-locked loop (PLL) to tune a LO over a given range of frequencies.
Building-block approach to RF design
It is advantageous to use existing commercial devices to deliver RF design solutions. CML’s CM97x and CM99x series of devices include integrated receiver, transmitter, transceiver, modulator/demodulator and PLL functions, which will speed the design process and reduce time-to-market. A summary of device specifications is as follows:
- CMX971 – A high-performance quadrature modulator featuring a wide operating frequency range. Control of the CMX971 may be either by serial bus or direct control. Programmable features include LO divider ratio (two or four) and optimised operation (for noise or linearity).
- CMX975 – An IC that expands the frequency reach of CML’s RF building blocks; it provides multiple functions: RF PLL/VCO, IF PLL/VCO, transmit up-convert mixer, Rx down-convert mixer and LNA. The RF high-frequency synthesiser employs a fractional-N design and operates up to 3.6 GHz using a fully integrated internal VCO or up to 6 GHz with an external VCO. The IF synthesiser employs an integer-N design and will operate at up to 1 GHz. It has an integrated VCO requiring only an external inductor to set frequency. The Rx mixer can be configured in image reject or normal mode, and the Tx mixer can be configured in sideband suppression or normal mode. The integrated LNA offers 18 dB of gain reduction in three steps.
- CMX99x – The CMX991 quadrature transceiver, CMX992 quadrature receiver, CMX993/993W quadrature modulator, CMX994A/E direct conversion receivers and CMX998 Cartesian feedback loop transmitter are a family of highly flexible ICs working in the RF frequency range 100 MHz to 1 GHz, with the CMX993/993W and CMX998 operating down to 30 MHz. These ICs, alone or in combination, address the needs of many over-air formats for data and encoded-voice operation in both constant-envelope and linear modulation systems. To produce savings in PCB costs, these products require a minimum of external circuitry and are available in compact VQFN packages. To enable the shortest design-in time CMX99x products are well supported by ready-to-use evaluation and demonstration aids along with a range of application information.
The following key benefits and advantages of using a building-block or modular approach to RF design, as opposed to the use of discrete components and circuits, can be simply summarised as follows:
- Shorter design cycle.
- Faster time-to-market and reduced time-to-money.
- Simpler testing of the finished design.
- Fewer components required to complete the design.
- Higher reliability.
- Enhanced performance
- Better control of tolerances.
- Lower-cost end products.
While the use of discrete components can offer some applications a greater degree of flexibility in most cases the use of integrated building blocks will yield some if not all of the above list of benefits. The technology and products delivered by CML are ideally adapted to this approach to RF design.