RFMD’s RF2051 Offers Higher Levels of Integration for Diversity Radios
By Mark Moffat and Thomas Eichhorst, RFMD®

Diversity reception is commonly used in wireless links that are susceptible to multipath effects. Multipath is a common problem in urban or indoor environments where the transmitted signal can reach the receive antenna after following a number of transmission paths. These multiple signals can suffer time delays, phase shifts, and attenuations that result in them adding destructively at the receiving antenna, causing abrupt fading and potential data or signal loss. An additional antenna located half a wavelength away will likely be experiencing very different signal conditions, particularly in the phase coherence of the multipath signals, that result in a much stronger reception. By selecting, or selectively adding, the outputs from the two diverse receiver channels, a more robust radio receiver can be designed.

Two radio systems where diversity is commonly employed to improve link quality are wireless microphone systems and MIMO (Multiple Input Multiple Output) systems for WLAN or WiMAX. Most wireless microphone systems now employ diversity reception to help maintain the radio link in the difficult environments of television studios or live concert venues. These spaces are typically cluttered with people and equipment that can cause interruptions to the radio signal with resulting audio quality problems. Additionally, the nature of multipath interference means that adverse signal conditions can be very localized within the performance area. By their very nature, wireless microphones are intended to be mobile, and therefore need to be resilient to these situations. One of the main reasons for the use of MIMO in WLAN and WiMAX wireless data systems is to increase the available data transmission rate for the system. In addition to exploiting multiple transmission paths, diversity is used to maintain signal strength.

Adding a second independent receive channel will inevitably mean additional hardware, including front end components (LNA, RF filter) as well as downconversion elements which require additional mixers and local oscillator (LO) generation. The challenge to the circuit designer is to provide this additional functionality while minimizing its impact on solution size, power consumption, and cost. In the case of wireless microphones, the microphone itself has to be very low power, but the receiver will typically be mounted within an equipment rack, where power consumption is not so critical. However, space is important within a studio or stage environment where many wireless microphone receivers need to be supported, so a compact solution is highly desirable.

Whereas highly integrated chipset solutions exist for higher volume WLAN and WiMAX wireless networking markets, there are no such application-specific products available for the lower-volume analogue wireless microphone market. Designers working on wireless microphone receivers are usually challenged to produce area- and cost-competitive designs using off-the-shelf LNAs, mixers, and PLL/VCOs. If we examine the simplified block diagram of such a system (also known as true diversity receiver), shown in Figure 2, it can be seen that two mixers are required–one for each channel–as well as a VCO, a PLL, and power splitter to drive the LO to each mixer. The RF design for this system is complex and area-intensive. The designer must also guarantee the overall functionality of the system for all performance combinations of the various components, which typically leads to inefficiencies.

However, there is one off-the-shelf component that offers higher levels of integration to the analogue wireless microphone system designer without sacrificing the flexibility to support various frequencies that can be employed worldwide for wireless microphone systems. The RF2051 from RFMD integrates two high dynamic-range mixers (30 MHz to 2.5 GHz) together with a monolithic VCO and fractional-N PLL that supports all LO frequencies from 300 MHz to 2.4 GHz. The PLL includes a crystal oscillator to allow the use of a low-cost crystal reference, and the fractional-N divider allows sub-Hz resolution for the required LO frequency, while its sigma-delta spreading function ensures reference spurs are kept to a very low level. The mixer linearity is programmable, allowing power consumption to be minimized. The LO signal is conditioned and amplified internally. The RF2051’s full duplex mode allows the LO signal to be routed to both RF mixers simultaneously, as required for a diversity receiver. The device is housed in a 5 x 5 mm QFN plastic package. Table 1 shows the RF2051 performance characteristics, and Figure 3 shows a block diagram for the part. Figure 4 shows some typical performance plots.

One feature of the RF2051 is that the mixers can be reconfigured according to the dynamic range requirements, allowing trade-offs between linearity and supply current to be made. The supply current per mixer is about 5 mA multiplied by the value programmed into mix_idd via the serial bus–thus a setting of mix_idd=001 is 5 mA per mixer, and mix_idd=100 is 20 mA per mixer. Varying the mixer current between 5 mA and 20 mA yields an IIP3 improvement from +10 dBm to over +20 dBm. Mixer isolation is very good for a monolithic implementation, with 70 to 80 dB of RF isolation between the two mixers. This should be adequate for most diversity receiver applications. The mixers have -3 to -6 dB of power gain. Additional gain is not included in the device to allow the designer to choose a specific gain block with the right performance characteristics for their application. Currently this application operates in the 470 MHz to 862 MHz band, but the synthesizer/VCO block in the RF2051 is very broadband, extending up to 2400 MHz, to provide a future-proof solution for prospective analog systems operating at higher frequencies, such as the L-band. Analogue wireless microphone systems use FM modulation, which requires a signal source with very low phase noise close to the carrier. The synthesizer/VCO block in the RF2051 produces a very stable, low noise signal with ideal characteristics for FM systems with frequency deviations of 50 KHz and above. Using the RF2051, wireless microphone designers can develop diversity receivers with a signal-to-noise ratio of better than 60 dB, which is the minimum requirement for high-end wireless microphone systems. The LO interface between the VCO and the mixers is optimized to reduce power consumption, and the overall performance of the frequency-conversion function is guaranteed and tested, avoiding some of the inefficiencies inherent in a properly toleranced discrete design.

Figure 5 shows a revision of Figure 2, a block diagram for an FM diversity wireless microphone, but using RF2051 instead of the discrete components that were previously employed. Visually, it is clear that the revised solution offers advantages in terms of integration and complexity, but there are also advantages in power consumption and cost.

Further savings in BOM cost and engineering time can be achieved if the same solution is being reused across a number of products from the same family. Implementing the RF2051 in a low-power CMOS process cuts the power consumption by 50 percent compared to traditional discrete designs, which is important when trying to incorporate multiple receivers in a single rack-mount unit.

Given the proven benefits of diversity radios, it is not surprising to witness the widespread interest in adopting these for various applications. Although wireless microphone systems offer a prime example, the same principles of operation and design can be applied to many other applications. With the RF2051, RFMD has developed a multipurpose, wideband diversity receiver that greatly simplifies the task of designing a compact, low-power, low-cost diversity radio.

About the Author
Mark Moffat is the director of the Emerging Products Line and has been with RFMD for nine years, working on products for the broadband and cellular markets. Thomas Eichhorst is the manager of Marketing and Applications of the Emerging Products Line and has been with RFMD for three years, launching and expanding the portfolio of integrated synthesizer products. For more information, please contact Thomas Eichhorst at teichhorst@rfmd.com.

RFMD®
www.rfmd.com
Email this article to a friend!