Tee Yong Lim, Product Marketing Manager at Peregrine Semiconductor discusses how UltraCMOS RF switches are offering a switching component that functions across a broad bandwidth providing good linearity and isolation
Global sales in support of broadband TV are surging. A recent report from ABI Research projects that Asia-Pacific revenues from IPTV delivery and fibre broadband are likely to increase by 34 percent, from $110billion to $147billion between 2010 and 2011.
Demand for broadband content is also growing in North America and Europe, with IMS Research expecting Western Europe to demonstrate 30.5 percent of the share of world VOD (video on demand) revenues, second only to North America’s 33.1 percent share.
In addition to this, global set top box (STB) shipments are expected to rise from 205 million in 2010 to 226 million in 2015. Broadband TV devices and STBs around the world are evolving in complexity, featuring dual 75ohm inputs and multiple tuners to support DVRs, data downloads, over-the-top (OTT) Internet content, and home-gateway applications.
For a large part of the world market, particularly in Western Europe, Brazil, Taiwan, India, and China, many TVs must support analogue, digital, and over-the-air (OTA) signals. These types of systems have very stringent requirements for isolation and linearity in order to ensure that signals do not interfere with each other or leak out through the antenna.
In these dual-input TVs (analogue and digital), it is critical to have a switching component that functions across a broad bandwidth and provides excellent linearity and isolation (in both powered and unpowered states) in order to minimise the impact on the signals making their way to the tuner(s).
In the past, the way to achieve the highest isolation and linearity was to use mechanical relays or multiple solid-state switches.
Engineers at Peregrine Semiconductor, recently applied the company’s UltraCMOS technology to develop solid-state PE42750 75-ohm single-pole, double-throw (SPDT) RF switches (figure 1). This technology provides the broadband performance of a mechanical relay in a solid-state device.
The new part offers low power operation (8uA typical power supply current from +3VDC at 250C).
The switch operates across 5 to 2200MHz and is targeted at broadband applications such as CATV, digital TV (DTV), multi-tuner digital video recorders (DVRs), STBs, PCTVs, and game consoles. Unlike GaAs solid-state RF switches that target this market, the UltraCMOS device ensures high isolation performance even when the system is in an unpowered state.
The UltraCMOS is silicon-on-sapphire (SOS) technology that provides high linearity and isolation. Since sapphire is a fully dielectric substrate that isolates each transistor, it eliminates parasitic drain capacitance and provides better isolation between circuit elements. Because there are no voltage variable capacitances or parasitic diodes in the junction wells, SOS enables good linearity and high isolation.
Linearity measurement
Linearity is a measure of how the part maintains the integrity of the signal, or how much distortion is added to the signal. Meeting performance specifications in an RF signal chain and ensuring a distortion-free picture requires high linearity for all of the components in the tuner signal path, including switches.
Linearity is particularly important in applications with a crowded spectrum, such as CATV, because nonlinearity at any frequency will cause harmonics to occur.
In a densely packed spectrum, any harmonics will occur directly on top of another channel, effectively cancelling out the signal and degrading or eliminating the picture entirely.
In an SPDT switch, linearity can be measured in input second order intercept point (IIP2). For instance the switch example mentioned here demonstrates +100dBm IIP2.
The other significant specification for a CATV SPDT switch is isolation. TVs today have multiple tuners and signal inputs, so high isolation is required for both the input and output to prevent signal degradation and poor signal quality.
As TVs shrink in size and become more complex, the PCBs in the TV become denser, and maintaining isolation becomes more difficult. This all combines to drive the need for higher isolation switches that maintain high isolation even when the power is off.
In the US, this issue caused the FCC to develop a spectral mask requirement (part 15.115) for isolation.
The PE42750 is designed to prevent CATV leakage through the antenna, even when the power is off because all of the ports in the switch are terminated when unpowered.
In terms of RFC to RFX isolation (which prevents channel to channel interference), the RF switch demonstrates ~10dB better isolation than the comparable GaAs part, which offers <1000MHz.
In terms of transient voltage or voltage ripple (noise added when switching RF paths between the two output ports) the switch provides 2mVpp while the comparable GaAs part is 70mVpp. The RF switch also offers considerably low power consumption, 8µA typical, from a 3VDC supply.
As consumer electronics designers look to scale back the traditional ‘power hungry’ nature of the television, this is a significant advantage. In addition, because it is manufactured using the UltraCMOS process rather than GaAs, the SPDT switch has inherent ESD advantages, which can improve robustness during manufacturing and consumer usage. For example, the device features Vs ESD protection of 2kV HMB and 1kV CDM on all ports, while a comparable GaAs part, such as the M/A COM MASW-00801, specifies ESD of <500V HBM.
As designers of cable television (CATV), digital and PC TV (DTV/PCTV), multi-tuner digital video recorder (DVR), set-top box (STB) and game console systems select devices for the RF signal chain, they will find that they can benefit greatly from the performance and reliability advantages offered by UltraCMOS switches.
