XVME-491 Receiver IC mixer, frequency synthesizer and IF amplifier
XVME-491 Wireless base stations were once packed into large Spaces with climate-controlled technology, but now they can be installed anywhere. As wireless network service providers try to achieve global signal coverage, base station component providers are under pressure to provide more functionality in a smaller package.
A pair of integrated circuits (ics) from Analog Devices offers a solution that redefines the meaning of receiver front-end mixers. In effect, the IC integrates within the mixer IC many components that were once attached to the mixer in the receiver, such as local oscillator (LO) and intermediate frequency (IF) amplifiers. With these ics, the size of cell sites can be significantly reduced, while also bringing the flexibility of software-defined radio (SDR) to deal with many different wireless standards.
XVME-491 The IC models involved here are the ADRF6612 and ADRF6614, which are designed to support RF ranges from 700 Mhz to 3000 MHz, LO ranges from 200 Mhz to 2700 MHz, and IF ranges from 40 Mhz to 500 MHz. They support low – or high-end LO injection and include an onboard phase-locked loop (PLL) and multiple low noise voltage controlled oscillators (VCO), all packaged in a 7 mm x 7 mm 48-pin LFCSP housing. The high level of integration and component density, combined with the diversity and programmability, can support a variety of different wireless standards, fully meeting the small batch production needs of modern microcells.
XVME-491 To better understand the space-saving benefits of these highly integrated mixer ics, it’s worth recalling the front end of a cellular base station circa 2010, as shown in Figure 1. The dual mixer architecture had a bandwidth range of about 1 Ghz and required multiple components to handle the cellular frequency range of the time, which was 800 MHz to 1900 MHz. The frequency synthesis is provided by a separate PLL and narrowband VCO module and requires a unique PLL loop filter for optimal performance. Dedicated VCO modules are used for each target band, resulting in an increase in the required board area within the base station.
XVME-491 In addition, these discrete components are connected to each other via low-impedance transmission lines, resulting in increased signal loss. As a result, a large current is required to drive the VCO output to a sufficient level so that the mixer can produce low phase noise and noise factor under signal blocking conditions.
Receiver ics with integrated VCO are not new. However, Global System for Mobile Communications (MC-GSM) wireless networks have been a challenge to achieve the broadband and low phase noise required by multi-carrier. The channel multiplexing scheme of GSM requires the receiving LO to have very low phase noise, especially in the case of the inter-channel offset frequency of 800 kHz, as shown in Figure 2. IF the excess phase noise of these interphase channels is mixed with an unwanted signal that is also under the off-balance condition of 800 kHz, the phase noise may be converted into an IF output, thereby reducing the sensitivity of the system.