Advances in integration for base station receivers

The increasing demand for data services on mobile phones puts continuous pressure on base station designs for more bandwidth and lower cost. Many factors influence the overall cost to install and operate additional base stations to serve the increased demand. Smaller, lower power electronics within a macrocell base station help to lower the initial costs as well as the ongoing cost of real estate rental and electrical power consumption for the tower. New architectures such as remote radio heads (RRH) promise to decrease costs even further. Tiny picocell and femtocell base stations extend the services to areas not covered by the larger macrocells. To realize these gains, base station designers need new components with very high levels of integration and yet they cannot compromise performance.

Integration in the RF portion of the radio is especially challenging because of the performance requirement. Over a decade ago, the typical base station architecture required several stages of low noise amplification, down-conversion to an intermediate frequency (IF), filtering and further amplification. Higher performance mixers, amplifiers and higher dynamic range analog-to-digital converters (ADCs) with higher sampling rates have enabled designers to eliminate down-conversion stages to a single IF stage today. However, component integration remains somewhat limited. Mixers are available with buffered IF outputs, integrated balun transformers, LO switches and dividers. A device with a mixer and a PLL for the LO represents a recent advance of integration. Dual mixers and dual amplifiers are available. As yet, no device is available that integrates any portion of the RF chain with the ADC on the same silicon. This is primarily because each component requires unique semiconductor processes. The performance trade-off associated with choosing a common process has been unacceptable for the application.

In parallel, the handset radio has evolved to highly integrated baseband and transceiver ICs and integrated RF front-end modules (FEM). RF functional blocks between the transceiver and antenna include filtering, amplification and switching (with impedance matching incorporated between components where needed). The transceiver integrates the receiver ADC, the transmit DAC and the associated RF blocks. Here the performance requirement is at a level such that a common process is viable. The FEM utilizes a system-in-package (SiP) technology to integrate various ICs and passives, including multi-mode filters and the RF switches for transmit and receive. Here, a common process was not viable but integration was still required.

The performance requirements for the RF/IF, ADC and DAC components in picocell and femtocell base stations tends to be much lower than for macrocell base stations because their range, power output and number of users per sector are lower than for macrocells. In some cases, modified versions of components for handsets can be used for picocell or femtocell base stations, providing the necessary integration, low power and low cost. Here, a common semiconductor process provides sufficient level of performance for all of the functional blocks in the signal chain.

Information is shared by www.irvs.info

Layout & bypass guidelines for high performance video amp/filter boards

Integrated circuit amplifiers are one of the most basic building blocks in any designer’s tool box and represent one of the most versatile products available.

An amplifier can provide a wide variety of functions such as driving ADCs, drive multiple video loads, operate as video or other types of filter, drive high speed instrumentation signals, and much more. They can also act as oscillators, but that can be actually be a problem since an amplifier should oscillate only when the designer wants it to.

However, an amplifier can have a mind of its own and oscillate at will if the board is designed incorrectly. So what should a designer do to protect against unwanted oscillation? The main thing is to recall the lessons of early electronics classes which taught that oscillation a function of capacitance, inductance, and feedback.

So, the key is to make any extraneous capacitive and inductive feedback paths are reduced or eliminated by designing the board carefully. This is especially important for higher speed amplifiers (greater than 50MHz).

Invisible sources of capacitance and inductance can come from the board, the load (especially if the load is capacitive), and/or the layout. Furthermore, currents flowing to bypass capacitors on different places on the board can take different paths which can lead to distortion.

So, ironically, some techniques for reducing distortion are the opposite of what is recommended to guard against oscillation. (A designer’s job is never easy, is it?) So what are some of the layout considerations to keep in mind to keep everything in balance and fight distortion and oscillation when designing in an amplifier or video filter?

Looking first at oscillation, when directly driving a capacitive load with an amplifier, the load together with amplifier's output impedance creates phase lag which can cause peaking or oscillation.

Some amplifiers have the ability to drive capacitive loads, but for those that cannot, a small series resistance (Rs) placed at the amplifier’s output can enhance stability and settling time (Figure 1 below).



Figure 1: A small series resistance (Rs) placed at the amplifier’s output can enhance stability and settling time.
When driving a transmission cable with an inherent impedance (such as a coax cable) as shown in Figure 2 below, resistors Rs and RL should be set equal to the cable’s impedance (Zo), and the capacitor C should be set to match the cable impedance over a wide frequency range to compensate for the amplifier's increasing output impedance (with increasing frequency).



Figure 2: Driving cable or transmission line.

This information is shared by www.irvs.info