Broadband communications equipment is increasingly pushing the envelope of mixed-signal technology in terms of speed, distortion, signal-to-noise ratio (SNR), and cost. Many broadband wireless systems require wideband, low-noise analog-to-digital converters (ADCs) and digital-to-analog converters (DACs) with 10 to 14 b of resolution. If architectures aren't carefully considered, these requirements can result in costly solutions.

Digital filtering and processing techniques can reduce the need for expensive analog circuitry. Such techniques include interpolation, decimation, and direct digital synthesis. For example, an architecture that uses multi-rate digital filters and gain stages can be used as part of an image-rejection architecture on both the receive (Rx) and transmit (Tx) paths. With this combination of features, it's possible to reduce component count and lower system cost.

TRANSMIT PATH
The transmit path of a digital transceiver ultimately requires a DAC to create the analog broadcast signal. When a signal is generated by a DAC or undergoes frequency translation using a mixer or upconverter, unwanted duplicate signals are produced. These signals are called images or aliases. Analog low-pass filters can remove these images and other spurious noise from the signal. Practical analog filters can be expensive, however. They also have limitations that require designers to make tradeoffs between transition band size, in-band attenuation, out-of-band attenuation, dimensions, and cost.

Fortunately, digital-processing operations can ease filtering requirements. One technique involves the initial separation of the digital baseband signal into its in-phase and quadrature (I&Q) components. Generally speaking, a great deal can be gained by working with these component signals and converting them to a single analog signal as close to the antenna as possible. Implementing this scheme requires a pair of DACs. The output of these DACs is fed to a quadrature mixer, thereby forming an image-rejecting quadrature upconversion architecture.

Quadrature modulation, which requires precise phase relationships, is not a new concept. More than 40 years ago, it was used to produce single-sideband radio signals. With analog circuitry, however, maintaining quadrature phase relationships over wide bandwidths is not easily accomplished. These techniques have mostly been used at low IF frequencies, with the aim of removing the redundant sideband and eliminating the "carrier."

The development of low-cost, integrated direct-digital-synthesizer (DDS) circuits has changed the game. Products such as the AD9879, for example, include a DDS that produces digitally precise quadrature output signals (with a typical accuracy of two-tenths of one degree). It operates from dc to greater than 70 MHz using a 210-MHz clock source. That clock source can be derived from a high-quality LO if divided-down appropriately. The quadrature phase error of quadrature modulators, like the one in the AD9879, is on the order of one degree over its output range.

An advantage of the DDS system is that its output frequency and phase can be precisely and rapidly manipulated under digital processor control. Other inherent DDS attributes include the ability to tune with extremely fine frequency and phase resolution. This would include frequency control in the millihertz (mHz) range, as well as phase control of less than 0.09°. The DDS system also has an innate ability to rapidly hop frequencies (up to 23 million output-frequency changes per second). These characteristics have combined to make the technology popular in digital communications systems.

Digital filtering of the baseband signal also can simplify the filtering required later in the signal chain. Generally speaking, increasing the DAC sampling rate can ease anti-alias filter rolloff requirements. While it may be possible to increase the sampling frequency of the DAC simply by using a faster DAC, a more economical alternative is to increase the effective sampling rate of the signal through interpolation.

The interpolation process involves the insertion of zero samples between data samples. The combination of these samples is fed to a finite-impulse-response (FIR) or cascaded-integrators (CIC) digital filter, which can have a low-pass or a bandpass characteristic. The new "zero-stuffed" sampling rate is used to update the DAC. This process results in a wider transition band, while easing the requirements of the analog smoothing filter at the output of the DAC. The aliases of the signal appear at a frequency that is higher than the original sampling rate by approximately the interpolation factor.

How do these techniques fit together? One implementation is shown below in Figure 1. This block diagram illustrates the transmit path of the AD9860 broadband-modem mixed-signal front end. The AD9860 architecture shows one method of implementing complex mixing. It also copes with the filtering and frequency-tuning issues that are encountered in the superheterodyne upconversion process. As mentioned earlier, quadrature upconversion requires separate I&Q signals. If I&Q signals are not available, they can be created with the use of a phase-shifting Hilbert filter from a low-IF real signal.

This example features a digital two-mixer architecture. When used in combination with the first mixer, the DDS provides fine frequency tuning. As mentioned previously, the complex low-IF signal is fed to an interpolation filter to increase the separation between the aliases. The signal is then mixed up to an IF that is the tuned frequency Fs/4 or Fs/8.

Note that a single mixer and DDS combination with a wider tuning range could have been employed in place of the second mixer. This simplified architecture, with a higher-resolution DDS running at a lower rate, is designed for low-cost integration and low power consumption. This approach comes at the expense of more complexity in the interpolation filters. They require a larger pass-band and a smaller transition band.