Direct conversion of RF signals to baseband has long been a goal of communications systems designers. The approach can eliminate expensive and bulky hardware, but at the cost of some trade-offs in performance. What follows is an examination of the trade-offs associated with designing a direct-conversion receiver (Rx) compared to a traditional superheterodyne architecture, as well as details on a new direct-conversion Rx integrated-circuit (IC) subsystem.

The function of a conventional superheterodyne Rx (Fig. 1, left) and a direct-conversion Rx (Fig. 1, right) is the same: translating and conditioning signals downward in frequency so that they can be sampled by low-frequency (baseband) analog-to-digital converters (ADCs). As is apparent from a comparison of the two block diagrams, the direct-conversion system achieves this function with considerably fewer components.

In a conventional single- or double-conversion superheterodyne Rx, the modulated RF signal is translated in frequency through one or more intermediate frequencies (IFs) before being converted back to its desired baseband format. At IF, the signal is filtered and amplified before being mixed to a lower frequency. In a direct-conversion Rx, the modulated RF signal is mixed with a local oscillator (LO) at exactly the same frequency.

The obvious reduction of component count, along with the elimination of expensive filtering has made direct conversion very appealing as an architecture for transmit and receive functions. However, it is only recently that components which facilitate its practical implementation have become available.

Although the direct-conversion approach reduces the component count, it also adds design challenges. In the superheterodyne approach (Fig. 1, left), by driving the mixer with a frequency-agile LO, the frequency of the desired signal or channel (which is generally varies in a multi-user system) is converted to a fixed frequency. Once the desired signal has been converted to a fixed IF, it can be processed by highly selective narrowband filtering using a surface-acoustic-wave (SAW) filter. In addition, all subsequent frequency translations can be effected using fixed-frequency LOs.

The other important function performed at IF in a superheterodyne system is signal amplification. Fixed-gain amplification, in the form of low-noise amplifiers (LNAs), is generally applied at RF, while signal leveling is generally accomplished through the use of variable-gain amplifiers (VGAs). Since it is easier to design a high-gain-range VGA at lower frequencies, and because many unwanted signal components have already been removed from the carrier by the time it is translated to IF, variable-gain amplification is generally performed at IF or baseband frequencies. This "distribution" of gain avoids a concentration of high gain at the Rx's front-end portion, which can cause saturation of subsequent stages, especially if large in-band or out-of-band signal blockers are present.

While the appeal of a direct-conversion Rx lies in its elimination of IF stages, therein lies its weakness. Because there is no longer an IF stage at which signal leveling and filtering can be conveniently performed, all signal conditioning must be performed either at RF or baseband. In a multichannel system, the capabilities of RF filters are limited (they cannot be narrowband), at best screening out-of-band interferers. SAW filters are available for some RF uses (through approximately 3 GHz), but they are generally more expensive than lower-frequency IF filters due to the higher quality-factor (Q) requirements.

A cost-effective means of realizing a direct-conversion Rx is through monolithic fabrication, by including as many of the required components on a single chip, such as the AD8347 direct-conversion Rx IC (Fig. 2) from Analog Devices (Norwood, MA). This Rx IC can demodulate signals from 0.8 to 2.7 GHz. The device consists of a number of subcircuits that can be configured separately. The IC includes all of the components required for amplification, downconversion, and filtering in a direct-conversion demodulation circuit.

Since gain control in a direct-conversion design must be implemented at RF and/or at baseband, the AD8347 employs three stages of VGA. Two stages are employed at RF and one at baseband, following the I/Q demodulator. The three VGA stages, which have a convenient linear-in-dB gain-control relationship, are controlled in parallel. Each of the two RF VGAs has a gain range from −10 to +13 dB. The combined gain range of the mixer and the baseband VGA is −10 to +14 dBm. The overall gain range from the RF input to the mixer output is therefore −30 to +40 dB.

The gain of the VGA is set by the voltage on the AD8347's VGIN pin, which is a high-impedance input port. Figure 3 shows a plot of the gain versus gain-control voltage, along with the linearity of the gain-control function. Note that the sense of the gain-control voltage is negative. As the gain-control voltage increases from +0.2 to +1.2 VDC, the gain decreases from +40 to −30 dB.