Mixed-signal products such as cellular phones consist of "important" digital functions that appeal to marketers and consumers and "unimportant" analog functions that have to relate to the real world. The struggle to build these analog subsystems, in turn, requires communication and cooperation between RF designers and their colleagues who design analog ICs. Unfortunately, these two creatures belong to different species. They look at problems in different ways, with different tools, and they speak totally different languages.

Every Transistor Tells A Story

Until recently, RF designers used RF transistors one at a time, painstakingly connecting them together with a network of inductors, capacitors, and an occasional resistor for biasing. RF designers would wring as much performance as possible out of each transistor, using S parameters to navigate through a Smith chart. For them, DC biasing remained a minor consideration that a tired professor explained on a rainy Thursday afternoon in Microwave Amplifiers class to an uncomprehending audience. System impedance was 50 ohms because the coax impedance was 50 ohms and dBm relative to 50 ohms was the standard unit of measure.

Integration And Simulation

The techniques and mindset of analog-IC designers, on the other hand, got their start with the first monolithic op amps and haven't changed since. Primary concerns are DC biasing, DC gain, and offset voltages. There is no standard impedance. ICs submit to the Laws of Ohm and Kirchoff as written down by his prophet Nagle. Circuits are a marriage of voltages and currents; impedances are the issue of that union and come in a variety of values, but generally not 50 ohms. Transistors are cheap and come in all shapes and sizes.

Whereas an RF designer considers a 20-transistor RF IC high integration, an IC designer thinks nothing of a 20-transistor bias circuit. Of course, those extra transistors do have benefits. An IC designer can coax a +24 dBm input-referred third-order intercept out of a mixer with just -10 dBm LO power. But the IC designer would have trouble communicating that accomplishment to the RF designer because the IC designer talks of peak LO drive in mV, harmonic distortion in percent, and impedance matching for an optimum noise figure.

Different Tools

An RF designer measures amplifier noise with a noise figure meter and an excess noise ratio source and gives the answer in dB. An IC designer shorts the same amplifier's input to ground, measures noise at the output, divides by the gain, divides again by the square root of the bandwidth, and gives you a figure in nV/ÃHz.

Every RF designer's bench includes a spectrum analyzer, signal generator (now synthesized), and RF power meter. On the IC designer's desk are a Sun Sparc station, SPICE-based analog simulator, and process models. The technician's bench has an oscilloscope, a low-distortion audio signal generator, and a DMM.

Different Solutions

One of the offshoots of the communications gap between RF and IC designers is each group's woeful misunderstanding of the other's needs and requirements. Wireless designs, for example, constantly drive toward integration and power reduction. Unfortunately, when an RF designer asks an IC designer to integrate certain functions and reduce power by 30%, the desired result often violates the laws of physics.

To truly get the worst of both worlds, ask an RF designer to specify a low-noise amplifier for an IC designer. The RF designer will specify 50 ohms at every port (although higher load impedances at the output port would greatly reduce power consumption), along with noise figures in dB and signal levels in dBm. Meanwhile, the IC designer's analysis will show why a one-transistor common-emitter amplifier biased by a 20-transistor bias circuit is the most elegant solution. The LNA's input impedance will depend on the bias current of the transistor and its biasing network, but will be "high" at DC. The input will terminate with a 50-ohm "impedance matching" resistor, immediately decreasing the S/N ratio.

If this design were for a cellular phone, somewhere in the background a system engineer would be doling out the scraps of power left over after the digital designers have eaten their fill of the battery pack. With the system engineer's expertise in signal analysis and ignorance of semiconductor physics, the target power budget will be 30% lower than the previous discrete design so that the company's marketing department can tout longer battery life. Of course, the power reduction can't come from the digital functions. (After all, the digital stuff is more important, right?) Given the current lack of communication between RF and IC designers, a potential disaster is brewing.

IC designers are now designing RF ICs. Traditional boundaries between these disciplines must disappear. IC designers have to learn to think like RF designers and vice versa. No tunnel vision, no blinders, no narrow boundaries. Only cooperation will lead to success.