TABLE OF CONTENTS Daniel M. Dobkin
It s easy to build a radio. When I was a kid, one could purchase crystal radio kits, consisting of a wirewound inductor antenna, a diode, and an earpiece; by tuning a plug in the inductor, one could pick up nearby AM radio stations quite audibly (at least with young pre-rock-amplifier ears). To build a good radio is harder, and to build a good cheap radio that works at microwave frequencies is harder still. That such radios are now built in the tens of millions for wireless local area networks (WLANs) (and the hundreds of millions for cellular telephony) is a testament to the skills and persistence of radio engineers and their manufacturing colleagues, the advancement of semiconductor technology, and the large financial rewards that are at least potentially available from the sale and operation of these radios.
The key requirements imposed on a good WLAN radio receiver are as follows:
Sensitivity: A good radio must successfully receive and interpret very small signals. Recall that the thermal noise in a 1-MHz bandwidth is about ?114 decibels from a milliwatt (dBm) or about 4 fW (4 10 ? 15 W). Depending on the application, the requirements on the radio may be less stringent than that, but in general extremely tiny wanted signals must be captured with enough signal-to-noise ratio (S/N) to allow accurate demodulation.
Selectivity: Not only must a radio receive a tiny signal, it must...
