Back in the Jurassic days of electronics, before Lee de Forest developed the three-element “grid audion” (a triode vacuum tube providing amplification) around 1906, circuits and channels were passive entities. If you needed to get a wireless signal from point A to point B but the received strength or SNR was too low (though the term SNR was not yet in use), your only option was to crank up the transmitted power. The digital side was different, though, to some extent: you could boost signals by using relay-based repeaters where a small signal would drive a sensitive coil, while the relay contacts switched larger voltages and currents and so boosted the on/off signal. While it was slow and mechanical, it was also effective.
Active analog devices with gain and amplification made an unimaginable difference; in a word, it changed “everything” and began the age of electronics as we know it today. Soon the basic vacuum tube lead morphed to more effective ones with multiple elements, improved noise performance, higher power, and application-specific designs. Passive was overtaken by active with good reason.
Still, passive components did not disappear. Even today, despite our wide selection of active devices and ICs, there is a place for passive functions such as basic filters (RC, LC, and RLC filters in numerous topologies). This may be due to reasons of cost, internal noise, power levels, operating power (passives need none), and other performance goals. Both passive and active components have relative pros and cons; otherwise, one would have wiped the other from the designer's resource kit and bill of materials (BOM).
In our fluid world of products and performance, devices such as RF mixers are an area where passive implementations have been the choice for many years as active devices could not match their performance in noise and distortion at higher frequencies. As a result, many designers choose to use a passive mixer but then supplemented with a stand-alone amplifier to get the mixer's performance along with the needed gain. After all, there is no free lunch in design; if you stay passive, you'll incur signal-path losses that you'll likely have to make up somewhere along the way.
That situation is changing. The vast growth of mass-market application above 1 GHz, along with IC process improvements has spurred the introduction of many active mixers to the market with performance as good as, and many times even better, than a passive mixer plus amplifier. The relationship between process and component advances, and market needs and volumes, is a case a beneficial positive-feedback loop. With the active mixer, designers get the traditional benefits of a single device such as reduced PC board real estate, plus guaranteed performance as there are no layout surprises as there might be between two separate components.
Ironically, while the trend towards increased use of active RF components may be putting passive ones in the shadows, “passive” itself is getting its own version of revenge and not taking this passively (so to speak). Engineers are very familiar with intermodulation distortion which occurs when signal passes through imperfect active components or even nonlinear ones such as diodes.
Now, passive intermodulation distortion (PIM) is becoming an increasing concern due to subtle nonlinearities in connectors, splitters, and other nominally benign interfaces at higher frequencies. Some applications, such as latest generation of cell sites, require PIM values below 100 dBc through the entire signal chain. Instrumentation vendors have developed PIM test sets and test suites including PIM versus time, swept PIM, and distance-to-PIM (DTP), while some cable/connector assembly vendors are adding PIM specifications to their data sheets.
Have you been dealing with the active versus passive RF-component decision? Has PIM become one of your concerns? What, if anything, are you doing about it?
Related and references