Editing PIN diode (section)

=== RF and microwave switches ===
[[File:Microwave Switch.png|thumb|right|A PIN diode RF microwave switch]]

Under zero- or reverse-bias (the "off" state), a PIN diode has a low [[capacitance]]. The low capacitance will not pass much of an [[RF signal]]. Under a forward bias of 1 mA (the "on" state), a typical PIN diode will have an RF resistance of about {{nowrap|1 [[ohm]]}}, making it a good conductor of RF. Consequently, the PIN diode makes a good RF switch.

Although RF relays can be used as switches, they switch relatively slowly (on the order of {{nowrap|tens of milliseconds}}). A PIN diode switch can switch much more quickly (e.g., {{nowrap|1 microsecond}}), although at lower RF frequencies it isn't reasonable to expect switching times in the same order of magnitude as the RF period.

For example, the capacitance of an "off"-state discrete PIN diode might be {{nowrap|1 pF}}. At {{nowrap|320 MHz}}, the capacitive reactance of {{nowrap|1 pF}} is {{nowrap|497 ohms}}:

:<math>\begin{align}
Z_\mathrm{diode} &= \frac{1}{2\pi fC}\\
&= \frac{1}{2\pi(320\times10^6\,\mathrm{Hz})(1\times10^{-12}\,\mathrm{F})}\\
&= 497\,\Omega
\end{align}
</math>

As a series element in a {{nowrap|50 ohm}} system, the off-state attenuation is:
:<math>\begin{align}
A &= 20\log_{10}\left(\frac{Z_\mathrm{load} + Z_\mathrm{source}} {Z_\mathrm{source} + Z_\mathrm{diode} + Z_\mathrm{load}}\right)\\
&= 20\log_{10}\left(\frac{50\,\Omega + 50\,\Omega} {50\,\Omega + 497\,\Omega + 50\,\Omega}\right)\\
&= {15.52}\,\mathrm{dB}
\end{align}</math>

This attenuation may not be adequate. In applications where higher isolation is needed, both shunt and series elements may be used, with the shunt diodes biased in complementary fashion to the series elements. Adding shunt elements effectively reduces the source and load impedances, reducing the impedance ratio and increasing the off-state attenuation. However, in addition to the added complexity, the on-state attenuation is increased due to the series resistance of the on-state blocking element and the capacitance of the off-state shunt elements.

PIN diode switches are used not only for signal selection, but also component selection. For example, some low-[[phase noise|phase-noise]] oscillators use them to range-switch inductors.<ref>{{cite web |url=http://www.herley.com/index.cfm?act=app_notes&notes=switches |title=Microwave Switches: Application Notes |website=Herley General Microwave |url-status=unfit |archive-url=https://web.archive.org/web/20131030102546/http://www.herley.com/index.cfm?act=app_notes&notes=switches |archive-date=2013-10-30}}</ref>