Technically speaking/RF switching with PIN diodes

If you have worked in the radio communications field for long, you remember when antenna switching was done by relay. In fact, many sets still use relay switching. Relays are mechanical devices (moving parts), and with any mechanical device comes wear and tear. Remember cleaning the relay contacts as part of preventive maintenance? Not only did the contacts become dirty and pitted, the contacts on some relays had a habit of becoming out of alignment. People using radio equipment with dirty or misadjusted relay contacts often would complain of an “insensitive receiver.” Usually, troubleshooting was easy: Simply measure the resistance of the contacts with an ohmmeter or bypass the relay temporarily to observe the result.

Besides the problems with dirt, wear and misalignment, the mechanical relay has the disadvantage of higher cost and larger space requirement. The space requirement is not as important in base stations as it is in mobile and portable equipment.

PIN diode The word PIN is an acronym for positive intrinsic negative, which refers to the diode’s construction. Heavily doped “N” and “P” sections are separated by an “intrinsic” section of silicon that acts as an insulator. (See Figure 1 below left.) When forward bias is applied to the diode, the intrinsic region is injected with carriers from the “N” and “P” sections to allow the intrinsic region to become conductive. A normal PN junction diode, shown in Figure 1A, has a fairly high junction capacitance and, therefore, would leak RF signals through the junction capacitance. The thickness of the intrinsic region of the PIN diode substantially reduces the capacitance. This makes the PIN diode quite suitable for RF-switching applications.

The forward-biased PIN diode behaves as a pure resistance at RF frequencies. The RF resistance of the PIN diode can be varied from less than 1V to more than 10,000V by controlling the bias current through the diode. Although it is true that most diodes behave in a similar manner, the PIN diode is specifically designed to cover a wide range of resistance with good linearity and low bias current. PIN diodes can be connected as shunt (parallel) or series devices and often are used in series-parallel combinations.

Typical circuits Basically, a PIN diode can be used to perform almost any type of RF switching. Some of the more common uses are described here in simplified form.

Antenna TX/RX switch. Figure 2 below shows how PIN diodes can be used for antenna TX/RX switching. When +12V is applied to the top of R2, bias current flows through R2, L1, D1, the quarterwave line section, D2 and R1. This flow causes both the PIN diodes, D1 and D2, to be forward-biased. Because D1 is forward-biased, the transmitter signal will pass on to the antenna.

With D2 forward-biased, that end of the quarterwave transmission line will be at a low impedance point. Because a quarterwave section of line causes an impedance transformation, the transmitter side of the line will present a high impedance to the transmitter output signal. This provides isolation between the shunt diode, D2, and the transmitter output to prevent the shunt diode from loading down the transmitter output. The isolation can be an actual quarterwave section of transmission line, a lumped constant circuit or a strip line on the circuit board. When +12V is removed from the top of R2, the bias current is interrupted, and the PIN diodes are then effectively open. This allows the signal from the antenna to reach the receiver virtually unattenuated.

Broadband bridged-T attenuator. Figure 3 above left shows a broadband bridged-T attenuator. With a high bias current through PIN diode D1 and a low bias current through PIN diode D2, the attenuation of the bridged-T network will be quite small. With a high bias current through D2 and a low bias current through D1, the attenuation of the bridged-T network will be extremely high. Thus, by varying the amount of control bias to the two diodes, the attenuation of the bridged-T network can be set to any desired level. This is a broadband attenuator because there are no frequency-sensitive components in the network.

Narrowband bridged-T attenuator. Figure 4 on page 59 shows a narrowband bridged-T attenuator. It is similar in operation to the broadband bridged-T attenuator shown in Figure 3, except that it contains a quarterwave section of transmission line in the leg of the attenuator. This makes the attenuator narrowbanded because the quarterwave section of transmission line is frequency-sensitive.

A/B switch. Figure 5 above left shows a simple A/B (RF input selector) switch that can be built using PIN diodes. The switch can be located some distance from the circuit being switched. For the “A” input to be active, diode D1 must be forward-biased, and for the “B” input to be active, diode D2 must be forward-biased. R1, R2, R3 and L1 are part of the bias circuit for the PIN diodes.

Simple shunt attenuator. Figure 6 above right shows a simple shunt attenuator using a PIN diode. As the +V is increased, the forward bias on D1 increases, thus reducing the RF resistance of D1, effectively placing capacitor C1 in shunt with the input to the RF amplifier. The higher the bias current, the lower the resistance of D1, and the more shunting effect C1 has on the input of the amplifier.

Summing up As you can see, using the PIN diode is simple. It can be used anywhere RF switching is needed. Just think of the PIN diode as a variable RF resistor, the resistance of which varies with forward bias. Problems with PIN diode switching circuits usually are caused by bad components in the biasing circuit or a defective PIN diode itself. Dc voltage, RF voltage and ohmmeter checks are the normal methods of troubleshooting PIN diode switching circuits. ‘Til next time-stay tuned! Bibliography Carr, Joseph J., Two-way Radio and Broadcast Equipment Troubleshooting and Repair, Prentice-Hall, Englewood Cliffs, NJ, 1989. Cooper, W.D., and A.D. Helfrick, Electronic Instrumentation and Measurement Techniques, Prentice-Hall, Englewood Cliffs, NJ, 1985. Haywood, Wes, Introduction to Radio Frequency Design, American Radio Relay League, Newington, CT, 1994. Patrick, Dale R., and W. Fardo Stephen, Understanding Semiconductor Devices, Prentice-Hall, Englewood Cliffs, NJ, 1989.