SP2T PIN Diode Switch

Rev. V1

MSW200x-200

11

11

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Transmit State

In the TX state, the series PIN diode between the

ANT and TX ports is forward biased by applying 0 V

to the TX bias input port (pin 1 of multi-pin connector

P1). The magnitude of the resultant bias current

through the diode is primarily determined by the

voltage applied to the ANT bias port (pin 3 of J1),

the magnitude of the forward voltage across the PIN

diode and the resistance of R1. This current is

nominally 100 mA. At the same time, the PIN diode

connected between RX and DC ports is also forward

biased by applying a higher bias voltage, nominally

28 V, to the RX bias port (pin 7 of P1) and 0 V to the

DC bias port (pin 5 of P1). Under this condition, the

PIN diode connected between the ANT and RX port

is reverse biased and the PIN diode connected

between the RX and DC ports is forward biased.

The magnitude of the bias current through this diode

is primarily determined by the voltage applied to the

RX bias port, the magnitude of the forward voltage

across the PIN diode and the resistance of R2. This

current is nominally 25 mA.

The RX series PIN diode, which is connected

between the ANT and RX ports, must be reverse

biased during the transmit state. The reverse bias

voltage must be sufficiently large to maintain the

diode in its non-conducting, high impedance state

when large RF signal voltage may be present in the

ANT-to-TX path. The reverse voltage across this

diode is the arithmetic difference of the bias voltage

applied to the RX bias port and the DC forward

voltage of the forward-biased transmit series PIN

diode.

The minimum voltage required to maintain the series

diode on the RX side of the switch out of conduction

is a function of the magnitude of the RF voltage

present, the standing wave present at the RX series

diode’s anode, the frequency of the RF signal and

the characteristics of the RX series diode, among

other factors. Minimum control voltages for several

signal frequencies are shown in the table “Minimum

Reverse Bias Voltage”, assuming the input power to

the RX or ANT port to be 100 W CW and the VSWR

on the ANT-TX path to be 1.5:1. It is important to

note that the evaluation board, as supplied from the

factory, is not capable of handling RF input signals

larger than 45 dBm. If performance of the switch

under larger input signals is to be evaluated, an

adequate heat sink must be properly attached to the

evaluation board, and several of the passive

components on the board must be changed in order

to safely handle the dissipated power as well as the

high bias voltage necessary for proper performance.

Contact the factory for recommended components

and heat sink.

Receive State

In the RX state, the series PIN diode between the

ANT and RX ports is forward biased by applying 0 V

to the RX bias input port (pin 7 of multi-pin connector

P1). The magnitude of the resultant bias current

through the diode is primarily determined by the

voltage applied to the ANT bias port (pin 3 of P1),

the magnitude of the forward voltage across the PIN

diode and the resistance of R1. This current is

nominally 100 mA. At the same time, the PIN diode

connected between RX and DC ports is reverse

biased by applying a high bias voltage, nominally 28

V, to the DC bias port (pin 5 of P1). A high voltage,

nominally 28 V, is also applied to the TX bias port

(pin 1 of P1). Under this condition, the PIN diode

connected between the ANT and TX port is reverse

biased thus isolating the TX RF port from the RX

signal path. The reverse voltage across this diode is

the arithmetic difference of the bias voltage applied

to the TX bias port and the DC forward voltage of

the forward-biased receive series PIN diode. The

minimum voltage required to maintain the series

diode on the TX side of the switch out of conduction

is a function of the magnitude of the RF voltage

present, the standing wave present at the RX series

diode’s anode, the frequency of the RF signal and

the characteristics of the TX series diode, among

other factors. For typical receive-level signals, this

diode is held out of conduction with a relatively small

reverse bias voltage. The values of the reactive

components which comprise the bias decoupling

networks as well as the signal path DC blocking are

shown in the table RF Bias Network Component

Values.

Reference Path

A reference path is provided on the evaluation

board, complete with bias decoupling networks, so

that the magnitude of the insertion loss of the micro-

strip transmission lines connected to the switch and

the associated bias decoupling components can be

measured and

removed from

the measured

performance of the switch.