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The effect of Lower Control Voltages on insertion loss in GaAs Fet RF antenna switches is mitigated using a negative voltage derived from the transmitter output.
Motorola Inc. May 2000
English (United States)
This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately
50% of the total text.
INSERTION LOSS IN GAAS FET RF SWITCH USING NEGATIVE VOLTAGE DERIVED FROM RF
effect of Lower Control Voltages on insertion loss in GaAs Fet RF antenna
switches is mitigated using a negative voltage derived from the transmitter
FEr RF switches are widely used in many portable radio products for their low
cost and ease of use/implementation. However, with today's modern radio
products operating at lower and lower supply voltages, the performance of these
devices becomes compromised.
operation at higher power levels is directly proportional to the control
voltages available. The result of this is increased insertion loss in the
transmit path, resulting in the need for extra RF power to be generated. This
causes greater temperatures, higher current consumption, and a corresponding
reduction in battery life.
paper describes a simple method for recovering this lost peIfonnance even at
low power supply voltages.
TO BE SOLVED
antenna switch designs use pin diodes, with the classic 1/4 wave track to
effect antenna changeover between Transmitter and Receiver paths. To simplify
and reduce the cost of low tier radio designs, a GaAs Fet RF Switch was used.
However, with radios which use 3 and 4 battery cells, the voltages available to
operate these switches can be 3V or less. The problem encountered was that even
though the devices are rated typically to 2Watts with only 3V available, at
power levels above 500mW the insertion loss increases rapidly to greater than
I.OdB, from a spec of less
0.2dB. Also, the isolation to the unselected Receive port is reduced to
typically 7dB from a specification of >20dB. For a 1 Watt Power Amplifier
this results in a power loss of typically 200mW, which can eliminate any 'head
room' present in the design, and reduce Power Amp efficiency significantly.
SOLUTION TO THE PROBLEM
type of switch mentioned above is generally operated using two control lines.
One line has >3V applied to it, with the other being taken to OV. To select