MOTOROLA
Technical
Developments
REDUCE
INSERTION LOSS IN GAAS FEr RF SWITCH USING NEGATIVE VOL TAGE DERIVED FROM RF
OUTPUT
by
Simon Jones
ABSTRACT
The
effect of Lower Control Voltages on insertion loss in GaAs FEr RF antenna
switches is mitigated using a negative voltage derived from the transmitter
output.
INTRODUCTION
GaAs
PET RF switches are widely used in many portable radio products for their low
cost and ease of use/implementation. However. with today's modem radio products
operating at lower and lower supply voltages, the performance of these devices
becomes compromised. Their 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.
This
paper describes a simple method for recovering this lost perforn1ance even at
low power supply voltages.
PROBLEM
TO BE SOLVED
Traditional
antenna switch designs use pin diodes, with the classic 114 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 2 Watts with only 3V available, at
power levels about 500mW the insertion loss increases rapidly to greater that
1.0dB, from a spec of less than O.2dB. Also, the isolation to the unselected
Receive port is reduced to typically 7dB from a
Motorola.
Inc. 2000
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.
PROPOSED
SOLUTION TO THE PROBLEM
The
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 QV. To select
the other p...
