9
iO1000 Detailed Service Manual - THEORY OF OPERATION
BALUN
The differential RF signal is converted to a traditional single-ended (unbalanced)
signal through the balun (balanced/unbalanced) circuitry. The balun is imple-
mented using multilayer ceramic technology.
SAW Filter
The signal is then routed to the SAW filter. The SAW filter further attenuates the
residual image frequency and any out-of-band noise that may be present.
RF Power Amplifier
The signal is then routed to the RF PA. The RF PA has an adjustable gain that can
be varied from 30 dB to 36 dB. The gain is adjusted by varying the control voltage
on pin 1 of the PA. The control voltage is supplied by a D/A converter in the
ADDAG. The gain is tuned at the factory and should not require any adjustment;
any change in the loop gain can result in the transmitter splattering into the adja-
cent channel. The PA is turned on by supplying the battery voltage to pins 3, 5,
and 7 of the PA through Q501. This is done only after the PA negative bias volt-
ages have been applied to pins 1 and 4.
Coupler
At the output of the PA is a high value inductor that is used to sample the signal
and provide the necessary feedback for the linearization. A directional coupler is
not needed because the isolator that follows provides the directivity. The sampled
signal passes through an attenuator and is input to the ODCT at pin 37. The sam-
pled signal is then mixed down to baseband in the downmixer, a quadrature mixer,
and the resulting output is an I and a Q signal which is then summed with the
input. This completes the closed loop.
