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AE/LZB 119 1890 R1A
TRANSMITTER BLOCK
Transmit Frequency Generation
The main VCO (G481) has a frequency range from
928.2625 Mhz to 947.237 Mhz. This signal is mixed in the
Ruth chip (N431) with the transmit offset frquency of
122.25 Mhz to generate the carrier and side band
frequencies. The carrier and upper sideband are suppressed
to leave only the desired transmit frequency. For example,
the VCO frequency of 937.75 Mhz mixed with the offset
frequency of 122.25 Mhz generates a transmit frequency of
860.5 Mhz.
Transmit Offset Synthesizer
The task of the transmit synthesizer is to supply the
power amplifier stage with the transmit signal at the chosen
transmitter frequency, 806.0125-824.9875 Mhz, in steps of
12.5 KHz. The reference for the transmit synthesizer comes
from an accurate 12.8 Mhz source (N511, 1.5 ppm) which is
divided down to 12.5 KHz in the synthesizer chip. The
transmit synthesizert creates a fixed 122.25 Mhz signal by
phase locking a down-divided VCO signal to the 12.5 Khz
reference signal. The local oscillator signal is then mixed
with the 122.25 Mhz signal to create the transmit frequency
signal.
The transmit VCO is located in Ruth (N431) and has a
sensitivity of about 3.4 MHz/V. An external tank circuit is
located off of pins 9 and 10 and is tuned by a varicap diode
(V431). One part of the VCO is sent through a buffer stage
into the PLL for phase locking. The main part of the signal
is fed to the transmitter mixer in Ruth. The receive signal is
then mixed down with the 122.25 Mhz signal to form the
transmitter frequency (F_Tx) at about 0 dbm and then fed to
the discrete driver stage in the transmitter block. A
precharge circuitry is used to keep the loop filter chanrged
to approximately 2.5 V in standby in order to get a shorter
lock in time.
Transmit Driver Amplifier
The transmit frquency (F_Tx) is next sent to a variable
gain driver amplifier. The transistor used (V302) is a bipolar
type in a common emitter configuration with a maximum
gain of 14 dB. Gain control is achieved by varying the
collector voltage of the RF transistor. A PNP pass element
(V3 11) and NPN driver (V3 12) is used for this purpose.
This configuration offers unconditional stability and good
dynamic range. The output and inputs are matched to 50
ohms. There is an attenuator (R302, R303, and R304) at the
input to increase reverse isolation. The amplifier is switched
on and off with VTx and the control signal VPAON.
8
Transmit Bandpass Filter (Z361)
The transmit frequency leaves the driver stage and is
sent through a bandpass filter (Z361) before it is sent to the
main power amplifier (N305). The bandpass filter is of the
SAW type with 20 dB of rejection out of band and an
insertion loss of approximately 2.5 dB.
Power Amplifier (N305)
The power amplifier is a 2 stage GaAs FET with
roughly 30 dB of gain. The gain and thus output power is
varied by means of the input drive level. The hybrid module
is matched to 50 ohms input and output impedance. Pin I is
the RF input, pin 2 is the negative supply (Vgg = -3.5 V) ,
pin 3 and 4 are the positive supplies (Vddl = Vdd2 =6.5
volts), and pin 5 is the RF output. The power amplifiers can
provide at least 32.5 dbm (1.8 W) of output power. The
extra power is needed due to the severe losses in the
duplexer.
DC-DC converter
The power amplifier and the display have to be biased
with a negative voltage, therefore N33 1 converts VDIG to -
3.5 V. It is extremely important that the negative voltage be
applied to the PA before the drain voltage is switched on.
The circuit is designed to provide the PA with a negative
voltage at all times.
Duplexer
The duplex filter consists of a 3-pole transmit section of
low pass characteristic with a notch at the receive frequency
and a 4-pole receive section filter all in one package.
The task for the transmit filter is to suppress wide band
noise evolving from the VCO and the power amplifier on
receive frequencies. It is also important to suppress
harmonics of the transmitter frequency by 45 dB or more.
The filter has at least 36 dB attenuation in the Rx band and
30 dB at the Tx harmonics in order to meet regulatory
agency requirements.
Power Detection
A schottky diode is used to rectify the RF (V3 13). This
diode is forward biased to avoid having the diode snap off
when detecting low levels of RF. Temperature compensation
is achieved by including a second schottky diode in series
with the POWLEV voltage on the input of the power control
loop.
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