User Manual

CIRCUIT DESCRIPTION
4-7
June 1997
Part No. 001-3412-002
Limiter-Amplifier
The output of Z252 is applied to a limiter-ampli-
fier circuit in U241. This circuit amplifies the 450
kHz signal and any noise present; then limits this sig-
nal to a specific value. When the 450 kHz signal level
is high, noise pulses tend to get clipped off by the lim-
iter; however, when the 450 kHz signal level is low,
the noise passes through the limiter. C275/C276
decouple the 450 kHz signal.
Quadrature Detector
From the limiter stage the signal is fed to the
quadrature detector. An external phase-shift network
connected to pin 8 shifts the phase of one of the detec-
tor inputs 90° at 450 kHz (all other inputs are
unshifted in phase). When modulation occurs, the fre-
quency of the IF signal changes at an audio rate as
does the phase of the shifted input. The detector,
which has no output with a 90° phase shift, converts
this phase shift into an audio signal. L253 is tuned to
provide maximum undistorted output from the detec-
tor. R255 is used to lower the Q of L253. From the
detector the audio and data signal is fed out on pin 8.
Audio/Data Amplifier
The audio/data output of U241, pin 8 is fed to the
audio amplifier U261. U261 amplifies and inverts the
detected audio/data signal and shifts the DC bias level
to +2.5V DC at the output on pin 13. The gain is set at
approximately 2.5 by R262/R263. R264/R265 pro-
vide a 1.3V DC reference bias voltage for U241. The
audio output of U261 is applied to J201, pin 13.
Receive Signal Strength Indicator (RSSI)
U241, pin 5 is an output for the RSSI circuit
which provides a voltage proportional to the strength
of the 450 kHz IF signal. The RSSI voltage is applied
to J201, pin 12.
4.4 TRANSMITTER CIRCUIT DESCRIPTION
4.4.1 BUFFER (Q501)
The VCO RF output signal is applied to R892,
R893 and R894 that form a resistive splitter for the
receive first local oscillator and the transmitter. The
VCO signal is then applied to a 50 ohm pad formed by
R501, R502, and R503. This pad provides attenuation
and isolation. Q501 provides amplification and addi-
tional isolation between the VCO and transmitter.
Biasing for this stage is provided by R504 and R505,
and decoupling of RF signals is provided by C503 .
Impedance matching to the predriver is provided by
L511 and C512.
4.4.2 PRE-DRIVER (Q511)
Pre-driver Q511 is biased Class A by R511,
R512, R513 and R515. L513, C517 and C518 match
Q511 to U521. R514 provides a resistive feedback
path to stabilize Q511 and C515 provides DC block-
ing. C516 bypasses RF from the DC line, and R513
provides supply voltage isolation and ties the +9V
transmit supply to the circuit.
4.4.3 FINAL (U521), COMPARATOR (U111C)
RF module U521 has an RF output of 1 to 5W
and operates on an input voltage from 10-16V.
Power control is provided by U581, U111, Q531
and a stripline directional coupler. The power is
adjusted by Power Set Control of U911 by R535 that
provides a reference voltage to U111C. U111C drives
Q531 and PA module U521.
One end of the stripline directional coupler is
connected to a forward RF peak detector formed by
R591, CR591, C591 and U581A. The other end of the
stripline directional coupler is connected to a reverse
RF peak detector formed by R593, CR592, C593 and
U581B.
If the power output of U521 decreases due to
temperature variations, etc., the forward peak detector
voltage drops. This detector voltage drop is buffered
by U581A and applied to inverting amplifier U111C
which increases the forward bias on Q531. The
increase on Q531 increases the power output level of
U521. If the power output of U521 increases, the for-
ward peak detector voltage increases and U111C
decreases the forward bias on Q531. The decrease on
Q531 decreases the output power of U521.
The output of CR591 and CR592 are fed to
U581A/B respectively. If the output of either buffer
increases, the increase is applied to the inverting input
of U111C. The output of U111C then decreases and