User Manual

CIRCUIT DESCRIPTION
4-10
Part No. 001-3422-003
Limiter-Amplifier
The output of Z241/Z242 is applied to a limiter-amplifier circuit in U241. This circuit amplifies the 450 kHz
signal and any noise present; then limits this signal to a specific value. When the 450 kHz signal level is high,
noise pulses tend to get clipped off by the limiter; 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 detector inputs 90° at 450 kHz (all other inputs are unshifted in phase).
When modulation occurs, the frequency 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 detector. R255 is used to lower the Q of L253.
From the detector the audio and data signal is fed out on pin 9. The audio/data output of U241, pin 9 is applied to
J201, pin 13.
Receive Signal Strength Indicator (RSSI)
U241, pin 5 is an output for the RSSI circuit which provides a current proportional to the strength of the
450 kHz IF signal. The voltage developed across R275 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 additional 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 and R512 and R515. L513, C517 and C518 match Q511 to U531.
R514 provides a resistive feedback path to stabilize Q511 and C515 provides DC blocking. C516 bypasses RF
from the DC line, and R513 provides supply voltage isolation and ties the +9V transmit supply to the circuit.