Johnson DM3474, Service Manual

Page: 34 / 64

CIRCUIT DESCRIPTION
4-10
Part No. 001-3474-002
Second IF Filter
The output of the internal double-balanced mixer is the difference between 52.95 MHz and 52.5 MHz which
is 450 kHz. This 450 kHz signal is fed out on pin 3 and applied to second IF filters Z241 and Z242. These filters
have passbands of 9 kHz (12.5 kHz BW), 15 kHz (20 kHz BW) or 20 kHz (25 kHz BW) at the
-6 dB points and are used to attenuate wideband noise.
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. C242, C243 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. L242 is
tuned to provide maximum undistorted output from the detector. R242 is used to lower the Q of L242. From the
detector the audio and data signal is fed out on pin 9.
Audio/Data Amplifier
The audio/data output of U241 on pin 9 is fed to the audio amplifier U261. U261 amplifies the detected
audio/data signal and shifts the DC bias level to 2.5V. The gain is set at approximately 1.5 by R261/R262. R263
and R264 provide a 1.9V DC reference bias voltage. The audio output of U261 is applied to J201, pin 13.
Receive Signal Strength Indicator (RSSI)
U241, pin 13 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 R241 is applied to J201, pin 12.
4.4 TRANSMITTER CIRCUIT DESCRIPTION
4.4.1 BUFFER (Q851)
The output signal is applied to a 50-ohm pad formed by R851, R852, and R853. This pad provides attenuation
and isolation. Q851 provides amplification and also additional isolation between the VCO and transmitter. Biasing
for this stage is provided by R854, and decoupling of RF signals is provided by C852. Impedance matching with
the transmitter is provided by L501 and C502.