9
RECEIVER
Receiver Front End
The signal from the antenna passes through the transmitter low pass filter and then goes to the
tunable bandpass filter L101 and L102. Q101 amplifies the signal about 15 dB before going through
another band pass filter and mixer matching stage. The receiver is turned on by Q107 and Q108
supplying RX_5v when RXEN of U301 goes high.
The amplified received input signal is applied to the base of mixer Q102. The 1st local oscillator
signal from the synthesizer module is buffered and filtered by band pass amplifier Q106 and then applied
to the source of Q102. L195, C114, C115 and C135 tune the drain output of Q102 to 43.65 MHz and
apply it to Y101 and Y103, a 43.65 MHz four-pole crystal filter. Q103 and associated components
amplify the 43.65 MHz IF signal and apply it to the input of the 2nd mixer at Pin 16 of U101.
FM Receiver Subsystem
A multi-function integrated circuit, U101 and associated components form the FM-receiver
subsystem. The subsystem performs the following functions as a 2nd mixer, IF amplifier and FM
detector. The second local oscillator at 43.2 MHz is applied to the 2nd local oscillator input at Pin 1 of
U101. The 43.65 MHz signal at Pin 16 and the 2nd local oscillator are mixed, with the resulting 450 KHz
output signal appearing at Pin 3. This signal is filtered by a 450 kHz 6-pole ceramic filter YF101 and
applied to the input of the limiting IF amplifier at Pin 5. IC101 pin 6 de-couples the IF amplifier. An
internal quadrature detector, whose center frequency is determined by the 450 kHz quadrature resonator
Y102, detects the FM IF signal. One input of the quadrature detector is connected internally to the IF
signal from pin11 while the other input is the phase-shifted signal from Y102 at Pin 10. Demodulated
audio appears at Pin 9, where a low-pass filter R323 and C309 removes spurious second IF output prior
to application to the voice, tone or squelch conditioning audio circuitry.
Two types of squelch circuits exist on the DR-142, an RSSI squelch and a noise squelch. Both
types can be used simultaneously or one or the other can be used. The RSSI (receive signal strength
indicator) squelch, which is typically set around –110 dBm, must be set to open at a higher level than the
noise squelch. This is necessary since the RSSI measures total power in the receiver IF band pass. All
background noise, which at VHF can be high, is seen as signal. The advantage of the RSSI squelch is
that it opens and closes the audio paths very quickly. The noise squelch has the advantage that it can be
set at a much lower level, typically –118 dBm for a 20 dB SINAD. It takes longer to open and close the
squelch. If both are used simultaneously, since they are wired ORed together at U301 pin 32, the squelch
will open quickly and close slowly for strong signals. If only RSSI is desired R145 can be removed.
Voice and Tone Conditioning in Receiver
Three post demodulation paths are provided in the DR-142. U304c provides DC level translation
to bias succeeding op amp stages at about 2.5 volts.
The audio path goes through a fourth order 300 Hz high pass filter U305c and d. C326, R353 and
R351 de-emphasis the audio. The 1 watt audio amp is turned on by Q311 and Q312. Error tones from
come through C325 from pin 13 of U301.
The data path goes through U304a and b. U305b is a second order 3000 Hz low pass filter.
U304a produces phase shift around 3000 Hz to equalize time delay. R337 and C316 provide another low
pass filter pole. U307a is an adjustable gain buffer stage. This stage is squelched by Q314.
Sub-audible signals go through a third order 250 Hz low pass filter U304d, R342, and C316. Pin
27 of U301 decodes the CTCSS or DCS signal. In the case of CTCSS an internal discrete Fourier
transform looks for the wanted tone. Decode bandwidth is about +/-2 Hz.
