Operations Manual

Page 16 RF Technology R50
5.5 Voltage Controlled Oscillator (Sheet 5)
5 CIRCUIT DESCRIPTION
Pin8 of U401 is the other recovered audio output which can be muted by pin5,
discriminated audio signal is amplified by U605A to a sufficient level for line level
detector, then fed into Audio Section (Sheet 6).
Pin7 of U401 is the Radio Signal Strength Indicator (RSSI) output, U402D is a buffer
for send RSSI voltage to MicroController.
Q405 and Q406 are amplifiers for impulse noise signal, the impulse noise is then
detected by D401 and U403, C489, R450 and U404 form a 2uSec noise blanking pulse
when the impulse noise is detected. Q408 converts the CMOS level to a required
switch level to control the noise blanker gate Q404.
U303B and Q407 work as gain control circuit to reduce the Intermodulation which is
produced from impulse noise amplifiers.
5.5 Voltage Controlled Oscillator (Sheet 5)
There are two similar VCO circuits in this sheet, the difference between these two
VCO is component values, so only one VCO circuit is illustrated in this section.
Q501, C532, C533, C534 and L501 are working as a Colpitts oscillator, the frequency
of the oscillator is decided by the tuning voltage of a varactor D501 which comes from
the loop filter of the 1st PLL. Q503 is used to reduce the noise on power supply. D503,
R517 and C519 are working as AGC circuit to improve output of the VCO signal.
MMIC amplifier MA501 provides about 20dB gain for the VCO signal and feeds it to
the PLL chip. MA502 and MA503 provide about 30dB gain of the VCO signal to
drive the double balanced mixer MX401, Q506 and those components of which value
with "*" are not used in this version, they will be used in case of higher drive power
which is required by some high LO level mixer.
5.6 Audio Processing Section (Sheet 6)
U605B, R642, R643, R644, C626, C627 and C628 are working as a 3KHz low pass
filter, in addition, the discriminated audio signal level from U605A is attenuated by
the low pass filter to fit the dynamic range limitation of CTCSS chip U602.
U602 is a CTCSS tone encoder and decoder. The integrated circuit is also capable of
generating and receiving DCS signals, but at this stage this has not been implemented.
The CPU accesses U602 via the serial bus using MOSI, SCLK, and the low active
Select signal CTCSS_SEL.
4MHz crystal X401 is used as the reference of the CTCSS decoding. The build-in high
pass filter of the audio path (Pin 10 to Pin 11) can eliminate the subtone, this filter can
be bypassed by the MicroController. The subtone output from Pin17 is amplified by
U605C, through C604 connected to the DB25 terminal on rear panel.