Ritron RIT44-465 Manual Do Utilizador
14
IC U102 is responsible for converting signals at the 1
st
IF to the 2
nd
IF where additional filtering and
amplification take place. The 43.65 MHz signals enters U102 at pin 16. It is then mixed with the 2
nd
local
oscillator signal at pin 1 to product a signal at the 2
nd
IF of 450 kHz at pin 3. The 2
nd
IF signal at pin 3 is
filtered by the FL103, the 2
nd
IF filter. This filter is a bandpass filter with a bandwidth of 4 kHz and an
insertion loss of 5 dB. The output of the filter re-enters U102 at pin 5 where it is amplified and then limited
to remove amplitude modulation components. The limited signal is then mixed against a 90 phase shifted
version of itself and filtered by Y101. This acts as an FM demodulator. The demodulated output exits
U102 at pin 9.
5.2.4
to remove amplitude modulation components. The limited signal is then mixed against a 90 phase shifted
version of itself and filtered by Y101. This acts as an FM demodulator. The demodulated output exits
U102 at pin 9.
5.2.4
1
ST
Local Oscillator/Receive Synthesizer
The local oscillator signal for the 1
st
mixer is generated by the receive synthesizer. The receive synthesizer
is based around a voltage controlled oscillator (VCO) and a synthesizer sub-system IC which handles the
digital portion of the synthesizer. The voltage controlled oscillator is an oscillator whose frequency can be
controlled by a voltage. The receive voltage controlled oscillator is formed around Q107 with varactor
diode D101 and D102 electronically controlling the frequency. A sample of the oscillator frequency is
sampled by R124 and C160 and sent to U104, an integrated monolithic Phase-Locked Loop (PLL)
synthesizer IC. Inside U104, the VCO frequency is divided by programmable digital dividers to a
frequency in the 50 kHz range. FL104, the 36.4 MHz receive reference oscillator’s frequency is also
divided down to the 50 kHz range. A phase/frequency detector compares the two divided down
frequencies and generates an error signal at U104, pin 2 which is filtered and used to pull the VCO on
frequency. By controlling the VCO divider number, the locked frequency can be changed. The divider
numbers are loaded into U104 by the microprocessor. The 20 MHz tuning range of the VCO is set by
C150. Because the VCO is locked to the receive reference oscillator, FL104, the frequency stability of
FL104 determines the frequency stability of the VCO and thus the 1
digital portion of the synthesizer. The voltage controlled oscillator is an oscillator whose frequency can be
controlled by a voltage. The receive voltage controlled oscillator is formed around Q107 with varactor
diode D101 and D102 electronically controlling the frequency. A sample of the oscillator frequency is
sampled by R124 and C160 and sent to U104, an integrated monolithic Phase-Locked Loop (PLL)
synthesizer IC. Inside U104, the VCO frequency is divided by programmable digital dividers to a
frequency in the 50 kHz range. FL104, the 36.4 MHz receive reference oscillator’s frequency is also
divided down to the 50 kHz range. A phase/frequency detector compares the two divided down
frequencies and generates an error signal at U104, pin 2 which is filtered and used to pull the VCO on
frequency. By controlling the VCO divider number, the locked frequency can be changed. The divider
numbers are loaded into U104 by the microprocessor. The 20 MHz tuning range of the VCO is set by
C150. Because the VCO is locked to the receive reference oscillator, FL104, the frequency stability of
FL104 determines the frequency stability of the VCO and thus the 1
st
local oscillator.
The output of the VCO is amplified by Q104 and Q106 to about 5 dBm to drive the 1
st
mixer. For
alignment purposes, a sample of the 1
st
local oscillator is available at TP101. For proper mixing operation,
the VCO is made to oscillate at a frequency 43.65 MHz below the receive frequency.
5.2.5
5.2.5
2
ND
Local Oscillator
To convert the 1
st
IF signals at 43.65 MHz to the 2
nd
IF frequency of 450 kHz, requires a 2
nd
local oscillator
frequency of 43.2 MHz (43.65 – 0.450). This is created by multiplying the 14.4 MHz output frequency of
FL105 by three in Q103.
5.2.6
FL105 by three in Q103.
5.2.6
Base band data
Discriminator audio from U102 passes to U103-C and it is amplified and through U103-D where is
equalized. It is then sent to the NXDN decoder IC U302 for demodulation. The main controller U301 reads
the data and performs needed tasks such as error correction, RAN and ID decoding.
5.3
TRANSMITTER
5.3.1
Transmit Synthesizer
The transmit synthesizer operates similar to the receive synthesizer except that the oscillator is locked to
the transmit frequency rather than offset by the 1
st
IF frequency. The transmit VCO is formed around Q207
and electronically tuned by D202 and D203. The oscillator frequency is sampled by R227 and C226 and
routed to U201, the transmit synthesizer IC. The oscillator frequency is divided down around 50 kHz and
compared in phase and frequency to the divided transmit reference oscillator signal. The error signal is
filtered and sent to the varactor diode D202 and D203 to lock the VCO on frequency. The reference is
created by dividing down the transmit reference oscillator frequency of 36.4 MHz from FL201. To
modulate the transmitter, the VCO must allow for frequency modulation other than what is required to put
it on frequency. This is effected by D204. The 20 MHz tuning range tuning range of the VCO is set by
routed to U201, the transmit synthesizer IC. The oscillator frequency is divided down around 50 kHz and
compared in phase and frequency to the divided transmit reference oscillator signal. The error signal is
filtered and sent to the varactor diode D202 and D203 to lock the VCO on frequency. The reference is
created by dividing down the transmit reference oscillator frequency of 36.4 MHz from FL201. To
modulate the transmitter, the VCO must allow for frequency modulation other than what is required to put
it on frequency. This is effected by D204. The 20 MHz tuning range tuning range of the VCO is set by