User's Manual

© RF Technology 2007
2.3.3.1 TX module
The TX module can be divided into the VCO, PLL, PA and the Data storage section.
The Voltage Controlled Oscillator (VCO)
The Voltage Controlled Oscillator uses a junction FET (Q2) which oscillates at the
required transmitter output frequency. Varactor diodes (D2, D9, D10, and D11) are used
by the PLL and bias control circuits to keep the oscillator on the desired frequency.
Transistor Q1 is used as an active filter to reduce the noise on the oscillator supply
voltage.
The VCO is keyed ON by the RISC processor through Q3, It is keyed ON when any of
the PTT inputs are active or self-calibrations, but OFF at all other times. The VCO
output is amplified by monolythic amplifier U4 before being fed to the PLL chip (U10).
The Phase Locked Loop (PLL)
The frequency reference for the PLL is from the Interface board via a 20pin connector.
A fractional-N PLL synthesiser (U10) is used in the TX module, this fractional-N
synthesiser provides very fine frequency resolution which enables the PLL used as a
FM modulator by modulating the PLL data. The modulation data is provided by DSP via
the serial bus. The phase detector output (charge pump) signal of U10 is smoothed and
filtered by the loop filter to form the tuning voltage for the VCO circuit.
The Power Amplifier (PA)
Amplifiers (U1, U2) increase the VCO output to a sufficient level to drive the power
amplifier (U3). The output power level of the PA is controlled by RISC processor via
bias pin of U3. The directional coupler (D3, D4) detects the forward and reverse power
components, the detected voltages are then amplified by U7 and U8 to provide
proportional dc levels to the RISC processor. The output from U3 is further filtered by
the low pass filter to reduce higher order harmonics. U1, U2 and U3 are not switched on
until the PLL has locked and had time to settle. This prevents any momentary off
channel transmission when the transmitter is keyed.
The Data Storage
Each TX module has an EEPROM for storing the individual module information such as,
TX module serial number, model name, frequency range, calibration data etc. This is
allows user to simply replace the TX module in the transceiver without redo the
alignment and calibration. The data is transferred between EEPROM and RISC
processor via the serial bus.
2.3.3.2 RX module
The RX module can be divided into the Font-end Amplifier, LO, PLL, IF amplifier and
the Data storage section.
The Front-end Amplifier