4.1 - 14 THEORY OF OPERATION (TRANSCEIVER)
Frequency Generating Section Detailed Circuit Description
For the Frequency Generating Section Block Diagram see Figure 4.1-4 on page 4.1-7.
This section describes the generating circuits that supply all the required frequencies for the
required transmitter and receiver functions. These circuits are described as follows:
• 16.8 MHz Reference Oscillator - TCXO.
• Main VCO and Main Synthesizer.
• Transmit frequency translation loop.
Reference Oscillator - TCXO
16.8 MHz TCXO (Y5363) is a temperature compensated crystal oscillator producing an accurate
and stable 16.8 MHz master clock to the terminal RF circuits. Its operating frequency is fine tuned by
means of a warp voltage signal generated by the LVFRACN IC (U5331, pin 25). On terminal power-
up, an initial warp voltage setting is programmed into the LVFRACN IC by the microprocessor via
the SPI bus, using a stored value programmed into the terminal codeplug during factory tuning.
While the receiver is registered to the terminal system, an automatic frequency control algorithm is
executed by the microprocessor, to fine tune the LVFRACN warp voltage to bring the TCXO
frequency into alignment with the terminal system frequency standard.
The output signal at pin 3 of the TCXO is passed to LVFRACN IC U5331, where it is used as a
reference for the main frequency synthesizer. The TCXO output is also buffered by Q5351, and
passed to other terminal circuits via REF16_8_OSC. The LVFRACN IC buffers the 16.8 MHz
reference, and provides it at FREFOUT (pin 41) for use by other terminal circuits.
Main VCO and Main Synthesizer
The synthesizer components are LVFRACN, Loop Filter, VCO and VCO Buffer IC (VCO BIC). It
produces the receiver first LO signal, as well as serving as the reference for the transmit frequency
translation loop.
The operating frequency of Voltage Controlled Oscillator (VCO) with active element Q5712 is
determined by resonator W5701, in conjunction with C5735, C5737, C5738, C5736, L5733, and
varactors D5731-5734. The operating frequency of the oscillator is varied by means of varactor
tuning voltage VCTRL, supplied by the LVFRACN U5331 IOUT signal (pin 43), and filtered by loop
filter components C5303, R5304, C5305, R5303, R5302, and C5301. The RF output of the VCO is
buffered by U5701
U5701 pin 12 provides a sample of the VCO output to the LVFRACN prescaler input (pin 32), where
the signal is divided down to a lower comparison frequency by means of a programmable frequency
divider. The divided down VCO signal is compared to a signal derived from the 16.8 MHz reference
clock to generate an error signal (IOUT) which is filtered by the loop filter and tunes the VCO to the
programmed operating frequency.
A LOCK_FN signal is available at LVFRACN pin 4 to provide feedback to the microprocessor that
the synthesizer has successfully locked to the programmed frequency.The “RX_OUT” signal from
buffer U5701 pin 8 is amplified by Q5701, and supplied to the first mixer LO port.
The “TX_OUT” signal from buffer U5701 pin 10 provides the frequency reference for the Transmit
Frequency Translation Loop.
6866539D31.book Page 14 Tuesday, March 20, 2007 6:32 PM
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