User's Manual

68P80801H45-1 1/20/2002 13
EBTS System Manual - Vol 2 Base Radio Exciter
QUAD Channel 900 MHz Exciter
request of the BRC, A/DC output signals enter the BRC via SPI lines. The
Controller periodically monitors all signals.
Some of the monitored signals include amplifier bias and synthesizer signals.
Low Noise Offset Direct Conversion Transmit (LNODCT) IC Circuitry
The Low Noise IC is a main interface between the Exciter and BRC. The BRC’s
Digital Signal Processor (DSP) sends digitized signals (baseband data) to the
Exciter over the DSP data bus.
The differential data clock signal serves as a 2.4 MHz reference signal to the Low
Noise IC’s internal synthesizer. The Low Noise IC compares the reference signal
with the outputs of Voltage Controlled Oscillators (VCOs). The Low Noise IC
might sense that a VCO’s output is out of phase or off-frequency. If so, then the
Low Noise IC sends correction pulses to the VCO. The pulses adjust VCO output,
thereby matching phase and frequency with the reference.
The Low Noise IC up-converts baseband data from the BRC to the transmit
frequency. The Low Noise IC also down-converts the Transmit signal from the
Power Amplifier to baseband data for cartesian feedback linearization.
The BRC uses the Serial Peripheral Interface (SPI) bus to communicate with the
Low Noise IC. The SPI bus serves as a general purpose, bi-directional, serial link
between the BRC and other Base Radio modules, including the Exciter. The SPI
carries control and operational data signals to and from Exciter circuits.
Synthesizer Circuitry
The synthesizer circuit consists of the Phase-Locked Loop (PLL) IC and associated
circuitry. This circuit’s controls the 1025 MHz VCO signal. An internal phase
detector generates a logic pulse. This pulse is proportional to the phase or
frequency difference between the reference frequency and loop pulse signal.
The charge pump circuit generates a correction signal. The correction signal
moves up or down in response to phase detector output pulses. The correction
signal passes through the low-pass loop filter. The signal then enters the 1025
MHz Voltage Controlled Oscillator (VCO) circuit.
1025 MHz Voltage Controlled Oscillator (VCO)
For proper operation, the VCO requires a very low-noise, DC supply voltage. An
ultra low-pass filter prepares the necessary low-noise voltage and drives the
oscillator.
A portion of the oscillator output signal enters the synthesizer circuitry. The
circuitry uses this feedback signal to generate correction pulses.
The 1025MHz VCO output mixes with the 90.3 MHz VCO output. The result is a
Local Oscillator [LO) signal for the Low Noise IC. The LNODCT uses this LO
signal to up-convert the programmed transmit frequency. The Low Noise IC also
uses the LO signal to down-convert the PA feedback signal.