User's Manual

10 68P80801E35-B 5/1/2002
Base Radio Exciter EBTS System Manual - Vol 2
Low Noise 800 MHz Exciter
LNODCT IC Circuitry
The LNODCT IC (Low Noise Offset Direct Conversion Transmit 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
LNODCT IC’s internal synthesizer. The LNODCT compares the reference signal
with the outputs of Voltage Controlled Oscillators (VCOs). The LNODCT might
sense that a VCO’s output is out of phase or off-frequency. If so, then the
LNODCT sends correction pulses to the VCO. The pulses adjust VCO output,
thereby matching phase and frequency with the reference.
The LNODCT IC up-converts baseband data from the EBRC to the transmit
frequency. The LNODCT IC also down-converts the Transmit signal from the
Power Amplifier to baseband data for cartesian feedback linearization.
The EBRC uses the Serial Peripheral Interface (SPI) bus to communicate with the
LNODCT IC. The SPI bus serves as a general purpose, bi-directional, serial link
between the EBRC 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 970 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 970 MHz
Voltage Controlled Oscillator (VCO) circuit.
970 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 970 MHz VCO output mixes with the 90.3 MHz VCO output. The result is a
Local Oscillator [LO) signal for the LNODCT IC. The LNODCT uses this LO
signal to up-convert the programmed transmit frequency. The LNODCT also
uses the LO signal to down-convert the PA feedback signal.