2: System Overview
2-2
PRC1099A-MS
2.2
DDS Synthesizer
The DDS synthesizer outputs two local oscillator frequencies and a beat
frequency oscillator (BFO) frequency that it derives from a single reference
oscillator. The first local oscillator frequency (LO1) is a variable output
ranging from 76.6 to 105 MHz in 1 Hz steps. The second local oscillator
(LO2) is a fixed output frequency is 73.35, and the BFO is either 1647 kHz
(LSB mode) or 1650 kHz (USB mode),
2.3
Processor Control
The processor controls the DDS synthesizer, display, antenna tuner and many
other functions using the firmware program stored in permanent read-only
memory (ROM). Programmable functions such as channel frequencies and
antenna tune information are stored in random access memory (RAM). A
lithium battery with a nominal life of ten years provides DC power backup for
the RAM.
The serial peripheral interface (SPI) bus delivers data and control signals to
the appropriate boards. The data is decoded on the individual boards. The
front panel controls also interface with the processor to control and configure
radio functions. The serial data extends to the Accessory connector to control
external accessories such as a RF power amplifier.
Figure 2-1 Frequency Conversion Plan
Receiver Input
1.6 to 30 MHz
Audio
Output
To final amplifier
1.6 to 30 MHz
Microphone
Input
Transmit Path
Receive Path
Low Pass
Filter
1.6 to 30 MHz
Mixer
Output
75 MHz
75 MHz
Crystal
Filter
Mixer
Output
1650 kHz
1650 kHz
Crystal
Filter
Product
Detector
Synthesizer
First Loop
76.6 to 105 MHz
in 1 Hz Increments
Low Pass
Filter
1.6 to 30 MHz
Mixer
Output
1.6 to 30 MHz
75 MHz
Crystal
Filter
Mixer
Output
75 MHz
1650 kHz
Crystal
Filter
Balanced
Modulator
Synthesizer
Second Loop
73.35 MHz
BFO
1650 KHz USB
1647 KHz LSB
