Specifications

4-12 888-2857-004 5/9/13
WARNING: Disconnect primary power prior to servicing.
Section 4 Theory of Operation
Platinum VAX-C Series
4.4.7 PFRU (Precise Frequency Reference Unit) Board
The Precise Frequency Reference Unit (board) performs four functions through four
circuits. They are as follows:
• Provides an optional GPS receiver unit
• 10 MHz reference oscillator
• 1st LO PLL
• 2nd LO PLL
4.4.7.1 PFRU Board 1st LO PLL for the DAC Clock Circuit
This PLL (Phase Lock Loop) oscillator provides a clock to the IF DAC (Digital to
Analog Converter). It is used to produce a 140 MHz analog IF output from the signal
processor board. The block diagram for local oscillator 1 is shown in Figure 4-5 on page
4-13.
Phase noise specification on DAC clock is given in Table 4-1.
This clock is sent to the signal processing board where it is used to generate the 140
MHz IF output signal, which is fed to the up/downconverter board. DAC clock
frequencies for various modulation methods is shown in Table 4-2.
Each modulation method produces an IF frequency which is equal to 1/4 of the DAC
clock frequency for that modulation system. Therefore, an offset frequency is produced
by the FPGA modulator to bring the IF frequency to 140 MHz. The required offset
frequency for each modulating system can be calculated using the following formula.
Table 4-1 Phase Noise Limit For DAC Clock
Hertz
10 -60 dBc/Hz
100 -90 dBc/Hz
1k -95 dBc/Hz
10k -100 dBc/Hz
100k -117 dBc/Hz
1M -135 dBc/Hz
Offset Frequency 140MHz
DAC Clock Frequency
4
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