Nautel XR6, Troubleshooting Manual, Page: 73 / 291

XR6/XR3 Troubleshooting Manual Detailed Circuit Descriptions
Issue 3.0 2009-07-15 Page 2-5
N divider control information
The 4f
c
/B frequency is divided by an 'N' factor, to provide 2f
PDM
output frequency between
126 kHz and 134 kHz (when E3 is set in
LOW PDM
mode) or between 245 kHz and 276 kHz (when
E3 is set in
HIGH PDM
mode) (see “N divider” on page 2-7 ). E3 should be set to
HIGH PDM
mode
when used in an XR6/XR3 transmitter. The microprocessor determines the value of N and outputs
the information at pins 2, 3, 4, 5, 6 and 7. The N divider uses this information as the N dividing
factor.
Direct digital synthesizer
The direct digital synthesizer consists of integrated circuit U5 (AD98525Q) and associated
components. It generates a frequency of 4f
c
based on information provided by microprocessor U4
(see “Microprocessor” on page 2-4 ). Integrated circuit U5 is a CMOS, numerically controlled
oscillator with a 48-bit phase accumulator and 12-bit digital-to-analog converter (DAC). The phase
accumulator, responsible for generating an output frequency is presented with a 48-bit value from the
Frequency Tuning Word 1 registers whose contents determine the FTW as follows:
FTW = (Desired Output Frequency x 2
48
)/SYSCLK
The 10 MHz clock input, coupled with U5's internal programmable reference clock multiplier, results
in a system clock of 50 MHz (i.e. SYSCLK = 50 MHz).
The 12-bit output from the phase accumulator is input to the DAC, which outputs a stepped sine
wave at a frequency of 4f
c
which is low-pass filtered to remove high frequency components.
Low pass filter
A low-pass filter comprised of C21, L1 and C23 removes the high frequency images present in the
DDS output signal. The output is a sine wave at a frequency of 4f
c
.
Digitizer
The output of the low-pass filter is connected to a digitizer circuit consisting of transistor Q4,
inverter U10:B and associated components. U10:B's output is a square wave at frequency of 4f
c
which
is applied to a divide-by-4 circuit and to the N divider circuit.
IPM correction
The incidental phase modulation IPM Correction circuit forms part of a dynamic loop that monitors
the I PM compensation input and compensates for any incidental phase modulation caused by the host
transmitter's RF power stages.