Keysight Technologies N5244/5B Service Manual page 186

Theory of Operations
Synthesized Source Group Operation
Version 7 Direct Digital Synthesizer (DDS) Assembly – Basic Operation
Table 5-2 on page 5-20 lists the L.O. harmonic number, the direct digital
synthesizer (DDS) frequencies (A15), the main source frequency (A5 and A10),
and the doubler frequencies (A7, A8, A12, and A13) within the analyzer for
each band. This table is referred to throughout this chapter and also appears
on the overall block diagram at the end of Chapter 4, "Troubleshooting."
The A15 DDS assembly provides stable reference frequencies to the rest of the
instrument. A high stability 100 MHz signal is generated on the DDS and used
to generate a 10 MHz reference signal. A high stability 100 MHz
oven-controlled crystal oscillator (OCXO) on the DDS, normally provides the
frequency standard. However, if a 10 MHz external reference signal is detected
at the 10 MHz EXT REF IN port on the rear panel, it is used as the frequency
reference instead.
Refer to the Note notice on page 5-1. The A15 13.5 GHz DDS assembly
produces an LO signal that is sent through the A25 LO multiplier/amplifier 26.5
1
board to the A27 and A28 mixer bricks (via the A25 HMA26.5 internal splitter ).
The frequency is synthesized such that the test signal output is a constant
signal. For analyzers with DSP version 5.0 or DDS assemblies, the IF is 7.438
MHz (at frequencies below 53 MHz the IF signal is 0.826 MHz). This signal is
signal is sent to the A16 SPAM board for digital processing.
The A15 13.5 GHz DDS assembly produces an incident signal that is sent
through the A5 26.5 GHz source board and then through the A7 and A8
doubler boards to the front panel outputs. Likewise, the A15 13.5 GHz DDS
assembly produces a separate incident signal that is sent through the A10 26.5
GHz source board and then through the A12 and A13 doubler boards to the
front panel outputs. Portions of these signals are coupled off and sent to the
A27 and A28 mixer bricks (A–D and R1–R4) where they are mixed with the LO
signal from the A25 HMA26.5 internal splitter 1 to produce the IF signal. For
analyzers with DSP version 5.0 or DDS assemblies, the IF signal is 7.438 MHz
(or 0.826 MHz).
The A15 13.5 GHz DDS assembly contains its own phase lock circuitry. The
A15 assembly produces either internal reference that is free running for the LO
Signal and test signals. Or if an external reference is available a phase locked
LO signal and it produces phase locked test signals based on the external
reference. Regardless of whether the internal or external reference is used each
DDS has its own set of digital registers that enable the output to be set to a
different frequency. This makes it possible for the LO signal to be tuned to a
different frequency than the test signal. With frequency offset mode disabled,
the LO signal is a frequency value higher than the test signal. Since the A15
13.5 GHz DDS assembly receives 10 MHz input reference signal from the exact
same source, frequency drift error is eliminated.
1. A26 splitter only applies to PNAs with serial number prefix <6021.
5-18 Keysight N5244B/45B Service Guide