Keysight Technologies E8257D Service Manual page 161

Troubleshooting
RF Path Description (Frequency Generation, Level Control, and Modulation)
amplified, and filtered before being routed to the A13 I/Q Multiplexer. The I/Q
signals are routed from the A13 I/Q Multiplexer to the I/Q modulators located
on the A8 Output and the A35 I/Q Modulator. These I/Q signals modulate the
main RF signal.
A13 I/Q Multiplexer
The A13 I/Q Multiplexer receives I/Q input signals from the A14 Baseband
Generator, external I/Q input signals from the front panel, and I/Q sense
feedback signals from the I/Q modulators on the A8 Output and A35 I/Q
Modulator. The A13 I/Q Multiplexer inverts and routes the I and Q signals to
the rear panel, where these signals can be used to modulate external
equipment. The A13 I/Q Multiplexer also routes the main internal I and Q
modulating signal into the I/Q modulators on the A8 Output and A35 I/Q
Modulator, which are used to modulate the main RF output.
The A13 I/Q Multiplexer has two identical paths in which to switch incoming
I/Q signals. Each path has individual gain control, and can be switched to a
summing node. The output of the summing node can be switched to the EXT
outputs (through additional gain control circuitry), and/or simultaneously to
the A8 Output and A35 I/Q Modulator.
The EXT IN I and EXT IN Q signals enter the A13 I/Q Multiplexer, and can be
switched into a 50 or 600 ohm input impedance.
An internal calibration source supplies the levels required to perform
independent self tests. This enables the firmware to check the A13 I/Q
Multiplexer by setting known levels and reading the various nodes without
actually needing an I/Q signal.
An Offset Sense signal that comes in from the I/Q Modulators on the A8 Output
is used to adjust for offsets between the I and Q signals. The Offset Sense
signal is compared to the I/Q signal output, and the result is used to control
the amplitude; the feedback signal minimizes quadrature and gain errors.
A14 Baseband Generator (Option 601/602)
The A14 Baseband Generator may be used as a dual arbitrary waveform
generator, a real time I/Q baseband generator, a noise generator, or a
multitone generator. Customer–supplied waveforms can be downloaded into
the waveform memory to generate customized formats.
The I/Q bandwidth is 80 MHz for internally generated signals, and 160 MHz for
externally generated I/Q signals. The A14 Baseband Generator can generate
symbol rates of up to 6.25 Msymbol/sec, and has 8 Msamples (Option 601)
and 64 Msamples (Option 602) of waveform memory.
Data is sequenced from waveform memory by the field programmable gate
array block (A1–FPGA), passed on to the format builder block
(A2–FPGA/Modulator/Filter), and finally to the output section. I and Q DACs at
the output convert the digital waveform data to analog. The I and Q output
DACs are driven by a sample clock generated by an on–board VCO. The VCO
164 E8257D/67D, E8663D PSG Signal Generators Service Guide