E8257D/67D, E8663D PSG Signal Generators Service Guide 159
Troubleshooting
RF Path Description (Frequency Generation, Level Control, and Modulation)
Frequencies 3.2 to 10 GHz pass through the A29 20 GHz Doubler. Frequencies
10 to 20 GHz are generated in the doubler by doubling the 5–10 GHz YO
signal. Frequencies > 3.2 GHz are routed from the A29 20 GHz Doubler to the
A30 Modulation Filter (in analog models) or to the 3 to 20 GHz I/Q Modulator
(in vector models).
A30 Modulation Filter
One function of the A30 Modulation Filter is to switch the highband and
lowband signals into a common path. Therefore the output frequency range of
the A30 Modulation Filter is 100 kHz to 20 GHz. For frequencies > 3.2 GHz, the
A30 Modulation Filter provides amplitude and pulse modulation circuitry,
power level control, and filtering. Frequencies < 3.2 GHz are simply switched
through the A30 Modulation Filter and are not filtered or modulated by any
circuitry in the A30 Modulation Filter.
The highband ALC diode within the A30 Modulation Filter provides level
control and amplitude modulation. With no bias applied to the ALC diode,
output power is at the maximum power level. In pulse operation, the pulse
diode must be biased on for the RF signal to pass through the pulse
modulation circuitry.
In 20 GHz analog signal generators, the A30 Modulation Filter output is routed
to the A24 Highband Coupler and A25 Highband Detector. In 40 GHz analog
instruments, two outputs from the A30 Modulation Filter are routed to the
A27 40 GHz Doubler, one input for frequencies ≤20 GHz to a bypass switch
and one to the 40 GHz Doubler circuit. In signal generators with output
frequencies above 40 GHz, the A27 40 GHz doubler is replaced with the
A36 Quadraplier. In these instruments two A30 Modulator outputs go to the
A36 Quadraplier. One of the A30 Modulator outputs is connected to the A36
Quadraplier bypass circuit and the other to the doubler circuit.
A35 3–20 GHz I/Q Modulator (Vector Models Only)
The A35 3–20 GHz I/Q Modulator adds I/Q information to the RF signal. In
20 GHz models, the A35 3–20 GHz I/Q Modulator is installed between the A28
20 GHz Doubler and the A30 Mod Filter. In models with output frequencies
above 20 GHz, the A35 3–20 GHz I/Q Modulator is installed after the A28
20 GHz Doubler with one output (11.4–19 GHz) routed to the A27 40 GHz
Doubler and the other output routed to the A37 Upconverter. The
A35 3–20 GHz I/Q Modulator provides the 90° delay circuitry required for I/Q
modulation. The I and Q inputs to the A35 3–20 GHz I/Q Modulator are from
the A13 I/Q Multiplexer. The A13 I/Q Multiplexer receives inputs from the
External I/Q Mod inputs and, if installed, the Option 601/602 Baseband
Generator.
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