E8257D/67D, E8663D PSG Signal Generators Service Guide
Troubleshooting
RF Path Description (Frequency Generation, Level Control, and Modulation)
1-100
• an A26 Microcircuit Interface Deck and front and rear panels that provide the digital interface
A6 Frac–N
The A6 Frac–N is part of the Synthesis Loop and the RF lowband path.
The RF signal from the A29 20 GHz Doubler is amplified, filtered, divided (dividers generate RF output frequencies
below 3.2 GHz), and then amplified again before being routed to the A8 Output.
The A8 Output provides the A6 Frac–N a pre–level bias voltage to maintain the required RF output level to the
A8 Output assembly.
Troubleshooting note: To check the A6 Frac–N RF output level to the A8 Output, remove the A8 Output from the
signal generator while the signal generator is powered on. If the A8 Output is removed while the power is on, the
pre–level bias is maintained on the A6 Frac–N, maintaining the RF power level on the A6 Frac–N output. Turn the
instrument off before reinstalling the A8 Output.
A8 Output
Analog
The A8 Output is in the RF lowband path; it provides:
• pre–level bias voltage to the RF output level control on the A6 Frac–N
For frequencies up to 3.2 GHz, the ALC diode controls the RF output power level. Without bias applied to the ALC
diode, the output power goes to the maximum level. In closed–loop operation, the A10 ALC uses a negative voltage
from both the A23 (< 2 GHz) and A24/25 (>2 GHz) coupler/detectors to control the ALC diode’s bias.
•pulse modulation
Pulse modulation (RF pulsed on) is achieved by applying +5 Vdc to the pulse circuitry on the A8 Output. +5 Vdc
turns the RF signal on; removing the +5Vdc turns the RF signal off. In instruments with Option UNW (fast pulse)
the pulse modulation input cable is moved from the A8 Output to the A12 Fast Pulse Modulator and the A8 diode
is biased on.
• amplitude modulation
• the circuitry to generate under–range frequencies (<250 kHz)
Under–range frequencies are the result of mixing the A6 Frac–N RF signal with a 1 GHz signal from the
A7 Reference and using the difference between the two signals for the output. The A8 Output signal passes
through the A23 Lowband Coupler/Detector to the A30 Modulation Filter.
Digital
In addition to providing the analog functionality described above, the A8 Output assembly used in vector models
contains an I/Q modulator that is driven with signals from the A13 I/Q Multiplexer to provide I/Q modulation
<3.2 GHz. When digital modulation is not selected, the I/Q modulator is bypassed.
A12 <3.2 GHz Fast Pulse Modulator (Option UNW)
Option UNW is a hardware/firmware (license key) option that provides improved pulse performance (see the data
sheet for specific pulse width performance). A firmware license key is required to turn Option UNW on; below 3.2 GHz
additional hardware is also required. The additional hardware for this option is installed in the lowband path between
the A8 Output and the A23 Lowband Coupler Detector. The pulse drive signal is electrically switched from the A8
Output to the UNW hardware, and the pulse diode on the A8 Output is biased on, allowing the RF signal to pass
through. The pulse shaper circuitry on the A11 Pulse Modulator needs to be set using a license key to provide
narrower pulse widths.
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