E8257D/67D, E8663D PSG Signal Generators Service Guide 157
Troubleshooting
RF Path Description (Frequency Generation, Level Control, and Modulation)
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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