E8257D/67D, E8663D PSG Signal Generators Service Guide
Troubleshooting
RF Path Description (Frequency Generation, Level Control, and Modulation)
1-104
FM/ΦM (Option UNT)
The FM/ΦM signal is routed from the A11 Modulation Generator to the A6 Frac–N. On the A6 Frac–N:
• FM rates < 4 kHz are applied to the pre–scale control circuit and the A6 Frac–N VCO. This generates a change in
the pre–scaled drive (feedback from the VCO) to the VCO’s phase detector, causing a VCO frequency change.
• FM rates 4 kHz to 100 kHz are applied only to the A6 Frac–N VCO.
• FM rates >100 kHz are routed from the A6 Frac–N to the A9 YIG Driver and applied to the FM coil in the
YIG Oscillator.
• For phase modulation, the FM drive signal is routed through an LCR circuit Other than the LCR circuit, ΦM
signals are routed the same as FM signals.
Pulse (Options UNU and UNW)
Pulse modulation signals from the A11 Pulse/Analog Modulation Generator depends on model and option can be
routed directly to the A8 Output and the A30 Modulation Filter or A12 Pulse Modulation (UNW) pulse diodes. Pulse
diodes must be biased on to pass RF signals. Circuitry on the A8 Output, A30 Modulation Filter, A36 Quadraplier, and
A37 44 GHz Upconverter keep the pulse diodes turned on in instruments that do not have modulation capability.
Because ALC bandwidth is limited, the ALC circuit can only level pulse widths > 1 microsecond. To level pulse widths
< 1 microsecond, use search mode. Search mode calibration sequencing is as follows:
1. Modulation is turned off.
2. The ALC feedback loop is closed and the RF power leveled.
3. The ALC feedback loop is opened and the integrated output voltage used to drive the ALC diodes is maintained at
the level determined during closed loop operation (step 2).
Changes in power level, frequency, or pulse settings cause the signal generator to repeat the search mode leveling
procedure and determine a new ALC diode drive level. Because drift in that leveling circuit can occur over time in
open–loop operation, it is not recommended to stay in search mode for extended periods of time without closing the
loop and refreshing the ALC diode drive voltage.
Digital Modulation
Digital modulation is produced using portions of the following major assemblies:
•A8 Output
• A13 I/Q Multiplexer
• A14 Baseband Generator (Option 601/602)
• A35 I/Q Modulator
Together, these assemblies generate and route the baseband signals to the I/Q modulators located on the A8 Output
and the A35 I/Q Modulator. The A8 Output modulates carrier signals form 250 kHz to 3.2 GHz, while the
A35 I/Q Modulator modulates carrier signals form 3.2 to 20 GHz. The A14 Baseband Generator (Option 601/602)
comprises data generation, burst control, and symbol building circuitry. The output section of the A14 Baseband
Generator consists of two 16–bit DACs: one for the I signal and one for the Q signal. The data produced from these
I/Q signals is converted to analog, 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.
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