Installation guide

Troubleshooting
Block Descriptions for Option 501, 502, 503, 504
1-123
A13 Output (Option 501, 502, 503, 504 or Option UNJ)
or A13 Output (Option UNB) The functions of the A13 Output include output power leveling, A17
Synthesizer (Option 501, 502, 503, 504) pre-leveling, pulse/burst, I/Q and amplitude modulation (AM).
The RF signal from the A17 Synthesizer (Option 501, 502, 503, 504) is amplified and filtered
by the A13 Output. After the first stage of amplification on the A13 Output, the RF signal is split with one
path becoming the coherent carrier reference. The coherent carrier reference is free of any AM or Pulse/Burst
modulation and is available at the COH Carrier output on the instrument’s rear panel. The other path
continues through the A13 Output. If I/Q modulation is enabled, the RF signal is routed through the I/Q
modulator, otherwise it is bypassed. Depending on the output frequency, different bandpass filters are
switched in to remove unwanted components from the RF signal. After filtering, the RF signal is split with a
portion of the signal being detected and becoming the pre-level bias voltage used by the A17 Synthesizer
(Option 501, 502, 503, 504).
Output power leveling is accomplished with an Automatic Leveling Control (ALC) loop. The ALC has two
modes of operation: ALC open or ALC closed. For normal operation, the ALC loop is closed and the output
power is leveled. For some modulation applications, the ALC is opened and an internal Power Search routine
may be used to achieve amplitude accuracy.
When the ALC loop is closed, a portion of the RF output is detected and compared to the sum of the ALC
reference voltage and the baseband AM. The difference is then applied to an integrator. The integrator output
will ramp either positive or negative, based upon the sign of the input signal. If there is no difference between
the detected voltage and the sum of both the ALC reference voltage and baseband AM, the integrator output
will remain unchanged. The output voltage ramp is converted to a current and used to bias pin diodes in the
ALC modulator. These pin diodes vary the amount of attenuation in the RF path and thus maintain leveled
output power.
The A13 Output has two independent integrators to facilitate fast power switching. The feedback detector
voltage is compared to one ALC reference voltage for one power level, while at the same time, a second
independent ALC reference voltage is set up for the second power level. When the user changes from one
power level to the other, the hardware switches between the two integrator circuits and their respective
references. This function and its related parameters are available through the front panel
Alternate Amplitude
softkey and this softkey is only present when an electronic attenuator is installed.
Depending upon the application, the user may change the ALC bandwidth from normal to narrow. The
defaults are normal if I/Q modulation is off and narrow if external I/Q modulation is on. The softkey is not
available if internal I/Q modulation is selected.
When AM is enabled, the baseband signal originates on the A18 Reference. The signal enters the A13 Output
as a Log AM signal. An Antilog circuit converts the Log AM back to Linear AM. As the baseband AM signal
changes amplitude, it causes the ALC modulator bias to change proportionately. This is due to the AM signal
being summed with the ALC Reference voltage prior to the integrator. Care must be taken to ensure that the
ALC doesn’t become unleveled due to a combination of high carrier power and large modulation depth. The
instrument's total output power equals the sum of the carrier power and the AM sidebands’ power. If this sum
is greater than the maximum output power rating, the ALC loop may become unleveled.
The ALC loop also contains a pulse/burst modulator. The pulse signal originates on the A18 Reference and the
burst signal originates on the A7 Baseband Generator (Option 001 or Option 002). The pin diodes within the
modulator are normally biased to provide minimum attenuation. When pulse/burst modulation is used, the
pin diodes are biased off and on thus providing maximum or minimum attenuation. This causes the RF signal
to pulse on and off. For narrow pulse widths, the ALC is opened. To maintain power level accuracy, a
Power Search routine may be used through the front panel. The Power Search routine shuts down the
modulation, momentarily closes the ALC loop, and levels. The ALC is then opened up and the modulation is
turned back on.
The I/Q modulator is also located on the A13 Output and may be switched in or out of the RF path. Baseband
modulation, for the I/Q modulator, is provided by the A10 I/Q Multiplexer. User-selectable lowpass filtering