Installation guide

Troubleshooting
Block Descriptions for Option UNJ and Option 506
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A16 Frac-N (Option UNJ or Option 506)
The A16 Frac-N is part of the Frequency Synthesis Block and the RF path. Depending on the desired output
frequency, the A16 Frac-N divides down the YO frequency. The A16 Frac-N also provides amplification and
filtering for the YO signal prior to it being routed to the A13 Output.
A pre-level bias voltage generated by the A13 Output is fed back to the A16 Frac-N. The pre-level voltage is
used to bias a pin diode and control the amount of RF amplification on the A16 Frac-N.
A13 Output
The functions of the A13 Output include output power leveling, A16 Frac-N pre-leveling, pulse/burst, I/Q and
amplitude modulation (AM).
The RF signal from the A16 Frac-N 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. The
coherent carrier is free of any AM or Pulse/Burst modulation and is available to the user at the COH Carrier
on the signal generator’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 A16 Frac-N.
Output power leveling is accomplished with an Automatic Leveling Control (ALC) loop. The ALC has two
modes of operation: either 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 loop is closed, a portion of the RF output is detected and compared to the sum of the ALC reference
voltage and 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 isn’t any difference between the
detected voltage and the sum of 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 the pin diodes in the
ALC modulator. The 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; 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 is on. The softkey is not available if
internal I/Q 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 the ALC
doesn’t become unleveled due to a combination of high carrier power and large modulation depth. The signal
generator’s total output power equals the sum of the carrier power and the AM sideband 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