Sc5506A Theory Of Operation; Output Amplitude Control - SIGNALCORE SC5506A Operating & Programming Manual

S C 5 5 0 6 A T O
H E O R Y O F P E R A T I O N

Output Amplitude Control

As shown in Figure 2, the SC5506A source architecture at a high level consists of an output amplitude
control section and a frequency synthesis section. The amplitude of the signal is controlled through the
use of digital step attenuators (DSAs) and a voltage controlled attenuator (VCA). The DSAs provide the
coarse-step tuning over a wide range while the VCA provides fine tune correction to the DSAs. The VCA is
part of the automatic level control loop (ALC), which additionally consists of an RF amplifier, a power
detector, and an integrator. The ALC loop can be closed or open. In the closed loop mode, the power
detector outputs a voltage proportional to the power it detects. This voltage is compared to that of the
reference ALC DAC voltage, which in turn is set for some calibrated power level. Voltage error between
the detector voltage and the ALC DAC voltage drives the integrator output in the direction that will vary
the VCA to achieve the desired output power level. When the ALC control loop is opened, the power
detector output voltage is grounded, and the integrator is configured as a voltage buffer that drives the
ALC DAC voltage to the VCA. In this mode, the ALC DAC voltage directly drives the VCA with voltage levels
that correspond to calibrated output power levels.
There are advantages and disadvantages to either of these two amplitude control modes. On one hand,
the open loop mode has an advantage over the closed loop mode when close-in carrier amplitude noise
is a concern. ALC loops do introduce some level of amplitude modulated noise onto the carrier signal, and
these levels may not be acceptable (although they are generally lower than the phase noise). SignalCore
offers the user the option to open the ALC loop to remove any unwanted AM noise that results from
closed loop control. Another side effect of the closed loop is that the frequency bandwidth of the ALC loop
may slow down amplitude settling. Typically, in order to keep AM noise low and close (in offset frequency)
to the signal, the loop bandwidth is also kept low. As a result, the settling time is increased.
On the other hand, a closed loop ALC provides better amplitude control over the entire frequency range.
With a temperature-stable ALC DAC, the closed loop will precisely maintain the power at the detector,
mitigating errors in the components prior to it in the signal path. Temperature-induced errors in
components and abrupt amplitude changes when switching filters in the filter banks contribute to errors
in the amplitude of the signal. However, these errors occur before the power detector and are
compensated by the feedback loop action. Errors in amplitude are thus confined to the output
attenuators, amplifiers, and the loop components. When the loop is opened, amplitude errors resulting
from all parts of the amplitude control section as well as the synthesizer section will affect the overall
output amplitude accuracy. In particular, when the filters within the filter bank are switched from one to
another, the signal experiences abrupt discontinuities in its amplitude which the open loop calibration
cannot appropriately account for in its correction algorithm.
Setting of the amplitude control components are performed automatically by the system, although the
user may choose to override the ALC DAC value if needed. In Figure 2, the labels in red indicate parameters
or devices which the user has direct control over.
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