Fluke 6080A Service Manual page 37

Output/Level Control
Level control is provided by two separate circuits: a step attenuator and a vernier level
DAC. The A20 Attenuator/RPP Assembly provides coarse level control in 6.02-dB
steps. Fine level control is provided by a vernier level DAC that varies the leveling-loop
control voltage. The controller microprocessor automatically controls the step
attenuator and the vernier level DAC. The microprocessor also applies level correction
to compensate for the signal generator frequency response.
Each signal generator has level correction data for both the A8 Output PCA and the
A20 Attenuator/RPP Assembly. The level correction data is stored in the compensa-
tion memory located on the A13 Controller PCA. The level correction data is based on
the measurements of each assembly during level compensation of the signal generator.
The level correction data is applied only to the vernier level DAC and does not affect
the coarse level control provided by the Attenuator/RPP Assembly. In other words,
all signal generators have the same attenuator pads inserted at a selected level even
though the correction data is different for each signal generator.
To improve level accuracy in relation to temperature, the signal generator uses a
software temperature-compensation technique. This technique uses data that is the
same for all signal generators.
Amplitude Modulation
The signal generator allows amplitude modulation depth programming from 0 to
99.9% with 0.1% resolution. Amplitude modulation depth is programmed using the
12-bit AM DAC. A nominal setting of 2997 on the AM DAC corresponds to 99.9%
AM depth.
The output of the level DAC is the leveling loop-control voltage. The signal generator
output signal is amplitude modulated by varying this control voltage with the
modulating signal. A 1V peak modulating signal from the internal modulation
oscillator or from the external MOD INPUT connector is applied to the AM DAC (a
multiplying digital-to-analog converter). The multiplying factor of this DAC,
corresponding to the programmed percentage of modulation, is calculated by the A13
Controller PCA.
The modulation signal from the AM DAC is summed with a fixed DC reference
voltage. The composite signal (DC plus modulation) is applied to the level DAC (a
level control-multiplying DAC). The multiplying factor for this DAC is also handled
by the A13 Controller PCA and corresponds to the programmed signal level. The
multipling factor also includes the level correction information stored in the
compensation memory.
The operation of the ALC loop causes the amplitude of the RF signal to conform to
this varying control voltage, thus causing amplitude modulation of the signal
generator output.
Pulse Modulation
Pulse modulation is accomplished by a single-pole single-throw GaAs FET switch
located at the input to the output amplifier. This switch can be driven by the internal
modulation oscillator or by an external signal. The GaAs FET switch provides a very
fast and high ON/OFF ratio RF pulse.
THEORY OF OPERATION
2-10.
2-11.
2-12.
2-5