THEORY OF OPERATION
3-2
The power supply
is
a
linear
design providing two
+
1 5
V,
-15V, +5
V,
+37
V,
+
1 8
V,
and 6
V
ac to the Generator. All the power supplies
are series-pass regulated except the 6V ac
filament supply and the + 1 8V supply, which
provides power
to the Attenuator relays and
Reverse Power
Protector (RPP) relays. A fuse/ filter/ line-voltage selector
allows
the
Generator to operate from any common
supply
voltage.
The dc fan is powered from the unregulated +5V supply. The fan
operates
only when line
power is available and the front panel POWER
switch is ON,
The
Option
-130 High-Stability Reference
operates whenever the instrument is plugged
into an active ac outlet, regardless of the position
of
the
instrument
POWER switch.
3-6.
FUNCTIONAL DESCRIPTION
The following paragraphs describe the key output parameters
of
the Generator: level,
amplitude modulation, frequency, and frequency
modulation.
3-7.
Level
Level control
is
provided by two separate circuits, a step attenuator and a vernier level
DAC. The 6.02 dB per step Attenuator/ RPP, (A2A5) provides coarse control. Fine level
control
is provided
by a
vernier
level DAC that varies the automatic level control voltage
(ALC). The microprocessor automatically controls the step attenuator and the vernier
level DAC. The microprocessor also applies level correction to compensate for the
Generator
frequency response.
Each Generator has level correction data for the Output and Attenuator
assemblies,
stored in the Output and Attenuator calibration
EPROMs.
The EPROMs are located on
the Controller assembly. The correction data
is
based on measurements of each assembly
during calibration of the Generator at the
factory.
This microprocessor level correction data is applied only to the vernier level DAC; it does
not affect the coarse level control provided by the Attenuator, In
other words, all
Generators have the same attenuator pads inserted at
a
selected
level even though the
correction data
is
different for each
Generator.
To improve level accuracy in relation to temperature,
the Generator uses a software
temperature compensation technique.
This
technique uses data that is the same for all
Generators
regardless
of
the
options
installed.
3-8.
Amplitude Modulation
The output of the level DAC is the ALC loop control
voltage.
The
Generator output
signal is amplitude modulated
by
varying this
control
voltage with the modulating signal.
A I
V
peak modulating signal from the
internal
modulation oscillator or from the external
MOD
INPUT connector
is applied to the AM DAC, a multiplying D-to-A Converter.
The multiplying factor of this DAC, corresponding to the programmed percentage
of
modulation, is factored by the Controller.
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 Controller
and corresponds to the programmed signal level. The multipling factor also includes the
level correction information stored in the calibration EPROMs,
The operation of the ALC loop
causes
the amplitude of the RF signal to conform to this
varying
control voltage,
thus
amplitude modulating the Generator output.
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