Fluke 6060B Instruction Manual page 106

THEORY OF OPERATION
3-55.
The modulation oscillator generates a leveled sine wave of 400 Hz or I kHz and is the
modulation source for the internal AM and FM functions. The oscillator is a level­
controlled Wien-Bridge type and consists of op-amps U405A, U405B. Frequency is
determined by the series RC time constant of the components between pins 5 and 7 of
U405B and by the parallel RC time constant of the components from U405 pin 5 to
ground. The modulation frequency control line, MF400L, originating at the latch U308,
selects either 400-Hz or 1-kHz operation, and is selected by switching resistors with
JFETs Q40 1 and Q403.
The amplitude of oscillation is controlled hy an ALC loop that varies the resistance on
U405B pin
Q402, is nominally 2K ohms. The oscillator signal amplitude is sensed by rectifier CR40 1 .
The average current through CR401 is made equal to the reference current in R416 by
integrator-amplifier U405A. Level adjustment is set by potentiometer R4 1 9 .
Temperature compensation i s provided hy R4 1 7, R418, and CR402.
3-56.
The FM modulation signal source and deviation control circuits are on the Output
assembly. Analog switches U40 1 A, U40 1 B, and op-amp U402A select the internal or
external modulating signal, or no modulation. The selected and buffered modulating
signal at U402A pin 7 is applied to FM DAC 0403. This DAC provides fine control of the
FM deviation. (The coarse control FM circuitry is part of the Synthesizer assembly). The
output of the DAC, at U405C pin 8, is the modulation signal multiplied by a factor
proportional to the 8-bit FM deviation control provided by the Controller.
3-57.
The AttenuatorjRPP Assembly, A2A5, consists of an Attenuator/ RPP PCA, A2A5A4,
in a metal housing mounted on the top side of the A2 module section to form a shielded
enclosure. The Relay Driver f RPP PCA, A2A5A5, is included in this assembly.
The Attenuator assembly controlled by the microprocessor provides coarse control of
the Signal Generator output level. The high-level signal from the Output PC assembly,
A2A4, is applied to the Attenuator which provides 0 dB to 138 dB of attenuation, in 6-dB
steps, to this signal before it goes to the Generator RF OUTPUT connector.
Compensation data for th
calibration EPROM located on the Controller PCA, A2A
uses this data to correct for the combined deviations of the attenuator sections in use. For
more details on level correction, refer to paragraph 3- 1 3 , Amplitud
The Attenuatorf RPP Assembly provides an attenuation range from 0 dB to 1 3 8 dB in
6-dB steps a nd consists of seven independently cascaded 50-ohm attenuation sections, a
6-dB, a 1 2-dB, and five 24-dB sections. Each section consists of a DPDT relay and a
three-resistor attenuator pad.
One relay position (when power is applied to the relay provides a straight path for the R F
signal, and the other position (no power applied t o the relay) inserts the attenuator pad
into the R F signal path, All seven relays are inside individual shielded compartments in
the Attenuator housing.
The control of the Attenuator relays is latched via U27, the open-collector drivers U30
and U 3 1 on the Controller PCA A2A 7 and transisto
DriverjRPP control PCA. For calibration and troubleshooting purposes, special
3-22
MODULATION OSCI LLATOR
6
to ground. This resistance comprised of R412 and the drain resistance of
FM DEVIATION CONTROL
Attenuator/RPP , A2A5
e
attenuator in each Generator is stored in the Attenuator
7.
The instrument program
Control.
e
r
drivers on the A2A5A5 Relay