Fluke 6060B Instruction Manual page 53

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Modulator attenuation is thus approximately proportional to the modulator control
voltage at the emitter of Q I 03. Proportionality is required to maintain constant leveling
loop bandwidth as modulator attenuation varies. Minimum attenuation is obtained with
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a modulator control voltage of IOV, while maximum attenuation is obtained with OV.
Comparator 0310A and associated components form an unleveled indicator circuit. The
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comparator senses the modulator control voltage at the emitter of Q
normally less than
voltage exceeds +
becomes inoperative (unleveled). This condition could be due to a fault or some abnormal
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operation such as over-modulation. In this case, the comparator output (ONL VLL) goes
low. The Controller senses this low and causes the front panel'ONCAL' indicator to flash
and displays an unleveled status if interrogated .
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LEVEL CONTROL
3-54.
The instrument output level is set by the level-control circuit. Inputs to this a udio signal
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processing circuit are the internal and external modulation signals, a de reference voltage,
and the digital control commands. The circuit output is the leveling loop control voltage
that provides vernier level control of the Generator output. Digitally encoded level,
modulation depth, and temperature-compensation information are provided by the
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Controller.
Selection of the internal or external modulating signal, or no modulation, is made by
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analog switches 0401 C, 0401 D, and Op-amp 0402B. The selected, buffered modulatio n
signal at 0402B pin I is applied to pin 4 of 0301, a dual S-bit DAC. 0301, with 03020,
acts as a digitally programmed variable attenuator and is labeled AM DAC.
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Binary AM depth control information from the Controller is applied to DAC 0301. The
output at 03020 pin 14 is the modulation signal scaled to the programmed AM depth.
This ac signal is summed by op-amp 0302B with a de reference voltage provided by
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CR403. The output at 0302B pin 7 is called the I+ AM signal. This signal provides the
desired AM depth when scaled by the LVL DAC and applied to the leveling loop. AM
depth adjustment is provided by potentiometer R42 1 .
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The instrument R F output amplitude is temperature compensated in a frequency­
dependent manner as follows. The I + AM signal is applied to pin 1 8 of dual S-bit DAC
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030 1 , the DAC B reference input. The DAC output, at 04050 pin 14, is the !+AM signal
attenuated by an RF frequency-dependent factor provided by the Controller using
constants stored in the Generator firmware. This voltage is applied to a
resistor/ thermistor network that includes R303, R305, R306, and RT30 1 .
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The network output is the I+ A M signal attenuated by an R F frequency and temperature­
dependent factor, and is applied to summing op-amp U302C. The
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applied to this summing amplifier. Thus, the voltage at U 302C pin 8 is the temperature­
compensated and scaled l +AM signal.
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This signal is applied to the reference input of Level DAC 0303. This 12-bit DAC, with
op-amp U302A, latches 0304, 0305, controls the Output assembly RF output amplitude.
The DAC output voltage, at 0302A pin
I multiplied by a factor proportional to the 1 2-bit level control number provided by the
Controller. This voltage is the leveling loop control voltage. The Generator RF output
level adjustment is provided by potentiometer R3 1 1 , and DAC offset voltage adjustment
is provided by potentiometer R309.
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IV, and the comparator output is high. If the modulator control
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V, the modulator attenuation is at a minimum, and the leveling loop
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is the temperature-compensated
THEORY OF OPERATION
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03. This voltage is
+AM signal is also
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+AM signal
3-21