Block Diagram - Keithley 148 Instruction Manual

MODEL 148 NANOVOLTMETER CIRCUIT DESCRIPTION
SECTION 3.
CIRCUIT DESCRIPTION
3-1. GENERAL.
a. The Model 148 consists of a chopper demodulator system followed by a
dc amplifier. Feedback is applied to the whole loop. (See Figure 11.)
b. A mechanical chopper converts the dc input signal to a 94-cps signal
'The ac signal is amplified, demodulated, dc amplified and applied co the
meter.
A feedback network samples the signal at the output and compares
it to the input.
The dc input signal and the feedback signal are compared
in the input transformer primary. The transformer increases the voltage-
difference signal between the two. The ac amplifier amplifies the dif-
ference signal; line-frequency sidebands of the 94-cps signal are filtered
out. The a~ signal is then demodulated by a saturated transistor switch
and enters a dc amplifier, which has a feedback capacitor to filter out
the demodulator ripple. The dc amplifier output is connected to the
meter, the output terminals and the feedback network. The feedback
resistors determine full-scale range. The zero suppress signal is con-
nected to the feedback point in the input circuit.
FIGURE 11. Block Diagram of Model 148 Amplifier Circuits.
c. The power source for the Model 148 is either line voltage or the
rechargeable battery. Voltage from either source is applied to a dc-to-
dc inverter and then to three highly regulated supplies. The three
supplies furnish power to the oscillator and the amplifier circuits.
There
is also a battery charging circuit to charge the battery when it is necess-
ary and when the line voltage is connected.
NOTE
The circuit designations referred to in this section are
for Schematic Diagrams 17351F, 17352D and 17353D found
at the back of the manual.
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