Section 4 THEORY OF OPERATION
The first portion of this section will deal with the signal flow through the instrument while later portions will cover
the operation of specific circuits in more detail.
Referring to
Figure D,
it is seen that the 270 interfaces with the pressure sensor via the sensor connector. This
connector also provides the preamp voltage, heater voltage and oscillator voltage to run the pressure sensor.
The pressure signal is applied to the zero circuit when it comes directly from the pressure sensor. Should it
come from the multiplex unit, the zero circuit is bypassed so that the zero controls in the multiplex unit override
the zero controls on the 270.
The output from the zero circuit is applied to the Input Selector switch. This switch connects the input of the
AC amplifiers to the pressure signal or to the internal cal signals. Another pole of this switch selects the gain
of the AC amplifiers.
The Input Selector switch applies the signal to the 1st AC amplifier where it is amplified and passed on to the
divider circuit and 2nd AC amplifier. The divider circuit controls the amplitude of the AC output signal by com-
mand of the range select circuit which in turn is controlled by the Input Selector switch.
The output of the 2nd AC amplifier is applied to the demodulator transformer which interfaces it to the demodulator
circuit. The demodulator fullwave rectifies the AC output and applies it to the DC amplifier. This demodulator
is synchronized to the oscillator to block quadrature signals.
The DC amplifier acts as an integrator and filters the input signal to produce a smooth DC output signal. This
circuit also has a Response Speed Selector switch which selects one of three possible bandwidths for this amplifier.
The signal from the DC amplifier goes to the Remote and Accessory connectors and is applied to the panel
meter via the Display Units Divider circuit. The panel meter is interfaced with a Digital Interface circuit which
converts the setting of the Input Selector switch, Display Units switch and the Sensor Range Selector switch
into the proper decimal point selection. The decimal point information plus the BCD output from the panel meter
is then applied to the BCD output connector.
Power for the heater and voltage for the electronics is supplied by the main power supply circuits. These sup-
plies also provide power for the oscillator which produces a highly regulated 10KHz sinewave output. This voltage
is used for excitation of the Pressure Sensor and for internal calibration signals as well as synchronization of
the demodulator. A more detailed circuit description follows.
—13 VOLT SUPPLY: See figure E - Section A
Both power supplies are identical floating 13 volt power supplies. The negative terminal of one is connected
to the positive terminal of the other and grounded. This produces the re13v and —13V at the other
remaining terminals.
The regulations are the three terminal adjustable type. An adjustment range of approximately 1V is obtained
from potentionmeters R19, R22. These regulators provide current limit and thermal overload protection. The
rectifiers CR4, CR5 protect against voltage reversal.
4-1
