(4) With the FUNCTION SELECT switch at SQ WAVE, CLSD LOOP, or BALANCE, -7-volt dc
from the power supply is applied through RI to one input of the four nor gates. Thus, the output of
each nor gate is controlled by the square wave signal applied to the other input. The in-phase square
wave from the op-amp is applied to nor gates U1A and U1C. The nor gates invert the square waves and
route them to self-demodulator U3A and cross demodulator U3B. The same signals are also applied to
nor gates U1B and U1D. In U1B and U1D, the signals are inverted to produce in-phase square waves
for self-demodulator U3D and cross-demodulator U3C.
d. Self-demodulator circuit.
(1) The self-demodulator includes out-of-phase self demodulator U3A, in-phase self-demodulator
U3D, self-amplifier uSA, and associated resistors and capacitors.
(2) Each demodulator operates like an electronic switch. When the demodulator control signal is
positive, the switch is closed. When the demodulator control signal is negative, the switch is open.
(3) Self-amplifier USA is an op-amp that operates with a gain near unity. Gain is set by the ratio
of R13 and R6. C3 and C19 integrate the pulsating dc output. R12 and R14 reference the input signals
(4) During the positive half cycle of reference voltage, demodulator U3D is closed and U3A is
open. The transducer signal is applied through R6 and U3D to the non-inverting input of U5A. During
the negative half cycle of reference voltage, demodulator U3A is closed and U3D is open. The
transducer signal is applied through R6 and U3A to the inverting input of U5A.
(5) When the extensible link is nulled, the transducer provides 0-volt and the output of U5A is 0-
volt. When the extensible link is extended, the transducer voltage is in-phase with the reference. The
output of USA is positive. When the extensible link is retracted, the transducer voltage is out-of-phase
with the reference. The output of U5A is negative.
e. Cross demodulator circuit.
(1) The cross demodulator circuit includes out-of-phase cross demodulator U3B, in-phase cross
demodulator U3C, cross amplifier U5B, and associated resistors and capacitors.
(2) Operation is the same as for those in the self-demodulator circuit (step d.) The cross
transducer signal is 180° out-of-phase with the self-transducer signal.
f. The servo amplifier circuit.
(1) The servo amplifier circuit includes servo amplifier U2 and associated resistors and capacitor.
This circuit algebraically adds the command signal from R22, the self-feedback signal from R9, and
the cross feedback signal from R21. The resultant output current from U2 drives the extensible link
(2) Servo amplifier U2 is an inverting op-amp. Resistor R4 keeps the non-inverting input of the
amplifier at 0-volt. The feedback loop for the amplifier includes resistor R8. Resistor R93 sets the ratio
between servo valve current and feedback current.
1-18. Metering Circuit (fig. 1-6).
Fig. 1-6 includes reference designations for SAS 1 and SAS 2
components. Only SAS 1 reference designations are used in the folIow-