Agilent Technologies 5517B Operation And Service Manual page 88

gated through A3U2A and A3U19C to the rear panel READY (and A3DS1) LED. Blinking continues until the
state machine enters the locked state.
8-102. When HTRQ goes high, A3U4A changes states from reset to count. Unless reset by HTR OK dropping low,
A3U4A counts for 100 seconds and then clocks U3A. This sets OPTICAL high and locks HTRQ on. The 100 second
period is sufficient to stabilize the heater temperature.
8-103. When OPTICAL goes high, the state machine enters the locking state and optical feedback controls the
heater. After this state has been held for 50 seconds, LTO |U1F (Pin 12)1 goes high and the controller enters the
locked state. If REF ON (a signal from the Reference Receiver) is low, the entire circuit will be reset by the Power-
on-Reset circuit. Otherwise A3U1F and A3U19E turn on the rear panel READY (and A3DS1) indicator LED
indicating that the HP 5517B is ready for use.
8-104. Liquid Crystal (D)
8-105. The liquid crystal, which is mounted within the A6 Sampler Assembly, connects to CMOS driver A3U8B via
two elastomeric connector strips.
A TILDE ("-") PRECEDING A SIGNAL NAME INDICATES NEGATIVE-TRUE LOGIC LEVELS.
8-106. When the PASSIVE/-ACTIVE signal at A3U6B (Pin8) is high, both outputs of A3U8B are high, thereby
placing zero potential across the liquid crystal. This is the passive state of the liquid crystal and causes the laser
beam polarization to rotate by 90 degrees.
8-107. When PASSIVE/-ACTIVE is low, A3U8B produces a 50 Hz, symmetrical square wave output to configure the
liquid crystal in the active state. During this state, the liquid crystal does NOT rotate the laser beam polarization.
8-108. Photodiode and Amplifier (D)
8-109. The polarizing filter, which lies between the liquid crystal and the photodiode (A4CR1), allows only one of the
two orthogonal components of the laser beam to strike tJ/ie photodiode. During the passive state of the liquid
crystal, the polarizing filter blocks the vertical component and the photodiode current is proportional to the
horizontal component. During the active state, the photodiode current is proportional to the vertical component. The
output of A3U11D (at TP16) is the photodiode current multiplied by an effective resistance of:
8-110. Warmup Error Amplifier and Comparator (J)
8-111. The bridge formed by A3R9 and the heater on one side, and A3R16, A3R18, and A3R19 on the other,
generates an error signal on the order of 1 mV/"C because the resistance of the heater increases with increasing
temperature. The Warmup Error Amplifier (A3U14) amplifies this error.
8-112. The output of the HTR OK comparator (A3U15) goes high when the error voltage nears zero. The
temperature at which HTR OK occurs is set by potentiometer A3R16 as described in the Reference Temperature
Adjustment in Section V.
8-113. Subtracting Sample and Hold (H)
8-114. During warmup mode, A3U2C output (DISABLE high) disables the heater (via the power amp) for 2.56
seconds out of every 25.6 seconds. During the 2.56 second disable, the heater temperature stabilizes and A3U12B
samples the output of U14. When A3U12B opens, the sampled voltage is held on A3C13 and buffered by A3U11B.
Switch A3U12C connects A3C14 and the input of A3U11C to ground.
8-115. During the optical mode, A3U11 D alternately drives one of the sample and hold circuits, then the other. One
sample and hold circuit (A3U12D, A3C14, and A3U11C) samples the output of A3U11D proportional to the
horizontal component. The other sample and hold circuit (A3U12A, A3C13, and A3U11B) is used for the vertical
component.
NOTE
(A3R20 + A3R22) A3R17 A3R20