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8-104. 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)] 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 U1F and U19E turn
on the READY indicator LED indicating that the HP5501B is ready for use.
8-105. Liquid Crystal Ⓓ
8-106. The liquid crystal, which is mounted within the A6 Sampler Assembly, connects to CMOS driver
U8B via elastomeric connector strips.
8-107. When the PASSIVE/~ACTIVE signal at U6B (Pin 8) is high, both outputs of U8B 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.
8-108. When PASSIVE/~ACTIVE is low, U8B 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.
8-109. Photodiode and Amplifier Ⓔ
8-110. The polarizing filter, which lies between the liquid crystal and the photodiode (A1CR1), allows
only one of the two orthogonal components of the laser beam to strike the 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 U11D (at TP16) is the
photodiode current multiplied by an effective resistance of:
8-111. Warmup Error Amplifier and Comparator Ⓙ
8-112. The bridge formed by A1R15 and the heater on one side, and A3R16, A3R18, and A3R19 on the
other, generates an error signal on the order of 1 mV/deg C because the resistance of the heater
increases with increasing temperature. The Warmup Error Amplifier, A3U14, amplifies this error.
8-113. 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
8-114. Subtracting Sample and Hold Ⓗ
8-115. 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 U12B 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.
8-116. During the optical mode, A3U11 0 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 A3U11 B) is used for the vertical component.
8-34
(R20 + R22) R17
R20
Section
5. NOTE: See
Section 9.1 Service Note
HP5501B
Service
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Troubleshooting
