HP 5501B, Operating And Service Manual, Page: 71 / 120

HP5501B
Service
8-11
8-37. When the state machine excites the liquid crystal into its active state (see Figure 8-8), the laser
beam passes through the liquid crystal unaltered. The polarizer blocks the horizontal component
and passes the vertical component onto the photodiode. The photodiode current is proportional
to the power in the vertical component and is amplified and converted to a voltage by the
photodiode amplifier. The voltage is held on the positive input of the subtracting sample and
hold circuit.
8-38. When the state machine places the liquid crystal in its passive state, the liquid crystal rotates the
polarization of both components by 90 degrees. In this state, the polarizer blocks the vertical
component and passes the horizontal component. The voltage out of the photodiode amplifier is
proportional to the horizontal component and is held by the negative input of the subtracting
sample and hold circuit. The error signal output of the subtracting sample and hold circuit is
proportional to any difference in power between the two components.
NOTE
A TILDE ("~") PRECEDING ALL SIGNALS INDICATES NEGATIVE-TRUE
LOGIC.
8-39. After the optical mode tuning has stabilized; LOCKED TUNED OPTICAL (LTO) signal is held true
enabling the reference frequency output. Proper laser tuning and reference receiver operation is
indicated by ~REF ON true. ~REF ON false will cause the state machine to reset to the warmup
mode.
Figure 8-8. Liquid Crystal Timing Diagram (Optical Mode)