Reference Receiver; Line Driver; Power-On Reset (Por); Laser Head Power - HP 5501B Operating And Service Manual

8-121. Reference Receiver Ⓛ
8-122. The reference receiver consists of photodiode A3CR5, an amplifier A3U17, a transistor level
shifter A3Q6, and associated passive components. The receiver amplifies and converts the
sinusoidal output from A3CR5 to a square wave. When ~LTO (LOCKED TUNED OPTICAL) goes
low, and the receiver amplifier has adequate signal level input, the receiver is enabled and
provides a low TTL signal ~REF ON. During the warmup and locking modes, the receiver
supplies no useful information and is therefore disabled (~LTO high).
8-123. A3CR5 is reverse biased to reduce its capacitance. The +15V bias voltage is filtered by A3R46
and A3C27. A polarizer, oriented at 45 degrees to both optical frequency components, passes
optical power at the reference frequency onto A3CR5. The reference frequency is the difference
in frequency between the two optical frequency components of the laser beam.
8-124. A3U17 contains two amplifiers. The gain of the first amplifier is set by A3R44. The gain of the
second amplifier is fixed. A peak detector circuit charges A3C28 to a DC value proportional to
the output of A3U17. With no current from the photodiode, the voltage on A3C28 is
approximately equal to the voltage on A3C24. When photodiode current is present, the change
in voltage on A3C28 is the amplitude of the signal coming out of the second amplifier. When the
amplitude exceeds 100 mV, the voltage at A3U17 Pin 2 (~REF ON) drops to TTL low. A3Q6,
A3CR6 and A3R45 shift the output of A3U17 to TTL levels. A3 capacitors C22, C23, C24, C26
and C29 are used for coupling and bypass. A TTL high on A3U17 Pin 11 (~LTO) gates off the
reference frequency output and raises A3U17 Pin 2 to Vcc.
8-125. Line Driver Ⓚ
8-126. A3U16 provides the necessary power to output the reference frequency signal to the system
electronics. A3L3 limits the current spikes in A3U16. The special grounding and decoupling of
the line drivers and receivers prevent crosstalk and oscillations.
8-127. Power-On Reset (POR) Ⓖ
8-128. When power is turned on, or REF ON drops low during the locked state of the optical mode, the
power-on reset circuit sets the state machine in preheat state.
8-129. Laser Head Power Ⓒ
8-130. +15V and -15V are supplied by the system power supplies via the rear panel POWER connector
A1J5. The +5V is generated by A3U7. A3CR2 protects A3U7. A3C5 stabilizes A3U7 and A3C4
filters A3U7 output. Output +15V is the supply after being filtered by A3L2 and A3C11. A3L1 and
A3C12 filter -15V. A3 capacitors C2, C3, C6, C16, C17, and C21 provide local decoupling of the
supplies.

8-131. A4 Sampler Board

8-132. The Sampler board connects to Controller/Reference board A3. The only active component on
A4 is A4CR1, a photodiode that provides a signal to U11D (Pin 13) on A3. Pads on the underside
of A4 connect to the liquid crystal via elastomeric connectors which project from the A6 Sampler
Assembly. Liquid crystal operation is discussed in the A3 theory section.
8-36
NOTE
A TILDE ("~") PRECEDING ALL SIGNALS INDICATES
NEGATIVE-TRUE LOGIC.
HP5501B
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