Agilent Technologies 5517B Operation And Service Manual page 89

8-116. The first counter (A3U6) in the divider section controls the liquid crystal and the sample and hold circuits. A
sample and hold is performed once per cycle of T1.28 alternating between the vertical and horizontal components
Figure 8-8
[see (Optical Mode Timing Diagram)]. Thus each component is sampled once every 2.56 seconds. The
liquid crystal is allowed to stabilize for 0.64 seconds before the switch to the sample and hold is closed. The hold
cycle begins when the switch opens at the end of a cycle of T1.28 and at the end of a half cycle of PASSIVE. The
present half cycle of PASSIVE/-ACTIVE determines which component is sampled. Op-amp A3U11A computes the
difference between the two sample and holds.
8-117. Power Amplifier
8-11 8. The output of A3U11A drives the power amplifier which in turn drives the laser tube heater. The frequency
response of the power amplifier is determined by the input filter (A3R31, R27 and C18) and the non-linear feedback
for op-amp A3U13 (A3R32, R33, R39, R40, R41, C20, CR3 and CR4).
8-119. A3Q5 provides level shifting to drive the base of the Darlington power transistor A3Q3. A3U19B disables the
output stage by grounding the base of A3Q5 when the heater error voltage is sampled during the warm-up mode.
8-120. Reference Receiver (L)
A TILDE ("~") PRECEDING ALL SIGNALS INDICATES NEGATIVE-TRUE LOGIC.
8-121. 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).
8-122. A3CR5 is reverse biased to reduce its capacitance. The +15V bias voltage is filtered by A3R46 and A3C27. A
polarizer, oriented at 45' 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.
8-123. 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 photo diode, 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 A3U1 7 Pin 2 to Vcc.
8-124. Line Driver (K)
8-125. A3U16 provides the neressary power to output the reference frequency signal to the system electronics. A3L3
limits the current spikes in A3U1(S. The special grounding and decoupling of the line drivers and receivers prevent
crosstalk and oscillations.
8-126. Power-On Reset (POR) (G)
8-127. 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.
8-128. Laser Head Power Supply Conditioning (C)
NOTE