HEWLETT-PACKARD 5501B Repair Manual

HP5501B
Service

8-26. Laser Tuning
8-27. The laser assembly produces stable frequency components only when the laser assembly cavity
length is properly controlled. The cavity length is determined by the separation of two mirrors
that are held against the ends of a glass rod. The HP 55018 uses a thermal tuning technique to
control the cavity length. Cavity length increases when the rod is heated and decreases when it
cools.
8-28. A heater coil wrapped around the glass rod is used to increase the cavity length. Rod cooling by
conduction to surrounding material is used to decrease cavity length. To properly control the rod
temperature, the rate at which it cools should approximate the rate at which it is heated. The HP
55018 raises the cooling rate by maintaining the rod at an elevated temperature.
8-29. Warmup Mode Tuning
8-30. One of two feedback loops used to control the laser assembly establishes the initial operating
temperature of the glass rod. When power is first applied to the HP 55018, the state machine
(see Figure 8-3) resets to its warmup mode. In the warmup mode, the state machine controls the
subtracting sample and hold so that the feedback is as shown in Figure 8-4. The warmup error
amplifier produces an error signal that is proportional to the difference between the rod
temperature and a reference temperature. The warmup mode feedback loop controls the rod
temperature until it equals the reference temperature. The reference temperature is adjustable
and factory set to be higher than the hottest expected ambient temperature.

Figure 8-3. Flow Diagram for State Machine
8-7