HP 5501A Operating And Service Manual page 22

Model 5501A
Theory of Operation
3-28. The polarized beam splitter allows a small portion of the f, component to be mixed with the f,
photodiode input. A small amount of f, signal is also combined with the f, photodiode sample. As a result,
each photodiode detects a difference frequency signal (approximately 2 MHz). One photodiode output
signal, designated comp-f, consists of a dominant f, signal, and a small amount off,. As a result the amplitude
of the comp-f, signal is a function of the f, component of the beam sample. The comp-f, amplitude is
determined by the f, level of the input beam sample. Comp-f, is applied to the negative difference
integrator input, while comp-f, is applied to the positive input. The difference integrator compares these
inputs and provides a resultant negative dc output signal. The magnitude of this signal depends on the
relative amplitude of the comp-f, and comp-f, signals. Typical levels range from -3 Vdc to -7 Vdc.
3-29. The FREQ potentiometer, A5R4, i s an offset adjustment that provides the appropriate difference
integrator output when equal comp-f, and comp-f, signals are applied. This adjustment compensatesforthe
fact that the two photodiodes are not perfectly matched.
3-30. A RETUNE CLAMP input signal from the A7 Control Assemby activates the clamp switch circuits; this
results in a -6 volts difference integrator output. This RETUNE CLAMP signal is provided when the manual
RETUNE button is pressed or when the RETUNE CMD signal is received. The clamp signal lasts 3 seconds,
after which time the automatic circuits resume control of the difference integrator output.
3-31. The dc difference integrator control signal is applied through an emitter follower to the PZT Power
Supply. The PZT Power Supply operates in a manner similar to the High Voltage Power Supply and consistsof
an oscillator and one voltage doubler. This power supply responds to control input by providing a dcoutput
that varies from 1 to 2 kV. This output directly controls the laser tube PZT,and ultimately causes equalization
of the laser beam f, and f, frequency components.
3-32. To demonstrate PZT control loop operation, consider an f, (center frequency) drift towards a lower
frequency (f,). The resulting increase in f, signal level is sensed by the Lock Reference Assembly circuits.
These circuits respond by applying a more negative PZT control signal to the PZT Power Supply, causing a
PZT voltage decrease. The tuned laser tube frequency is inversely proportional to PZT control voltage input.
A decreasing PZT voltage therefore tunes the laser tube towards a higher frequency. As a result, the f,
amplitude decreases and the f, component becomes equal to the f, component signal level.
3-33. The Lock Reference Assembly also performs the function of providing the system with a reference
signal. The comp-f, signal is applied to an over-driven R F amplifier. The resulting output is applied to a
differential line driver and a detector circuit. The detector provides a dc signal which is proportional to the
RF comp-f, signal strength. When this dc output signal exceeds +0.3volt, a sufficient signal requirement is
satisfied. As a result, the threshold detector output goes high enabling the differential driver which
produces a true R E F OK signal. The enabled driver provides complementary (REF and REF) reference signals
to the transducer accessory modules.
3-34. Control and Diagnostic Monitoring
3-35. The A6 Control Assembly generates diagnostic signals and provides timing and control signals forthe
retune function.
3-36. RETUNE FUNCTION (refer to Figure 3-1 and 7-4). The retune function is initiated when the RETUNE
pushbutton, S1, is depressed or an external RETUNE CMD signal
i s
received from a transducer controller.
The high-to-low transition of this signal clears the tune fault latch and sets the tune latch on the Control
Board. The tune latch then provides a low RETUNE output signal that activates the error gate, resulting in
a
true (high) ERROR diagnostic output signal. The logic high RETUNE signal, (also generated by the tune latch)
provides drive that lights the RETUNE LED indicator. The ERROR diagnostic signal stays high and the RETUNE
indicator remain lit during the complete retune cycle. If the laser head successfully retunes, the ERROR
signal goes low and the RETUNE indicator goes out.
3-37. At time t, (the positive transition of the RETUNE CMD signal) the retune timing circuits generatea 3-
second RETUNE CLAMP signal which i s applied to the Lock Reference Assembly. The Lock Reference
Assembly tuning circuits respond to this input by ultimately driving the PZT-controlled laser frequency
towards center (f,). Upon termination of the RETUNE CLAMP signal (3 seconds after t,), the automatic PZT
tune circuits resume control of the laser tuning. Successful retuning is accomplished when the comp-f, input
signal to the Lock Reference Assembly provides proportional dc drive that exceeds a +0.3 Vdc threshold
level. When his condition is satisfied, a low R E F OK signal is generated. This signal is applied to the Control