Miscellaneous active electrical nonlinear devices – circuits – and – Specific signal discriminating without subsequent control – By pulse width or spacing
Reexamination Certificate
1999-09-01
2001-04-17
Lam, Tuan T. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific signal discriminating without subsequent control
By pulse width or spacing
C327S077000, C327S089000
Reexamination Certificate
active
06218870
ABSTRACT:
FIELD OF THE INVENTION
The invention generally relates to the field of precision measurements. More specifically, the invention relates to laser interferometer measurement systems which measure by detecting zero crossings of low intensity signals, while minimizing the effects of noise. The invention has particular applicability to the field of integrated circuit (IC) fabrication.
BACKGROUND OF THE INVENTION
A laser interferometer is a measuring tool that uses wavelengths of a periodic phenomenon, such as laser light, for accurately measuring distances. An interferometer includes one or more receivers for detecting characteristics of the light, such as the phase of the light waves. In particular, counting cycles of the light wave plays an important role in the measurement.
Noise can cause an error in the cycle count. Such an error is also referred to as a “Glitch”. Glitches are undesirable, because they reduce the accuracy of the measurement being taken. Therefore, it is an important objective of interferometer measurement system designers to eliminate or minimize glitches. This can be done by keeping the magnitude of the light high enough that the interferometer measurement is relatively insensitive to noise. However, other design considerations favor a reduced light magnitude.
A laser interferometer measurement system that reduces glitches would allow laser signals of smaller intensity to be used. Also, glitch reduction would result in more accurate and efficient positioning.
SUMMARY OF THE INVENTION
There is provided, in accordance with the invention, a circuit and method, for use with a laser interferometer measurement system, for receiving low intensity signals. The circuit and method employ hysteresis to reduce sensitivity to noise conditions which conventionally would result in glitches. Measurement error caused by glitches is advantageously reduced below that of conventional circuits and methods.
A circuit according to the invention includes inputs for receiving analog detection signals such as sinusoidal signals, and comparators for producing digital versions of the detection signals. The digital versions may bounce between high and low during transitional periods in which noise on the analog signal would cause glitches in the zero crossing detection
In accordance with the invention, the inputs of the comparators have hysteresis circuitry which causes the signal transitions to take place at a phase of the analog signals which is displaced from the phase at which the signal actually crosses zero.
High- and low-true versions of the digital detection signals are logically combined to produce a first signal which is active only while the input signal moves in one direction, and is masked from the active state while the input signal moves in the other direction. Likewise, a second signal is produced which is active the opposite way. Because of the phase displacement resulting from the hysteresis circuitry, the logical combination as able to achieve this masking.
The first and second signals respectively set and reset a latch. The output state of the latch represents the threshold crossing. Because of the logic masking of activity on the second signal due to noise on the input signal, once the first signal puts the latch in one state, the latch is not immediately put back to the other state by the second signal. Rather, even if the input signals bounce because of the noisy threshold crossing, the latch remains in the one state until the input signal changes direction, at which time the first signal is masked but the second signal is enabled. Accordingly, the latch masks out the bouncing, which would be detected as erroneous zero crossings. The result is that clean transitions of the first and second signals are produced.
There are further provided a circuit and method as above, but further including circuitry for disabling the hysteresis functionality for frequencies above a threshold, where the hysteresis is not needed.
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patent: 6091239 (2000-07-01), Vig et al.
Hewlett-Packard Company, “High Performance Laser Interferometer Positioning for VMEbus Systems”, Product Overview, Aug. 1996, pp. 1-11.
Hewlett-Packard Company, Operating Manual, “HP E 1707A Dynamic Receiver” “HP E 1708A Remote Dynamic Receiver”, Jul. 1997.
Agilent Technologie,s Inc.
Lam Tuan T.
Pintner James C.
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