Automatic focussing systems

Details Automatic focussing systems Automatic focussing systems

1977-08-01

1979-09-25

Blum, Theodore M.

Electric lamp and discharge devices: systems

Cathode ray tube circuits

Cathode-ray deflections circuits

250311, 250396R, H01J 2958

Patent

active

041692403

ABSTRACT:
An automatic focusing system is described for use with scanning systems having a poor signal to noise ratio video signal obtained by repetitive scanning. Typically the system is applicable to a scanning electron microscope.
A preliminary filtering of the video signal is performed using a filter which eliminates video signal content corresponding to the fundamental and low harmonics of the repetitive scanning.
Basic noise reduction is achieved using an N-path filter (typically N=8) commutated at a frequency equal to N.times.line repetition frequency of the electron beam scanning. The output signal is rectified and averaged to give a d.c. level proportional to the high frequency content of the video signal clustered around harmonics of the line scan repetition rate. This d.c. level is maximized by making stepwise adjustments in the current flowing in the focusing coil of the electron beam scanner and comparing d.c. levels before and after adjustment.
Because of the attenuation of the N-path filter to signal content above 4.times.the commutation frequency, a low pass filter having a cut off frequency of about 4.times.the commutation frequency is incorporated, in the preliminary filtering. This effects a further reduction in the noise content in the signal which is supplied to the rectifying and averaging circuit.

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