Continuously running equipment is often encountered in which signals are produced in synchronism with its operation. For example, demodulation signals are often needed for modulating systems, especially for multiple-beam spectral photometers, regardless of whether the modulating systems are driven by synchronous motors, as they generally have been (e.g., DD-PS No. 65 468) or by stepping motors (e.g., U.S. Pat. No. 4,386,852, DD-PS No. 228 058). In U.S. Pat. No. 4,386,852, a higher-frequency timing signal is disclosed for the purpose of producing therefrom, by means of a divider circuit, the control frequency of the stepping drive, and also for the purpose of forming a demodulation signal by means of an additional divider synchronized by the optical signal by means of a null detector.
This system, which is usable only for single-beam apparatus, also has the important disadvantage that, in the case of higher absorptions of the optical sample, the demodulation is impaired by noise.
In DD-PS 228 058, the pulses of the stepping drive control frequency are fed to the stepping drive, and are also carried through an adjustable delay circuit followed by pulse shortening. They control a counter logic such that the demodulation signals, which serve for the analog processing of the measurement, will be in phase with the electrical analog signals formed from the optical signals.
An optocoupler cooperating with the modulating mirror supplies for this purpose a synchronization signal which at the beginning of the modulation period resets the counter logic. This eliminates the disadvantages of U.S. Pat. No. 4,386,852, but since adjustment can be made with the adjustable delay circuit only within one step, it is necessary to pre-adjust the modulating mirror and optocoupler with a stepping motor energized in a defined manner, or to assemble them together in an apparatus with sufficient accuracy.
In addition to the complexity involved in the construction of the apparatus, the necessary pre-adjustment interferes with service to the modulating system including the optocoupler.
Also, in the case of aging phenomena, phasing errors occur between optical signals and analysis signals as the load torque changes at the stepping motor, and necessitate manual readjustment.
It has been proposed (GDR Pat. No. 242,089), for synchronization with respect to a given required-phasing signal, to make the stepping motor run one step per modulation cycle slower or faster by means of control frequencies differing from the synchronous frequency, until the required-phasing signal and sensor signal are sufficiently in agreement. Here the same disadvantages are encountered as in DD-PS No. 228 058.
It is furthermore generally known to operate a stepping drive not by pulses of a control frequency, but for example to produce commutator signals by means of a computer, whereby the windings of the stepping motor are switched.