Frequency range of a pen-writing oscillograph is limited to be from DC to about 80Hz because of the response of the galvanometer. If the high frequency signal is known to be periodic, it can be sampled and its frequency can be converted to a low frequency preserving its waveform. Thus the input signal can be recorded by a pen-writing oscillograph. The input waveform is necessary to be continuously sampled for a long time in the pen-writing oscillograph. In a sampling oscilloscope, however, the sampling operation is not continuous, because each sampling pulse is taken at a time progressively delayed with respect to some reference point in the input waveform and the delay time is not infinite.
In this paper, a new sampling technique for a pen-writing oscillograph is discussed. In this technique, sampling pulses are periodically taken. The frequency of the output signal after sampling is equal to the difference between P·F_s and the frequency of the input signal. F_s is the sampling frequency and P is a positive integer. The input waveform can be continuously sampled for a long time. However, the frequency drift of the output signal increases as the frequency of the output becomes lower.
In this paper, this new sampling technique is applied to a chopper amplifier and a frequency control system is contrived in order to compensate the frequency drift.
By the pen-writing oscillograph with this chopper amplifier, it is possible to record from DC to periodic signals of high frequency.