Applications for a Sampling Oscilloscope: Part 2

Measurements on Signals below the Noise Level

The use of a recorder for handling the output signals of the PM 3400 oscilloscope has the advantage of acting as a low-pass filter that effectively reduces noise and jitter. It has been found possible to measure signals that lie considerably below the noise level in this way. Recording signals on an X-Y recording via a sampling oscilloscope will give a considerable reduction of the noise because the X-Y recorder acts as an integrator. If, however, the noise signal contains a component which is coupled in frequency with the trigger signal, this part of the signal is recorded without attenuation.

As the repetition rate of the trigger signal determines the sampling frequency, the samples will follow low-frequency noise components. Such noise will thus give a residual signal on the recording.

An extra integrating network between the oscilloscope and the recorder can be used to give further attenuation of these If noise components, but the signal will then have to be scanned more slowly. Otherwise, the signal itself will be affected by the integrating network.



Double exposure. (Upper trace) A pulse signal with a signal-to-noise ratio of about 1. (Lower trace) The signal is attenuated 10 x (-20 dB) while the noise remained the same.


The Sampling Scope in the Development of Magnetic-Bubble Devices

Introduction Magnetic bubbles- cylindrical magnetic domains with a diameter of the order of 5 µm – in garnet films show promise for applications in such devices as computer memories and display and recording systems. Such bubbles are surrounded by a circular magnetic-domain wall. It is important to be able to measure the velocity of domain walls and bubbles in the development work on such devices, and the Philips sampling oscilloscope PM 3400 has been playing an essential role in such measurements.

The Experiment The position of the wall (set up for a measurement of the velocity of a magnetic domain wall) is made visible in a polarizing microscope by the Faraday effect. The displacement of the wall can be followed by measuring the intensity of the light transmitted in the microscope with a photomultiplier.  The photomultiplier output is amplified and passed to the sampling scope.

The signal-to-noise ratio can be improved by a factor of about 30, by using the sampling scope as a boxcar integrator. In this mode, the horizontal sweep is driven externally by a ramp voltage obtained from a battery and a motor-driven potentiometer (scanner).  The motor speed is adjusted so that a complete horizontal scan is completed in about 5 minutes. With this slow sweep, the improvement in the signal-to-noise ratio can be obtained simply by feeding the vertical output voltage of the oscilloscope to the Y input of an X-Y recorder via an RC network with a typical time constant of 1 s. This network then serves as a noise filter. The X output of the oscilloscope is connected directly to the X input of the recorder.


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