The Sampling Oscilloscope Part 2

Loop Gain and Smoothing

To ensure smooth operation, the loop gain of the signal path through the transformer, the ac amplifier, the memory, and the feedback attenuator must be unity. When the loop gain is less than unity, more samples are required to ensure a correct reproduction of the input waveform. For example, with only a few samples per centimeter, the shoulder of a displayed square-wave signal is likely to be rounded.

However, this allows the signal-to-noise ratio to be improved. When a signal has a noise content, and when a reduction in loop gain makes more samples necessary to reproduce the input waveform, the noise will be averaged out at the memory capacitor. The reduction in loop gain is easy to obtain. By closing the electronic switch, the feedback capacitance of the integrator is increased, requiring more samples to reach the same output voltage. Therefore, a smoothed picture is the result. It must be kept in mind that the horizontal steps of the successive samples on the screen must be set to a maximum per division. With the Philips PM 3400 oscilloscope, it is possible to have over 1000 samples per centimeter. In this case, an integration over 30 samples gives a horizontal deflection of 0.3 mm on the screen, which equals one spot diameter. To prevent influencing the loop gain at the various settings of the input attenuator (part of the ac amplifier), the feedback attenuator is mechanically coupled to the former.



Time-Base Controls and Their Function

The time-base controls of the PM 3400 oscilloscope can be explained further. Part of the vertical input signal is taken off by the trigger take-off transformer and supplied to the trigger circuit. This circuit can also accept trigger signals from an external source (EXT TRIG IN). The pulses from the trigger start a ramp generator. The slope of the ramp generator is determined by the position of the TIME/DIV switch. In the comparator stage, this ramp voltage is compared to a voltage derived from the staircase generator. When the ramp voltage equals the staircase voltage, the comparator supplies a strobe pulse which is fed to the sampling pulse generator and to the pump circuit. Here it must be noted that the maximum voltage reached by the staircase generator remains constant and thus independent of any setting of the controls. This maximum voltage is matched to fit the screen width.

The number of steps over the screen is controlled by controlling the height of each step of the pump generator; in this way, the number of samples per division are adjusted. The output of the staircase generator is fed to the comparator via an attenuator and a variable-voltage source. A time-scale magnifier is controlled by the attenuator. If the staircase voltage is attenuated before it is compared with the ramp, the ramp voltage reaches this lower staircase level more quickly and a sample is taken earlier. But, since the staircase generator still gives the same step, the next step (= sample) on the screen represents a shorter time also. It will be clear that by adding a dc voltage in series with one input of the comparator, a shift in horizontal direction or time position will result on the screen. At the end of every sweep, the staircase generator is reset (internally).

Simultaneously, a blanking pulse is fed to the blanking mixer and in turn to the CRT. Thus, blanking between each sweep is accomplished. The pump circuit additionally supplies pulses to the blanking mixer. These pulses are used for blanking during the staircase transients. Other horizontal modes, such as single scan, x EXT, and manual scan, can be obtained also by means of the HORIZONTAL MODE switch.


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