Oscilloscopes for Radio Amateurs by Paul Danzer (N1II)

Author:Paul Danzer (N1II) [Danzer (N1II), Paul]
Language: eng
Format: mobi
Publisher: ARRL, the national association for Amateur Radio
Published: 2015-04-14T07:00:00+00:00

Chapter 7

Let’s Put a Scope to Work

Oscilloscopes are generally used for one of three types of measurements: amplitude (voltage), duration or time (frequency), and calculations. Most older scopes did not have the ability to do calculations, but as discussed in preceding chapters, as long as you have information in digital form and are using a digital processor (in other words a small computer), the oscilloscope can do a variety of calculations.

Amplitude Measurements

When most people think of using an oscilloscope, they envision a screen with a plot of the amplitude or voltage of a signal on the vertical axis of a screen, along with time on the horizontal axis. To make this primary plot on the display, there are several steps you have to take. It is very possible to overlook one of these steps, in which case the displayed waveform may mislead you and not represent the true picture of the signal.

First pick a voltage scale, and if there is a variable scale control, make sure the voltage scale selection switch is in the calibrated position. Looking at Figure 7.1 you can see scale selection switches for channel 1 and channel 2. Notice that the center knob (labeled VAR) for channel 1 is in the full clockwise position, corresponding to the calibration position on the panel. The VAR knob for channel 2 is not in the same position, so as a result the voltage scale selected for channel 1 is correct but the scale selected for channel 2 is incorrect. It is very easy and very common to overlook checking this setting.

Check your probe compensation. Compensation, as noted in a prior chapter, only holds for the X10 setting on the probe switch. If you are looking primarily at sine waves and RF, it is doubtful that (except for extreme maladjustment of the compensation screw) this adjustment will make much of a difference. Digital signals are a different story — just look at Figure 7.2! With compensation maladjustment, square waves, pulses, sawtooth and other linear waveforms may be shown far from their true waveshape.

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