Instruction manual

HB 08-18-2010 12
7 Analog versus Digital Scopes
In an analog scope the input voltage, after amplification or attenuation, is applied to the
vertical deflection plates of the scope. The scope trace is smooth. In a digital scope, such
as the SWS scope, the input signal is sampled at the “sampling rate” and the scope trace is
these values usually connected by straight lines. You may have noticed this fact in looking at
the SWS scope trace. One of the important specifications for a digital scope is its sampling
rate, which may be hundreds of kHz or higher. This number is the maximum sampling rate
and is the rate used at the highest sweep speeds. In reality, the actual sampling rate is lower
and is usually adjusted so that there are about a few hundred to a thousand samples per
sweep. If the sampling rate is not high enough severe distortion of the signal can result. To
get a reasonable portrayal of the signal the sampling rate should be at least ten times the
frequency of the signal.
In the SWS scope the sampling rate is adjusted automatically and is displayed at the
bottom of the scope near the Trigger button and sweep speed. For the SWS scope determine
the sampling rate for a sweep speed of 1 ms/div and then calculate the samples per sweep.
A div or division is a large division as indicated by extended lines on the graticule.
A digital scope is often a storage scope. The sampled voltages are stored and the traces
viewed after the input voltage has ceased to be sampled. In an analog scope the trace will
disappear once the signal is no longer applied to the scope.
8 Optional Exercises- NO Credit
If you fully understand the operation of the scope and have extra time, feel free to explore two
additional features of the Wavetek generator; sweep and modulation. Look at the waveforms
on the scope and adjust the controls so as to highlight the features of these waves. Although
you can use any waveform you want, it is suggested that you start with sine waves and with
setting of the TIME/DIV control of 1 ms.
• SWEEP In the sweep mode, the wavetek repetitively sweeps from a lower frequency to
a higher frequency. Set the frequency knob of the Wavetek at about 1 kHz and pull out
this knob to start the sweep function. The higher frequency is selected by this knob.
You will find that the two knobs labeled SWEEP TIME and MOD/DEPTH-SWEEP
RATE affect the waveform. Can you figure out exactly what these knobs do? Does it
make any difference whether these knobs are in or out? When you are finished, put the
frequency knob back in.
• MODULATION A “carrier (wave)” is a single frequency (sinusoidal) electromagnetic
wave. For radio broadcasting in the U.S. the carrier frequencies for AM stations lie
between 520 and 1610 kHz, and the carrier frequencies for FM stations lie between
87.5 and 108.0 MHz. Information, such as speech or music, has to be encoded on the
carrier wave. The process of encoding the information is called modulation. When
the amplitude of the carrier wave is varied it is called amplitude modulation or AM.
When the frequency of the carrier wave is varied it is called frequency modulation
or FM. Modulation at a single frequency corresponds to a pure tone. For example,
broadcasting “middle C” would require a sinusoidal modulation of about 261.6 Hz for
both AM and FM stations.