Operating instructions

University of Saskatchewan
Electrical Engineering Laboratory Equipment Manual
Rectangular
Pulses or transients
Special purpose window for waveforms that do not
have discontinuities (same as no window)
The FFT zoom level (X1 / X2 / X5 / X10) changes the horizontal magnification of the
FFT display.
The FFT display is calibrated in dBV (decibels referenced to one volt rms). The level of
a signal in dBV is given by the following formula:
Signal Level (dBV) = 20 log (V
rms
/1 V)
Where V
rms
is the rms voltage of the signal.
The oscilloscope display changes slightly when the FFT function is active. At the top of
the screen is a readout that displays the frequency corresponding to the middle graticule. The
upside-down arrow at the top of the screen indicates the position of this frequency. The
horizontal position knob moves the display left or right, and the corresponding middle graticule
frequency at the top of the screen will change accordingly. At the bottom of the screen near the
middle is the frequency/div setting of the display. Next to that, in parenthesis, is the current
sampling rate used in computing the FFT.
At the bottom left of the screen is the vertical scale in dB/div. The volts/div knob of the
source channel for the FFT now controls the vertical dB/div setting. The choices are 1, 2, 5, 10
and 20 dB/div. The vertical position knob of the source channel for the FFT controls the vertical
position of the display.
The cursors may be used to determine signal levels and frequencies of the FFT spectrum.
Ensure that the source for the cursors is set to MATH. Two types are then available: magnitude
and frequency. As with the voltage and time cursors, the positions of the two cursors are
displayed (in dB and Hz), as well as the difference between the two. Again, as with the voltage
and time cursors, their positions are controlled by the two vertical position knobs for each
channel. Don’t be fooled by the readout for the magnitude cursors: the scope tells you the
position of each cursor in dB, but it’s really dBV, so keep that in mind. To convert signal levels
in dBV to rms volts,
Signal Level (V
rms
) = 10
(Signal level (dBV)/20)