Transcript Magnitude and Phase Measurements

Magnitude and Phase
Measurements
Analog Discovery
Set up the oscilloscope to measure the signal of the reference voltage (the input voltage
from the arbitrary function generator, in this case) and the output signal from the
operational amplifier.
Set the Source of the trigger signal , located on the toolbar directly above the graph of the
signals, to be equal to the channel of your reference signal, which is Channel 1 in this
example). The Condition can be either Falling or Rising and the Level can remain at 0 V.
Set the time division for the two channels to a value that is
reasonably small to maximize the measurement accuracy. In this
example, the Base selected is 200 us/div as the signals have a
frequency of 1 kHz. Slightly less than two periods of the signals on
the two channels is displayed. A smaller Base could be selected.
Move one of the X cursors on
to the plot by click-anddragging on the X in the lower
left corner of the graph. Since
the voltage of the reference
signal should be 0 V at time is
equal to 0 s, as set by the
trigger function, you only have
to measure the time when the
second signal crosses the xaxis (i.e., has a value of 0 V).
Note that this may be
impossible to position the
cursor to exactly the point
where the value is equal to 0 V
due to digitizing errors. This is
one reason why you should
select the smallest possible
Base when performing this
measurement.
Record the time displayed with the cursor.
Make sure that you have positioned your cursor to the
point that is closest to time equal 0 s. Time = 0 ms is in
the middle of the graph in this example.
• Before you can determine the phase angle of the signal measured on
Channel 2, you must also determine the period of the two signals. The
period of both signals must be the same when calculating phase angles
and when determining which signal leads or lags the other.
– The period can be measured by moving the cursor to an appropriate position
on the sine wave on Channel 1 (e.g., first time when the signal of Channel 1 is
its maximum value after time = 0 s is equal to ¼ of T, the period of the signal).
– Or, you can use the Measurement option on the upper toolbar of the scope
program to determine the period of each signal. The steps to perform to use
the Measurement option are described on Slides 19-20 in
Intro_AnalogDiscovery_Scope.pdf, which was posted in Module 7.
• The phase angle of the signal on Channel 2 using the signal on Channel 1
as the reference can then be calculated using the technique described in
Phase Delay.pdf, which is posted in this week’s module (-61.2o in this
example).
• The amplitude of the two signals may be measured using the Y cursors.
Or, you can use the Measurement option on the upper toolbar of the
scope program.
Measurements vs. Frequency
It is often more interesting to measure the amplitude of one signal and the phase
angle of that signal with respect to another over a range of frequencies. A plot of the
amplitude and phase angle as a function of frequency is known as a Bode plot.
To set up the oscilloscope to make a Bode plot, go back to the main window of the
program. Select Bode Transfer Function from the pull-down menu at the lower right
of the window.
Channel 1 is assumed to be the reference signal in the Analog Discovery Bode plot.
The magnitude of Channel 1 is referenced to the magnitude of the output of the
arbitrary waveform generator, which you specify on the upper toolbar of the Bode
Transfer Function GUI. The magnitude of the signal on Channel 2 is referenced to the
magnitude of Channel 1. The magnitudes are plotted in dB as a function of frequency.
Select the Start and Stop frequencies for the measurement using either the dropdown menu or by typing in a value into the box or by using the sliders to the right.
The Steps and Max-Gain should be chosen to obtain a reasonably noise free plot in a
relatively short period of time.
A single set of measurements (Single) or repeating set of measurements (Run) can be
made. The option is located in the upper left side of the toolbar.
The cursor on the Bode plot is activated by right clicking in the
graph and selecting Hot Track.
A black line will appear of the graph, which you can drag to the
frequency at which you want to make a measurement. Labels
on the cursor will display the frequency at which the amplitude
and phase measurements are shown.
Or, you can click on File/Export. A pop-up window will open that contains a list of the
data measured at each of the frequency steps used to form the Bode plot. Note that
the number of data points that will be collected is set by the value of Steps in the
upper toolbar of the Bode Transfer Function. You can either find the values of interest
from the list displayed or export the list as well as the image of the Bode plot.