RF Systems Lab

Lab 1: Test and Measurement

1.0 Oscilloscope Basics

Section 1.0 guides the student through set-up and basic functions of the oscilloscope including voltage scale, time scale, and triggering. The student is asked to adjust the voltage and time scales to appropriate values without using the autoscale function. To this, the time scale is set to approximately half the anticipated period of the signal being displayed, and the voltage scale is set to approximately half of the anticipated amplitude. The trigger level is set to produce a clear waveform that is static and unflinching using the trigger level knob.

1.1 Simple Sine Waves

Next, the student becomes familiar with the function generator by producing a sine wave with peak-to-peak voltage equal to 2V and no offset. The frequency is set to 10kHz and the second channel on the oscilloscope is connected to the second channel on the function generator via a BNC-to-BNC cable. On the oscilloscope, the time scale is set to 40us/div and the voltage scale is adjusted to ~1V/div. Figure 1 below shows the waveform on the o-scope with these settings.

The cursors are then added to the o-scope view, and the vertical and horizontal cursor functions are explored. Figure 2 and Figure 3 below show the cursors on the waveform from above. Additionally, in Figure 3, the frequency measurement is added to the menu output at the bottom of the scree.

Pop Quiz!

Given a 10kHz signal, what would be a good setting for the timescale? 25-100us

Finally, the frequency generator is disconnected from the o-scope and the digital multimeter is used to probe the output of the function generator, which reads at about 1.69V. This value doesn't make sense as an RMS value or amplitude, so it likely has a large error from the DMM attempting to read AC.

1.2 Amplitude Modulation

AM Radio signals are amplitude-modulated, meaning that the intelligence signal is "carried" by the carrier wave. The Arbitrary Function Generator is used to create an amplitude modulated signal, which is then viewed on the o-scope as shown in Figure 3.

Pop Quiz!

Since you want to see the intelligence signal of the modultaed waveform, what would be a good setting for the timescale of a signal with a 1 kHz intelligence frequency and a 1 MHz carrier frequency? .25-1ms

The Spectrum Analyzer is then utilized to view the spectrum of the AM waveform. The carrier frequency is set to 1230kHz with an amplitude of 100mVpp and a 1kHz intelligence signal. The display is configured to show 1.23MHz as the frequency of interest on the graph. The waveform spectrum in Figure 4 is produced and can then be used to find the power levels in Tables 1 and 2. Another use of the Spectrum Analyzer would be in applications where it is necessary to measure circuit response to a variety of frequency inputs.

1.3 Fast Fourier Transform (FFT) Method

The final segment of the laboratory experiment involves employing an alternative method for frequency analysis known as the Fast Fourier Transform (FFT). This analysis utilized the standard oscilloscope channels, so the function generator was reconnected to the first channel of the oscilloscope. The function generator settings were readjusted to generate a 50 kHz sine wave with an identical 10 mV amplitude. The modulation depth was also restored to 50%. To facilitate the FFT computation, the oscilloscope featured a new button labeled 'Math.' Within the 'Math' menu, students selected the FFT option and modified the horizontal scale to 12.5 kHz, resulting in the subsequent output shown in Figure 5. The power levels in Tables 3 and 4 are generated from this output.