Frequency modulation (FM) is the encoding of information in a carrier waveform by varying the instantaneous frequency of the carrier while the amplitude of the carrier wave remains constant. In analog applications such as signal processing or telecommunications, the difference between the instantaneous and the base frequency of the carrier is directly proportional to the instantaneous amplitude value of the modulating signal.
A convenient means of generating a frequency-modulated signal is with an instrument such as Teledyne LeCroy’s WaveStation 3000 waveform generator. In this Application Brief, we will show how to manipulate three key attributes of a frequency-modulated signal: deviation, frequency, and shape. We will also cover use of Spectrum Analysis software, measurements, and trends with Teledyne LeCroy’s HDO6000 oscilloscopes.
Connect the Channel 1 output of a WaveStation 3000 waveform generator to an input channel of a suitable oscilloscope and apply power to both instruments.
For this Application Brief, the WaveStation 3000 is set to produce a 5-MHz sine wave with an amplitude of 150 mV pk-pk. Any output waveform can be frequency modulated, however. Pressing the Mod (Modulation) button on the WaveStation’s front panel activates the Modulation menus. Press the Type button and then press the FM button. Press the Shape button and set the shape of the modulation signal to a sine wave. Next, press the FM Freq button to adjust the amount of modulation and set it to 200 mHz. Finally, press the FM Dev button and adjust the depth of FM deviation to 2 MHz. The screen of the WaveStation 3000 appears in Figure 1.
One may easily change the shape of the frequency modulation on the WaveStation. Referring back to Figure 1, press the Shape button below the display (Figure 2).
From here, the user may change the waveshape of the modulation any of the types shown in Figure 2. Pressing the More button opens a second page of options, which include a noise waveform and arbitrary types. The latter in turn provides choices between the instrument’s array of built-in arbitrary waveforms and any previously stored arbitrary waveforms.
As an example of how the modulation shape may change, select square wave as the modulation shape (Figure 3). Note that the overlaid yellow waveform, which denotes the shape of the modulating signal, is now a square wave instead of a sine wave.
Change the modulation shape back to a sine wave to return to the previous setup.
To more effectively present the effects of changes in FM modulation parameters when viewing waveforms on a Teledyne LeCroy HDO6000 oscilloscope, use the instrument’s Measurement functions. To do so, tap the Measure drop-down menu at the top of the oscilloscope’s screen and then tap Measure Setup. In the main dialog, check the Show Table box at far left and the Statistics On and Histicons button at far right.
Tap the P1 tab in the Measure Setup dialog and set Source1 to Channel 1 (C1). Then touch the Measure button and select Amplitude from the Select Measurement pop-up. Similarly, configure the P2 tab to measure frequency of C1. Then, under Actions for P2, select the Trend button and assign it to Math trace F1. The result will be as shown in Figure 4. At top is Channel 1 with the frequency-modulated sine wave. Below that is Math trace F1, which plots the trend over time in the frequency measurements taken from C1. Note that the frequency trend follows the repetitive contraction and expansion in the frequency of the frequency-modulated sine wave.
Further, it is interesting to note that the trend plot takes the shape of whatever the modulating signal happens to be. In the case of Figure 4, the modulating wave is a sine wave, so the trend plot displays a sine-like pattern. Were the modulating signal changed to a square wave, the trend plot would take on more of a square shape. This demonstrates the correlation between the modulating wave and the trend plot.
Next, change the frequency of the modulation by pressing the FM Freq button. The default value is 100 Hz; our example uses an FM frequency of 200 mHz. Use either the knob or keypad to enter a value of 1 Hz for the frequency of the modulation signal (Figure 5). Note that the trend plot shows the frequency changing much more often when compared to Figure 4.
Reset the FM frequency value to 200 mHz. Now press the FM Dev (deviation) button to observe the effect of a change in that value. The FM deviation is the maximum instantaneous difference between the FM modulation frequency and the nominal carrier frequency, which in this case is the 5-MHz sine wave. For this demonstration, FM deviation is set to 2 MHz. Change the value to 3 MHz with either the knob or keypad. The resulting waveform appears in Figure 6. Note that in Figure 5, with an FM deviation value of 2 MHz, minimum and maximum frequencies were about 3 MHz and 7 MHz, respectively. In Figure 6 with the higher FM deviation value of 3 MHz, those values are now 1 MHz and 9 MHz. This can also be seen from the deeper trough in the frequency trend plot (F1).
Oscilloscopes such as Teledyne LeCroy’s HDO4000 and HDO6000 series offer spectrum analysis software for examining signals in the frequency domain, the former as an option and the latter as standard. This can be especially useful when working with frequency modulated signals (Figure 7).
In the screen capture above, the spectrum view (Sp) of the source signal (C1) indicates the frequencies with peak amplitudes, which are also shown in tabular format at top left. Above C1 is the 2D Spectrogram view of the spectrum. A live view of this setup would show the peak frequency traveling from left to right and back again as the modulation changes. With Peaks turned on in the Peaks/Markers tab of the Spectrum Analyzer dialog, note an instantaneous peak at 3.17 MHz and its first harmonic at 6.3 MHz.
A full complement of measurements may be taken on the frequency-modulated input signal, including such common parameters as amplitude, frequency, rise/fall times, and many others. Parameters that are more complex may be measured as well, such as period@level, or the duration of each full cycle at a specified slope and level.
Because it can generate a practically infinite variety of FM waveforms, the WaveStation 3000 is a highly capable and versatile means of generating frequency-modulated signals for design and debugging applications. When coupled with an HDO oscilloscope, the WaveStation 3000 provides a complete solution for FM-signal generation, measurement, and frequency-domain analysis.