Question:

How do I generate an Amplitude Modulated (AM) signal using the Agilent 33120A? 

Answer:
Amplitude modulation (AM) changes the amplitude of the carrier waveform as the amplitude of the modulating signal varies.

Getting Started:

Carrier Waveform
The original waveform whose amplitude parameter is modified.
Carrier Waveform Shape
The shape of the original signal. Can be sine (default), square, triangle, ramp, or arbitrary.
Carrier Waveform Frequency
The frequency of the original signal. The frequency range for the Agilent 33120A is from 10 mHz to the maximum specified frequency of the waveform with a default of 1 kHz.
Modulating Waveform
The signal that determines the changes to the original waveform (carrier). Can be an external or an internal-external signal. Default is internal-external.
Modulating Waveform Shape
Shape of the modulating signal. Can be sine (default), square, triangle, ramp, noise, or arbitrary.
Modulating Waveform Frequency
The frequency of the modulating waveform. This will determine how long it will take for the carrier to repeat its pattern. Can be set from 10 mHz to 20 kHz, with a default of 100 Hz.
Modulation Depth
The extent of the amplitude variation expressed as a percentage. At 0% modulation, the output amplitude is half of the set amplitude. At 100% modulation, the output amplitude equals the selected value. Modulation depth can be set from 0% to 120% modulation.
Front Panel Example We wish to output a 5 Vpp, 5 kHz sine wave amplitude modulated signal. Our modulating waveform will be a triangle wave with a frequency of 200 Hz. Our modulation depth will be 80%.
  1. Select the function, frequency, and amplitude of the carrier. Press the [SINE] key on the front panel to select the sine wave (this is actually the default). Next, adjust the frequency to 5 kHz using the arrow keys, the knob, or the enter number methods. Adjust the amplitude to 5 Vpp in the same manner.
  2. Turn on AM. [SHIFT] [AM] Note that the AM annunciator turns on. At this point, the carrier is frequency modulated with the default AM parameters (100 Hz sinewave with 100% modulation depth).
  3. Select the function, frequency, and modulation depth of the modulating signal.
    1. Function Press the [SHIFT] [RECALL MENU] to go directly to the AM SHAPE command in the menu. Press the down arrow, and use the left or right arrows until TRIANGLE appears on the display. Press [ENTER] to change the shape of the modulating signal and exit the menu.
    2. Frequency Set the modulating frequency by pressing [SHIFT] [FREQ] . Note that the AM annunciator flashes, indicating that the displayed frequency is for AM. Adjust the frequency to 200 Hz using the arrow keys, the knob, or the enter number methods. Press the [ENTER] key to return to the normal display.
    3. Modulation Depth Set the modulation depth by pressing [SHIFT] [LEVEL] and entering the number as before to obtain 80% modulation.
The modulated signal is now ready to view on the oscilloscope. When the modulating waveform (the triangle) is at its peak value, the amplitude of the carrier will be at 2.25 V peak. Similarly, when the modulated signal is at its lowest value, the carrier will have an amplitude of 0.25 V peak. Programming Example Below is a sample program in Agilent Basic for the front-panel example. The example set up a 5 Vpp, 5 kHz sine wave amplitude modulated signal. The modulating waveform is a triangle with a frequency of 200 Hz and a modulation depth of 80%. 
     10 !

     20 CLEAR 7                       ! clear bus interface

     30 ASSIGN @Gen TO 710            ! assign I/O path to default address

     40 OUTPUT @Gen;"*RST"            ! reset the function generator

     50 !

     60 OUTPUT @Gen;"FUNC:SHAP SIN"   ! carrier waveform shape

     70 OUTPUT @Gen;"FREQ 5000"       ! carrier frequency is 5 kHz

     80 OUTPUT @Gen;"VOLT 5"          ! carrier amplitude is 5 Vpp

     90 OUTPUT @Gen;"AM:INT:FUNC TRI" ! modulating waveform shape

    100 OUTPUT @Gen;"AM:INT:FREQ 200" ! modulating frequency is 200 Hz

    110 OUTPUT @Gen;"AM:DEPT 80"      ! modulation depth is 80%

    120 OUTPUT @Gen;"AM:STAT ON"      ! enable AM 

    130 !

    140 END
Theory An amplitude modulated signal is represented by the equation v(t) = Ac[1 + am(t)]cos(2*pi*fct) where  
Ac = the signal amplitude
a = modulation index (0=<a=<1)
m(t) = normalized modulating signal (maximum value is 1)
fc = carrier frequency

As shown in the figure, the modulating frequency shows up as the envelope of the carrier. The modulation percentage determines the maximum and minimum of the envelope, Vmax and Vmin. The maximum of the envelope occurs when the modulating signal is at its most positive value, +1, and the minimum occurs when the modulating signal is -1. Vmax = Ac (1+a) Vmin = Ac (1-a)

Ac is equal to one-half the signal amplitude set on the Agilent 33120A. This is because the Agilent 33120A automatically reduces its peak-to-peak amplitude by one-half when amplitude modulation is selected so that a 100% modulation depth signal can be output.

Solving for the modulation index,

a = Vmax-Vmin