Agilent Technologies Appendix C, Agilent E1441A Function Generator Tutorial, For wideband FM

In amplitude modulation, the amplitude of the carrier varies between zero and twice its normal value for 100% modulation. The percent modulation depth is the ratio of the peak information signal amplitude to the constant. When amplitude modulation is selected, the Agilent E1441A automatically reduces its peak-to-peak amplitude by one-half so that a 100% modulation depth signal can be output. Amplitude settings are defined to set the 100% peak-to-peak amplitude independent of the modulation depth setting. Vrms and dBm amplitude settings are not accurate in AM since signals are very complex.

Frequency Modulation (FM) Frequency Modulation is a process of producing a wave whose frequency varies as a function of the instantaneous amplitude of the modulating information signal. The extent of carrier frequency change is called deviation. The frequency deviations are caused by the amplitude changes of the modulating information signal. You can set the amount of the peak frequency in FM with the deviation parameter.

In frequency modulation, “100% modulation” has a different meaning than in AM. Modulation of 100% in FM indicates a variation of the carrier by the amount of the full permissible deviation. Since the modulating signal only varies frequency, the amplitude of the signal remains constant regardless of the modulation. The function generator uses the deviation parameter to describe the peak frequency change above or below the carrier in response to a corresponding amplitude peak of the modulating signal. For FM signals, the bandwidth of the modulated signal can be approximated by:

BW ≈	2 x (Deviation + Information Signal Bandwidth)	For wideband FM
BW ≈	2 x (Information Signal Bandwidth)	For narrowband FM

Narrowband FM occurs when the ratio of the deviation frequency to the information signal bandwidth is approximately 0.01 or less. Wideband commercial FM radio stations in the United States use a 75 kHz peak deviation (150 kHz peak-to-peak) and audio signals band-limited to 15 kHz to achieve 200 kHz channel-to-channel spacing from the 180 kHz bandwidth.

Frequency Sweep The Agilent E1441A performs phase-continuous frequency sweeping — stepping from the sweep start frequency to the sweep stop frequency with between 2,048 and 4,096 discrete frequency steps. The direction of frequency sweeps can be varied by setting the stop frequency either above or below the start frequency. Individual frequency steps are either linearly or logarithmically spaced based on the sweep mode setting. Like FSK modulation (described on the next page), the sweep function is also a special case of frequency modulation (FM). All of the FM operations described on the previous page also apply to sweep when the following translations are applied:

Start Frequency + Stop Frequency

Carrier Frequency = --------------------------------------------------------------------------------------------

2

Start Frequency – Stop Frequency

Deviation = ------------------------------------------------------------------------------------

2

The modulation waveshape for sweeps is a ramp wave or exponential wave for linear or log sweeps, respectively. The logic sense of the ramp or exponential modulation signal (positive or negative ramp) is selected when the stop frequency is either larger or smaller than the start frequency. Like the FM function, changes to sweep parameters cause the generator to automatically compute a modulation signal and download it into modulation RAM. Similarly, the sweep time parameter adjusts the period of the modulating waveform. The sweep function also allows triggered operation. This is like frequency modulating with a single cycle burst of the modulating signal beginning when a trigger is received. Trigger signals can come

Appendix C Agilent E1441A Function Generator Tutorial 159