Agilent Technologies E1441A user service Deviation = Hop Frequency – Carrier Frequency, Appendix C

from the front-panel Ext Trig terminal or from BUS triggers.

A sine wave sweep from 50 Hz to 5 kHz with linear 1 second sweep time.

Frequency Shift Key Modulation In Frequency-Shift Keying modulation (FSK), the function generator's output frequency alternates between the carrier frequency and a second “hop” frequency. The rate of frequency hops is controlled either by an internal source or from an external logic input. FSK is essentially a special case of frequency modulation (FM) where the hop frequency is another way of specifying both the deviation and the modulating signal shape.

The modulating signal shape is always a square wave with an amplitude of zero to +1. The deviation is either positive or negative depending on whether the hop frequency is larger or smaller than the present carrier frequency (as shown below). The internal FSK rate generator specifies the period of the modulating square wave signal. When selected, the external FSK input replaces the internal FSK rate generator to directly control the frequency hop rate. A TTL “low” input always selects the carrier frequency and a TTL “high” always selects the hop frequency. The logic sense of the external FSK input can effectively be changed by interchanging the carrier and hop frequency values.

Deviation = Hop Frequency – Carrier Frequency

An FSK waveform with a 3 kHz carrier waveform and 500 Hz "hop" waveform (the FSK rate is 100 Hz).

Burst Modulation In burst modulation, the function generator turns the carrier wave output “on” and “off ” in a controlled manner. The carrier output can be controlled using either triggered or externally- gated methods.

When configured for triggered operation, the function generator can output a carrier waveform with a user-specified number of complete cycles. Each time a trigger is received, the specified number of complete cycles is output. You can also specify a starting waveform phase in triggered operation. Zero degrees is defined as the first data point in waveform memory. The function generator will output the start phase as a dc output level while waiting for the next trigger. Output dc offset voltages are not affected by burst modulation — they are independently produced and summed into the function generator's output amplifier.

A three-cycle bursted sine wave with 100 Hz burst rate.

In gated burst mode operation, the front-panel Burst terminal is used to directly (and asynchronously) turn off the waveform DAC output. The burst count, burst rate, and burst phase settings have no effect in this mode. When the burst signal is true (TTL “high”), the function generator outputs the carrier waveform continuously. When the burst signal is false (TTL “low”), the waveform DAC is forced to a zero output level. Like triggered burst operation, the output dc offset voltage is not affected by the external burst gate signal.

For triggered burst operation, the function generator creates an internal modulation signal which is exactly synchronous with the carrier waveform. This internal modulation signal is used to halt waveform memory addressing when the last data point is reached. This modulation signal effectively “gates” the output “on” and “off” for the specified number of carrier wave cycles. The modulation signal is then triggered by another internal burst rate signal generator which controls how often the specified carrier burst is output. In external triggered burst operation, the modulation signal trigger source is set to the function generator's front-panel Ext Trig terminal. This source replaces the internal burst rate signal generator for pacing triggered bursts.

Changes to the burst count, burst rate, burst phase, or carrier frequency will cause the function generator to automatically compute a new modulation signal and download it into modulation RAM. It is not possible for the function generator to burst single cycles for all carrier frequencies because the internal modulation signal generator is not as capable as the main carrier signal generator.

160 Agilent E1441A Function Generator Tutorial Appendix C