Agilent Technologies E1439 manual Magnitude trigger and magdwell time

Using the Agilent E1439

Magnitude trigger and magdwell time

Magnitude trigger and magdwell time

The magnitude trigger operates on the magnitude of a (possibly filtered) signal. For a real signal, the magnitude is merely the absolute value of the signal. For a complex signal, the magnitude is the square root of the sum of the squares of the real and imaginary parts of the signal.

Because the magnitude trigger can operate on the filtered signal, the trigger can be more selective regarding what signals will cause a trigger than the ADC trigger. Only signals in the filter bandwidth around the center frequency will be considered when determining when a trigger occurs. Signals outside the filter's passband will be filtered out before the magnitude trigger circuit and will not cause any triggers to occur.

The magnitude trigger's behavior can be modified by the magDwell time. The magDwell time is the number of samples that a signal's magnitude must be low (i.e., below the magLevel threshold) before the magnitude trigger circuit will recognize the signal as being low. This can facilitate triggering off of a burst signal; for example, a tone burst or a TDMA burst. Due to the zero crossings within the tone burst, the ADC trigger can not reliably trigger on the leading edge of the burst. If you set the magDwell time longer than any potential drop outs within a burst and shorter than the gap between bursts, the magnitude trigger can easily catch the leading edge of a tone burst.

For a magnitude trigger with positive slope, the signal must be low for at least a magDwell number of samples. After that, the module will trigger the next time the signal goes above the magLevel threshold. For a negative slope, the module will trigger the first time that the signal is low for at least a magDwell number of samples after being high. Note that in this case, the trigger will occur a magDwell period of time after the end of the tone burst. You can use a negative trigger delay to compensate for this and to capture the end of the tone burst.

A	B	C	D
Signal
Envelope	Possible		Negative
or	Positive	Positive	Trigger
Level	Trigger		Point
	Points	Trigger
		Point
magDwell times:				Time
				High
Output of magnitude
comparators				Low

A.Time A is less than the magDwell time. The magnitude trigger does not recognize the signal as being low.

B.Time B is longer than the magDwell time. The magnitude trigger does recognize the signal as being low and a positive trigger may occur on the rising edge at the end of B.

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