Agilent Technologies VXI E1439 - Manual (Page 38)

28
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. 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.
A B C D
Signal
Envelope
or
Level
Possible
Positive
Trigger
Points
Negative
Trigger
Point
Positive
Trigger
Point
Time
High
Low
magDwell times:
Output of magnitude
comparators