IF Mag Trigger Style (Trigger)

The IF Mag trigger style is available when using zoomed (real and imaginary) time data. The IF Mag trigger only triggers when the signal is within the specified frequency span, as shown in the following block diagram:

$image\trig_IFmag_wmf.jpg$

The IF Mag trigger is usually used with amplitude modulated (AM Amplitude Modulation - CW modulation using amplitude variation in proportion to the amplitude of the modulating signal. Usually taken as DSB-LC for commercial broadcast transmissions and DSB-SC for multiplexed systems.) or burst signals. This trigger style triggers the VSA under the following conditions:

the input signal is within the current span
the input signal is varying in amplitude
the trigger conditions are met.

The trigger conditions that can be specified are level, slope, delay, and hold-off.

With IF Mag, the magnitude of the IF signal must vary to trigger the VSA. The magnitude of the IF signal varies when the input signal switches on and off or if the input signal is AM modulated.

Trigger hold-off is often used with IF Mag. If using an IF burst to trigger the VSA and the IF burst contains more than one valid trigger edge, use trigger hold-off to ignore all but the first trigger edge.

Viewing the IF Signal

The following steps show how to view the IF Mag trigger signal. This is useful for determining the best values for trigger level and slope.

Make sure Digital Demodulation and Analog Demodulation are off.
Click MeasSetup > Frequency:
1. In the Time Data box, select Zoom.
2. In the Center box, set the center frequency to the frequency of the carrier.
Click Trace > Format. In the Format box, select Lin Mag.
Click Trace > Data and select a time domain display (Main Time).

There are some considerations to be aware of when viewing IF Mag in the time domain. See below for details.

Considerations

With IF Mag, the magnitude of the IF signal must vary to trigger the VSA. The magnitude of the IF signal varies when the input signal switches on and off or if the input signal is AM modulated.
Trigger conditions (level, slope, delay, and hold-off) must be met to trigger the VSA.
Use trigger hold-off if there are multiple, valid trigger signals within the IF signal. With trigger hold-off, configure the VSA to ignore extra trigger signals.
To get an accurate time-domain display of the IF Mag trigger signal, make sure the input signal is within the specified frequency span.
If the DC level is too high, the DC level introduces a second waveform that makes the IF Mag trigger signal look like a mixed signal. This problem is most apparent with DC coupling, but may also be apparent with AC coupling.
To avoid problems caused by DC level, set the stop frequency high enough that DC is out of band. Usually, setting the stop frequency to 0.1´ span is sufficient; however, it may need to be set as high as 0.3´ span.