The Sample Period The sample period is the interval at which the digitizer takes readings. The sample period is derived from a reference which can be the digitizer’s internal 20 MHz oscillator, or a reference from an external source (see "The Digitizer Reference Clock").

Setting the Sample When the trigger source is TIMer, the sample period is set with the Period command:

TRIGger[:STARt]:TIMer1 <period >

The TIMer trigger source derives the sample period from the reference source (see "The Digitizer Reference Clock"). The period is a 1, 2, 4, 10, 20, 40 through 1E8, 2E8, 4E8 multiple of the reference source. Programmed sample periods that are not an exact 1, 2, 4, ... multiple are rounded to the nearest multiple of the source. If the digitizer cannot produce a sample rate that is within 1% of the specified sample rate, bit 2 of the Questionable Signal Status Group condition register is set.

The sample period required can be determined two ways. First, to take a given number of samples of a signal with frequency (f), the sample period is computed as:

<period > = signal period / number of samples

The reference is then divided by a value (N) which gives the sample period. For example, assuming the digitizer’s 20 MHz oscillator is used and given the following:

input signal frequency = 1 kHz number of samples = 100

then the <period > specified by TRIG:STAR:TIM1 is:

1 ms / 100 = 10 μs

122 Understanding the HP E1429 Digitizer

Chapter 3

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Image 122
HP E1429A manual TRIGgerSTARtTIMer1 period