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In addition to the time record length limits, there are limits to the allowable
range of spans. These are represented as vertical lines on the operating region
graphs (Figure 4). The upper span limit is set by the digitizer’s sample rate. For
the highest sample rate supported by the digitizer this boundary may be further
limited by the analog performance of the digitizer’s front-end. To zoom in on a
signal of interest by reducing the span, traditional VSA instrumentation reduces
the effective sample rate through the use of decimating fi lters while keeping
the ADC at full rate. This technique allows for maximum alias protection. Unlike
traditional VSA instruments, the Acqiris digitizers have no decimating fi lters.
To compensate for this shortcoming, decimating fi lters have been built into the
89600 VSA software. The maximum amount of decimation allowed by the 89600
VSA software sets the minimum span for any given digitizer sample rate.
The operating region depicted in Figure 4 is determined by constraints imposed
by both time record length and span. The Acqiris digitizers support many sample
rates. For each sample rate, the operating region shifts slightly on the operating
region graph. As the sample rate decreases, the operating region shifts slightly
up and to the left. So, for any given digitizer, there will be a family of overlapping
operating regions, one for each supported sample rate.
In general, these regions would be the same for every digitizer that supports a
given sample rate. The only difference is when the maximum span at the maxi-
mum sample rate is limited by analog performance of the digitizer’s front-end
rather than just by the sample rate.
In addition to the time record length limits, there are limits to the allowable
range of spans. These are represented as vertical lines on the operating region
graphs (Figure 4). The upper span limit is set by the digitizer’s sample rate. For
the highest sample rate supported by the digitizer this boundary may be further
limited by the analog performance of the digitizer’s front-end. To zoom in on a
signal of interest by reducing the span, traditional VSA instrumentation reduces
the effective sample rate through the use of decimating fi lters while keeping
the ADC at full rate. This technique allows for maximum alias protection. Unlike
traditional VSA instruments, the Acqiris digitizers have no decimating fi lters.
To compensate for this shortcoming, decimating fi lters have been built into the
89600 VSA software. The maximum amount of decimation allowed by the 89600
VSA software sets the minimum span for any given digitizer sample rate.
The operating region depicted in Figure 4 is determined by constraints imposed
by both time record length and span. The Acqiris digitizers support many sample
rates. For each sample rate, the operating region shifts slightly on the operating
region graph. As the sample rate decreases, the operating region shifts slightly
up and to the left. So, for any given digitizer, there will be a family of overlapping
operating regions, one for each supported sample rate.
Taken altogether, these constraints form an operating region that the 89600 VSA
software must remain within (Figure 6). To complete our understanding there are
a couple of additional points regarding sampling mode and alias exposure that
must be presented.
The graphs presented in this section describe discrete operating regions that are
a function of sample rate. In full rate or user rate sample modes a single operat-
ing region curve fully describes the constraints imposed on the measurement.