Main Time Length (Time)

The Main Time Length parameter determines the length of the main time record.

By default, the Main Time Length is limited by the frequency span and number of frequency points. If a value is entered for Main Time Length that exceeds what is allowed by the current Span and number of Frequency Points, the VSA sets Main Time Length to the maximum allowable value. To increase the maximum value of Main Time Length, reduce the Span and/or increase the number of Frequency Points.

It is recommend to use the Auto Frequency Points mode which provides the most flexibility in specifying a ResBW or Main Time Length for a given span and window type.

Time record length and resolution bandwidth are inversely proportional, as shown by the following formula. Changing the time record length changes ResBW and vice-versa.

where:

Time Record Length = Main Time Length

ResBW = Resolution bandwidth

Also, see Equivalent Noise Bandwidth.

When the time record length length is changed, the VSA automatically sets ResBW Mode to Arbitrary and ResBW Couple to Fixed to allow:

Entering arbitrary values for Main Time Length.
Changing the span without changing Main Time Length.

When time gating is on, ResBW is determined by gate length, not Main Time Length. In this case, changing gate length changes ResBW and vice-versa.

Minimum and Maximum Time Record Lengths

The maximum time record length is determined by the number of Frequency Points and the measurement's frequency Span (which determines the sample rate, which determines the time between samples).

The minimum time record length is determined by the maximum ResBW setting (0.3 * Span).

For a more in depth explanation of these relationships, see Understanding Time and Frequency parameters.

Maximum time record length

The time record length (T seconds) is limited by the number of Frequency Points and the frequency Span.

where: FPFrame Protocol = number of Frequency Points

This can be derived through the following steps:

TP (max) = a * (FP - 1)

T (max) = TP (max) * Dt = (a * (FP - 1)) / (a * span) = (FP - 1) / span

where

TP = number of points in time record

Dt = 1 / f

f = a * span

a = 1.28 (for zoom data, i.e. complex time samples)

a = 2.56 (for baseband data, i.e. real time samples)

Minimum time record length

Minimum time record length is dependent on the resolution bandwidth to span ratio:

The maximum resolution bandwidth allowed by the VSA is 0.3 * span.

For a given window type, the minimum resolution bandwidth, while not directly affected by Span and number of Frequency Points, may be limited by these parameters.