Agilent Technologies N8201A Service Manual download pdf (Page 78)

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78 Agilent N8201A Performance Downconverter Synthetic Instrument Module, 250 kHz to 26.5 GHz

3 Auto Couple

Manual: FFT

Manually selects FFT analysis, so it cannot change automatically to swept.

While Zero Span is selected, this key is grayed out. The status of the FFT & Swept selection

is saved when entering zero span and is restored when leaving zero span.

Tip: Making Gated FFT Measurements

The process of making a spectrum measurement with FFTs is inherently a “gated” process,

in that the spectrum is computed from a time record of short duration, much like a gate

signal in swept-gated analysis.

The duration of the time record is 1.83 divided by the RBW, within a tolerance of about 3%

for bandwidths up through 1 MHz. Therefore, unlike swept gated analysis, the duration of

the analysis is fixed by the RBW, not by the gate signal. Because FFT analysis is inherently

faster than swept analysis, the gated FFT measurements can have better frequency

resolution (a narrower RBW) than would swept analysis for a given duration of the signal to

be analyzed.

FFT analysis in the N8201A usually involves making auto ranged measurements, and the

time required to autorange the FFT can be both long and inconsistent. The N8201A

hardware automatically sets the ADC Ranging to Bypass when any trigger, except Free Run

is selected.

To make a gated FFT measurement, set the N8201A as follows.

1 Press Auto Couple, FFT & Sweep to select Manual: FFT.

2 Set the resolution bandwidth to 1.83 divided by the required analysis time, or higher, by

pressing BW/Avg, Res BW.

3 Set the trigger source to the desired trigger, by pressing Trig.

4 Set the trigger delay to observe the signal starting at the required time relative to the

trigger. Negative delays are possible, by pressing Trig, Trig Delay.

Key Path: Auto Couple, FFT & Sweep

Remote Command:

[:SENSe]:SWEep:TYPE AUTO|FFT|SWEep

See “FFT & Sweep" on page 76.

Example:

SWE:TYPE FFT

1 2 ... 73 74 75 76 77 78 79 80 81 82 83 ... 621 622

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Agilent Technologies N8201A Service Manual Page 78

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