Agilent Technologies N8201A Service Manual download pdf (Page 79)

Auto Couple 3

Agilent N8201A Performance Downconverter Synthetic Instrument Module, 250 kHz to 26.5 GHz 79

FFTs/Span

Displays and controls the number of FFT segments used to measure the entire Span. This

key is unavailable (grayed out) unless Sweep Type has been set to FFT. If Sweep Type is set

to Auto and FFTs are selected, FFTs/Span is still unavailable, and the number of FFTs

automatically selected is shown. If Sweep Type is set to Manual:FFT, FFTs/Span becomes

available. Press FFTs/Span and an integer can be entered. The N8201A will try to use the

number entered, but it may need to use more due to hardware or software limitations.

An FFT can only be performed over a limited span or segment (also known as the FFT width).

Several FFT widths may need to be combined to measure the entire span. The “FFT Width”

is (Span)/(FFTs/Span), and affects the ADC Dither function. (See Auto Couple).

FFT measurements require that the signal level driving the A/D converter in the IF be small

enough to avoid overloading, and that the gain that controls that signal level remain fixed

during the measurement of an entire FFT segment. This constraint can allow higher

dynamic ranges in swept mode in some cases, but increasing FFTs/Span can restore that

dynamic range to FFT measurements, at the expense of losing some of the speed

advantages of the FFT.

For example, in pulsed-RF measurements such as radar, it is often possible to make high

dynamic range measurements with signal levels approaching the compression threshold of

the N8201A in swept spans (well over 0 dBm), while resolving the spectral components to

levels below the maximum IF drive level (about –8 dBm at the input mixer). But FFT

processing experiences overloads at the maximum IF drive level even if the RBW is small

enough that no single spectral component exceeds the maximum IF drive level. If the user

reduces the width of an FFT using the FFTs/Span function, an analog filter is placed before

the ADC that is about 1.3 times as wide as the FFT segment width. This spreads out the

pulsed RF in time and reduces the maximum signal level seen by the ADC. Therefore, the

input attenuation can be reduced and the dynamic range increased without overloading the

ADC.

Further improvement in the dynamic range is possible by changing the ADC gain. In swept

analysis in N8201A, the gain is normally auto ranged such that it can track the signal power

as the N8201A sweeps through CW-like signals. Since FFT processing cannot autorange the

gain within the measurement of a single FFT segment, the autoranging advantage is lost for

single FFT measurements. But if the segments are reduced in width by using more

FFTs/Span, then individual FFT segments can use higher gains, improving the dynamic

range.

Additional information about selecting FFTs/Span can be found in a product note, "PSA

Series Swept and FFT Analysis," literature number 5980-3081EN, available on-line through

http://www.agilent.com.

Key Path: Auto Couple, FFT & Sweep

State Saved: Saved in Instrument State

Factory Preset: 1

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Agilent Technologies N8201A Service Manual Page 79