Real-Time vs. Batch-Mode Instruments
The Analog Analyzer is a real-time in stru ment. That is, it re turns read ings
con tin u ously, whether or not a sweep is in prog ress. These read ings in clude
level and fre quency, as well as func tions such as dis tor tion or phase. Dur ing a
sweep, read ings from the au dio an a lyzer are di verted from the panel me ters to
the sweep en gine for graph ing. Af ter the sweep, the read ings are once again
dis played in the panel me ters.
By con trast, the FFT Spec trum An a lyzer is a batch-mode in stru ment. It does
not gen er ate read ings if a sweep is not be ing per formed. When a sweep is
started, the Spec trum An a lyzer first ac quires a seg ment of au dio by stor ing it
in mem ory. This is in di cated (briefly) on the Sta tus bar as DSP Ac quiring
Data. Once the ac qui si tion is com plete, the Spec trum An a lyzer pro cesses the
data. This is in di cated as DSP Trans forming Data (again, briefly) on the Sta -
tus bar. Once pro cess ing is com plete, the con trol soft ware fetches a batch of
read ings from the Spec trum An a lyzer. At that point, the data is graphed.
The FFT Concept
The FFT (fast Fou rier trans form) is an ef fi cient way to de com pose a pe ri -
odic sig nal into its com po nent fre quen cies. It trans forms a time-domain sig nal
(such as you might see in an os cil lo scope trace) into a fre quency-domain rep -
re sen ta tion.
As an ex am ple, a high-quality power am pli fier fed with a si nu soi dal in put
will pro duce har monic dis tor tion at its out put that is in vis i ble in an os cil lo -
scope trace. If an FFT is per formed on the out put sig nal, how ever, the in di vid -
ual dis tor tion har mon ics can be seen. The pat tern of har mon ics pro vides
in sight into the mech a nisms re spon si ble for the dis tor tion.
The FFT pro cess con sists of the fol low ing steps:
§
Trig gering. The sys tem waits for a trig ger event from the se lected
source, for in stance, a pos i tive-going zero cross ing of the an a log gen er a -
tor.
§
Ac qui si tion. The sys tem stores the in put au dio in mem ory un til the se -
lected num ber of au dio sam ples has been re ceived. The more au dio data
ac quired, the more fre quency points there will be in the re sult, and hence
the higher the res o lu tion will be. How ever, lon ger ac qui si tions take more
time.
§
Win dow ing. Be fore trans for ma tion, the au dio data is shaped in time by
mul ti ply ing it with a pre-defined en ve lope. This in creases the ac cu racy
of the am pli tude val ues re turned from the FFT.
Chapter 5: Quick Guides The FFT Spectrum Analyzer
46 Getting Started with Your 2700 Series Instrument
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