Autoranging
When autoranging is on, the analyzer continuously monitors the amplitude of
the input signals and, if necessary, automatically changes the input range. If
the input signal increases enough to exceed the current input range, the
analyzer changes to a less-sensitive input range. When autoranging occurs,
the analyzer displays the message. If you are making
an averaged measurement and autoranging occurs, the analyzer begins a new
series of averages.
You can set the input range manually when you want to maintain a specific
input range setting. Ideally, the signal peak should fall in the upper half of the
currently-selected input range.
If you set the input range too low (more sensitive than necessary), the
analyzer’s input circuitry will introduce distortion into the measurement. But if
you set the input range too high (less sensitive than necessary), the resulting
loss of dynamic range will introduce additional noise—in some cases, the
increase in the noise floor may even obscure low-level frequency components.
If you use a fixed input range, there is approximately 2 dB of headroom at the
top of each range before the input channel distorts. So if you are using a fixed
input range, an overload condition does not occur unless you exceed this level.
This is important to know, particularly for rotating machinery measurements
where transient signals often exceed the desired input range.
Configuring the source
The analyzer’s source can supply a number of different waveforms. For
example, you can select any of the following waveforms with the FFT Analysis
mode (keep in mind that not all source waveforms are available with each
instrument mode):
• random noise
• burst random
• periodic chirp
• burst chirp
• pink noise
• fixed sine
• arbitrary source (optional)
Agilent 35670A
Setting Up a Measurement Operator's Guide
Part I-10
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