Processing Details - Agilent Technologies 8753ET User Manual

While only a single flow path is shown, two identical paths are available, corresponding to channel 1 and
channel 2. Each channel also has an auxiliary channel for which the data is processed along with the
primary channel's data. Channel 3 is the auxiliary channel for channel 1, while channel 4 is the auxiliary
channel for channel 2. When the channels are uncoupled, each channel is processed and controlled
independently.
Data point definition: A "data point" or "point" is a single piece of data representing a measurement at a
single stimulus value. Most data processing operations are performed point-by-point; some involve more
than one point.
Sweep definition: A "sweep" is a series of consecutive data point measurements, taken over a
sequence of stimulus values. A few data processing operations require that a full sweep of data is
available. The number of points per sweep can be defined by the user. The units of the stimulus values
(such as power, frequency, and time) can change, depending on the sweep mode, although this does not
generally affect the data processing path.

Processing Details

The ADC
The ADC (analog-to-digital converter) converts the R, A, and B inputs (already down-converted to a fixed low
frequency IF) into digital words. (The AUX INPUT connector on the rear panel is a fourth input.) The ADC
switches rapidly between these inputs, so they are converted nearly simultaneously.
IF Detection
This detection occurs in the digital filter, which performs the discrete Fourier transform (DFT) on the digital
words. The samples are converted into complex number pairs (real plus imaginary, R+jX). The complex
numbers represent both the magnitude and phase of the IF signal. If the AUX INPUT is selected, the
imaginary part of the pair is set to zero. The DFT filter shape can be altered by changing the IF bandwidth,
which is a highly effective technique for noise reduction.
Ratio Calculations
These calculations are performed if the selected measurement is a ratio of two inputs (for example, A/R or
B/R). This is a complex divide operation. If the selected measurement is absolute (such as A or B), no
calculations are performed. The R, A, and B values are also split into channel data at this point.
Sampler/IF Correction
The next digital processing technique used is sampler/IF correction. This process digitally corrects for
frequency response errors (both magnitude and phase, primarily sampler rolloff) in the analog
down-conversion path.
Sweep-to-Sweep Averaging
Averaging is another noise reduction technique. This calculation involves taking the complex exponential
average of several consecutive sweeps. This technique cannot be used with single-input measurements.
Pre-Raw Data Arrays
These data arrays store the results of all the preceding data processing operations. (Up to this point, all
processing is performed real-time with the sweep by the IF processor. The remaining operations are not
Operating Concepts
Processing
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