7-6
Operating Concepts
Processing
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 A UX INPUT connect or
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 tec hnique c anno t b e us ed
with single-input measurements.