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While only a single flow path is shown, two identical paths are available, corresponding to
channel 1 and 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.
lF Detection
This detection occurs in the digital
hlter,
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
hlter
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.
Application and Operation Concepts
6-6