222 Keysight EXG and MXG X-Series Signal Generators User’s Guide
Basic Digital Operation for N5172B/82B with Options 653/655/656/657
Using Finite Impulse Response (FIR) Filters in the Dual ARB Real-Time Modulation Filter
Setting the Oversample Ratio
The oversample ratio (OSR) is the number of filter coefficients per symbol. Acceptable values range
from 1 through 32; the maximum number of taps allowed by the table editor is 1024.
The actual limits on OSR, number of coefficients, and number of symbols depends on the feature
with which the FIR is used. Refer to Table 8-4.
For modulation filters, if the oversample ratio is different from the internal, optimally selected one,
then the filter is automatically resampled to an optimal oversample ratio.
For this example, the desired OSR is 4, which is the default, so no action is necessary.
Displaying a Graphical Representation of the Filter
The signal generator has the capability of graphically displaying the filter in both time and
frequency dimensions.
1. Press More 1 of 2 > Display Filter > Display FFT (fast Fourier transform).
Refer to Figure 8-20 on page 223.
Modulation filters are real and have an oversample ratio (OSR) of two or
greater.
Equalization filters are typically complex and must have an OSR of one
(refer to “Using the Equalization Filter” on page 217 and to “Setting the
Real-Time Modulation Filter” on page 232).
Table 8-4
Filter Type Oversampling Ratio
(OSR)
Number of Taps
(Maximum)
Symbols/Coefficients
(Maximum)
Equalization
a
a. When I/Q timing skew, I/Q delay, or the ACP internal I/Q channel
optimization features are active, the effective number of taps for the
equalization filter are reduced.
1256--
ARB Custom
Modulation
b
b. The filter may be sampled to a higher or lower OSR.
≥ 2 -- 512/1024
Dual ARB
Real-Time
Modulation
c
c. The filter will be decimated to a 16 or lower OSR depending on the
symbol rate.
≥ 2 -- 32/1024
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