296 Keysight EXG and MXG X-Series Signal Generators User’s Guide
Using Custom Digital Modulation for N5172B/82B with Option 431 and 653/655/656/657
Using Finite Impulse Response (FIR) Filters with Custom Modulation
Duplicating the First 16 Coefficients Using Mirror Table
In a windowed sinc function filter, the second half of the coefficients are identical to the first half in
reverse order. The signal generator provides a mirror table function that automatically duplicates
the existing coefficient values in the reverse order.
1. Press Mirror Table. The last 16 coefficients (16 through 31) are automatically generated and
the first of these coefficients (number 16) highlights, as shown in Figure 9-34 on page 296.
Figure 9-34
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 combination of symbols and oversampling ratio allowed by the
table editor is 1024. The instrument hardware, however, is actually limited to 32 symbols, an
oversample ratio between 4 and 16, and 512 coefficients. So if you enter more than 32 symbols or
512 coefficients, the instrument is unable to use the filter. 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 > Display Filter > Display FFT (fast Fourier transform).
Refer to Figure 9-35 on page 297.
Modulation filters must be real and have an oversample ratio (OSR) of two
or greater.
FIR table coefficient values,
may be from the factory
default values or entered by
the user.
Use the Goto Row menu
to move around and
make changes to the FIR
Values coefficient table.
For details on each key, use key help as described on page56.
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