164 Chapter 6
Custom Arb Waveform Generator
Working with Filters
Optimizing a Nyquist or Root Nyquist FIR Filter for EVM or ACP (Custom Realtime I/Q Baseband only)
1. Preset the instrument: Press
Preset.
2. Press
Mode > Custom > Real Time I/Q Baseband > Filter > Optimize FIR For EVM or ACP.
The FIR filter is now optimized for minimum error vector magnitude (EVM) or for minimum
adjacent channel power (ACP). This feature applies only to Nyquist and root Nyquist filters; the
softkey is grayed out when any other filter is selected.
Restoring Default FIR Filter Parameters
1. Preset the instrument: Press Preset.
2. Press
Mode > Custom > ARB Waveform Generator > Digital Mod Define > Filter > Restore Default Filter.
This replaces the current FIR filter with the default filter for the selected modulation format.
Using a User–Defined FIR Filter
FIR filters can be created and modified by defining the FIR coefficients or by defining the
oversample ratio (number of filter coefficients per symbol) to be applied to your own custom FIR
filter.
To Modify Predefined FIR Coefficients for a Gaussian Filter Using the FIR Values Editor
You can define from 1 to 32 FIR coefficients, where the maximum combination of symbols and
oversample ratio is 1024 coefficients. While the FIR Values editor allows a maximum filter length of
1024 coefficients, the PSG hardware is limited to 64 symbols for real–time and 512 symbols for
arbitrary waveform generation (the number of symbols equals the number of coefficients divided by
the oversample ratio).
If you enter more than 64 symbols for real–time or 512 symbols for arbitrary waveform generation,
the PSG cannot use the filter; it will decimate the filter (throw away coefficients) until the required
condition is met and then use the filter, but fine resolution may be lost from the impulse response.
FIR filters stored in signal generator memory can easily be modified using the FIR Values editor. In
this example, you will load the FIR Values editor with coefficient values from a default FIR filter (or,
if one has been defined, a user–defined FIR file that has been stored in the Memory Catalog), modify
the coefficient values, and store the new file to the Memory Catalog.
1. Press
Preset.
2. Press
Mode > Custom > Arb Waveform Generator > Digital Mod Define > Filter
or Mode > Custom > Real Time I/Q Baseband > Filter
3. Press Define User FIR > More (1 of 2) > Load Default FIR > Gaussian.
4. Press
Filter BbT > 0.300 > Enter.
5. Press
Filter Symbols > 8 > Enter.
6. Press
Generate.
NOTE The actual oversample ratio during modulation is automatically selected by the instrument. A
value between 4 and 16 is chosen dependent on the symbol rate, the number of bits per
symbol of the modulation type, and the number of symbols.
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