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Chapter 6 125
Custom Arb Waveform Generator
Working with Filters
Working with Filters
This section provides information on using predefined (page 126) and user-defined (page 127) FIR filters.
NOTE The procedures in this section apply only to filters created in either the Custom Arb
Waveform Generator or Custom Real Time I/Q Baseband mode; they do not work with
downloaded files, such as those created in Matlab.
The
Filter menu selections enable you to apply a filter to the generated signal, define a finite impulse
response (FIR) filter, change a Root Nyquist or Nyquist filter alpha, change a Gaussian filter BbT, or restore
all filter parameters to their default state. In Custom Real Time I/Q mode, you can also optimize a FIR filter
for Error Vector Magnitude (EVM) or Adjacent Channel Power (ACP)
Predefined Filters (
Filter > Select)
•
Root Nyquist is a root-raised cosine pre-modulation FIR filter. Use a Root Nyquist filter when you want to
place half of the filtering in the transmitter and the other half in the receiver. The ideal root-raised cosine
filter frequency response has unity gain at low frequencies, the square root of raised cosine function in
the middle, and total attenuation at high frequencies. The width of the middle frequencies is defined by
the roll off factor or Filter Alpha (0 < Filter Alpha < 1).
•
Nyquist is a raised cosine pre-modulation FIR filter. You can use a Nyquist filter to reduce the bandwidth
required by a signal without losing information. The ideal raised cosine filter frequency response
comprises unity gain at low frequencies, a raised cosine function in the middle, and total attenuation at
high frequencies. The width of the middle frequencies is defined by the roll off factor or Filter Alpha (0
< Filter Alpha < 1).
•
Gaussian is a Gaussian pre-modulation FIR filter.
•
User FIR enables you to select from a Catalog of FIR filters; use this selection if the other predefined FIR
filters do not meet your needs. For more information, see
Define User FIR, below.
•
Rectangle is a rectangular pre-modulation FIR filter.
•
APCO 25 C4FM is an APCO 25-specified C4FM filter; this is a Nyquist filter with an alpha of 0.200 that is
combined with a shaping filter.
Filter Parameters
•
Define User FIR is available for when the predefined FIR filters do not meet your needs. You can define
FIR coefficients and set the oversample ratio (number of filter coefficients per symbol) to be applied to a
custom FIR filter.