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Фрагмент инструкции
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. Chapter 6 125 Custom Arb Waveform Generator Working with Filters • Filter Alpha enables you to adjust the filter alpha for a Nyquist or root Nyquist filter. If a Gaussian filter is used, you will see Filter BbT; this softkey is grayed out when any other filter is selected. •(Custom Realtime I/Q Baseband Only) Optimize FIR for EVM ACP enables you to optimize a Nyquist or root Nyquist filter for minimized error vector magnitude (EVM) or for minimized adjacent channel power (ACP); the softkey is grayed out when any other filter is selected. • Restore Default Filters replaces the current FIR filter with the default FIR filter for the selected format. Using a Predefined FIR Filter Selecting a Predefined FIR Filter 1. Preset the instrument: Press Preset. 2. Press Mode > Custom > ARB Waveform Generator > Digital Mod Define > Filter > Select or Mode > Custom > Real Time I/Q Baseband > Filter > Select > 3. Select the desired filter. If the filter you want is not in the first list, press More (1 of 2). Adjusting the Filter Alpha of a Predefined Root Nyquist or Nyquist Filter 1. Preset the instrument: Press Preset. 2. Press Mode > Custom > ARB Waveform Generator > Digital Mod Define > Filter > Filter Alpha or Mode > Custom > Real Time I/Q Baseband > Filter > Filter Alpha 3. Enter a new Filter Alpha value and press Enter. Adjusting the Bandwidth-Bit-Time (BbT) Product of a Predefined Gaussian Filter 1. Press Mode > Custom > ARB Waveform Generator > Digital Mod Define > Filter > Select > Gaussian or Mode > Custom > Real Time I/Q Baseband > Filter > Select > Gaussian 2. Press Filter BbT. 3. Enter a new Bandwidth-Bit-Time (BbT) product filter parameter and press Enter. 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. 126 Chapter 6 Custom Arb Waveform Generator Working with Filters 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 me...
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