Alpha / BT (Digital Demod)

Range: non-Gaussian filters: .05 to 1
Gaussian filters: .05 to 100
Minimum increment: .01

Alpha / BT determines the filter characteristics of the Nyquist and Gaussian filters used by the VSA's digital demodulator.

If one of the VSA's Nyquist (cosine) or Gaussian filters is selected, the filter's characteristics must be specified. For Nyquist filters, the "alpha" of the filter must be specified; for Gaussian filters, the Bandwidth Time Product (BT Bandwidth Time Product) of the filter must be specified.

The value that is specified for Alpha / BT is used by both the measured filter and the reference filter. For example, if a value of 0.35 is entered, the VSA sets the alpha or BT of the measured and reference filters to 0.35.

Your communications system should specify the "alpha" or "BT" for filters in the system. Set the value of Alpha / BT to match that of your communications system.

Because the filter alpha/BT can easily be changed with a minimum amount of delay, the filters can be manually adjusted to achieve a minimum EVM Error vector magnitude (EVM): A quality metric in digital communication systems. See the EVM metric in the Error Summary Table topic in each demodulator for more information on how EVM is calculated for that modulation format. (Error Vector Magnitude).

Alpha

Alpha describes the shape of a Nyquist (cosine) filter. Alpha is also called the roll-off or the excess bandwidth factor. A higher value for alpha increases the bandwidth that is used in excess of the theoretical minimum.

Modulation theory states that the minimum bandwidth needed to transmit a signal is equal to half of the symbol rate. However, to realize this system would require a perfect brick-wall filter. This filter is equivalent to an alpha of 0. This would give the best possible spectral efficiency. Unfortunately, this filter is unrealizable.

In practice, radios typically use a filter alpha of about 0.3. This means that they take up 30% more bandwidth than the theoretical minimum.

Filter length

When filter alpha < 0.4, extended filter length (40 symbols) is used for the following formats:

BPSK Binary phase shift keying - A type of phase modulation using 2 distinct carrier phases to signal ones and zeros., QPSK Quadrature phase shift keying, 8-PSK Phase Shift Keying: A broad classification of modulation techniques where the information to be transmitted is contained in the phase of the carrier wave., DQPSK Differential Quadrature Phase Shift Keying: QPSK modulation using differential encoding of the digital information stream., D8PSK
All QAM Quadrature Amplitude Modulation
All DVB-QAM
16-APSK, 16-APSK w/DVB, 32-APSK, 32-APSK w/DVB, Custom APSK

Otherwise, normal filter length (20 symbols) is used.

BT

BT (Bandwidth Time Product) describes the shape of a Gaussian filter. BT indicates the filter roll-off (or excess bandwidth) of the Gaussian filter.