Symbol Rate (Digital Demod)

The Symbol Rate parameter sets the symbol rate for the VSA's digital demodulator. Set this parameter to match the symbol rate of the system.

When the Digital Demodulation measurement type is first selected, the default Symbol Rate is Frequency Span/2.

In digital modulation, the Symbol Rate determines the rate (frequency) at which symbols occur. A symbol may consist of one or more bits as determined by the modulation format. For example, in a BPSK Binary phase shift keying - A type of phase modulation using 2 distinct carrier phases to signal ones and zeros. system, each symbol represents 1 bit; in a QPSK Quadrature phase shift keying system, each symbol represents 2 bits.

Symbols are valid only at the timing instant when the receiver interprets the signal. This timing instant is called the detection-decision point.

The VSA's demodulator uses the Symbol Rate to determine the frequency of the detection-decision points. It is important that the Symbol Rate is set to match exactly the symbol rate of the system because the symbol clock frequency is not estimated.

The more complex the modulation format, the more critical it is that the Symbol Rate be exact. Specifying an incorrect symbol rate introduces errors into the demodulation process.

If a Symbol Rate that is slightly different than the symbol rate of your signal is entered, the 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) is typically small at the center of the Result Length and increases linearly towards the ends of the Result Length.

Symbol Rate and Frequency Span

The Symbol Rate determines the maximum Frequency Span (information bandwidth) that can be measured. For QAM Quadrature Amplitude Modulation and 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. signals, the Symbol Rate also determines the minimum Frequency Span that achieves published specifications.

The following formulas show how the symbol rate determines the maximum frequency span that can be measured:

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The next formulas show, for some signals, how the Symbol Rate can be used to determine the minimum Frequency Span that achieves published specifications. The first formula is valid only for QAM and PSK signals that use the Raised Cosine or Root Raised Cosine filters at the transmitter. The second formula is valid only for VSB signals.

$image\minSpan1_wmf.jpg$

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where: alpha is the roll-off factor set with Alpha / BT

For all other signals, there is no "easy" formula to help determine the minimum span that achieves published specifications. However, it can be ensured that the measurement achieves published specifications by selecting a Frequency Span that contains all energy of the transmitted signal.

To see if your signal falls within the current frequency span, view the spectrum of the signal. If necessary, modify the Frequency Span until it includes all of the signal's energy.

To achieve published specifications, all of the signal's energy must fall within the span. If a Frequency Span or Symbol Rate is selected that causes the Frequency Span to be narrower than (Symbol Rate) ´ (1 + alpha), the VSA displays DATA?.