Troubleshooting (Digital Demodulation)

This topic contains information to help troubleshoot measurements when using Digital Demodulation. Also see Setting up a Digital Demod Measurement for additional measurement information.

In VSA version 20.00, the default values when first selecting the Digital Demodulation measurement type were changed for the following settings:

Symbol Rate - Changed from 24.3 kHz kiloHertz: A radio frequency measurement (one kilohertz = one thousand cycles per second). to Frequency Span/2.

Pulse Search - Changed from ON (selected) to OFF (cleared).

If you use the previous default values for existing measurements, you may get unexpected results unless you change manually these settings to their previous values.

PROBLEM	POSSIBLE CAUSE	SOLUTION
Demodulation fails to lock on signal	Signal not present	Check connections
	Carrier too far from center frequency	Adjust Center frequency
	Input is over-loaded or under ranged	Adjust input Range
	Frequency span is too narrow	Increase frequency Span
	Signal is pulsed	Set Pulse Search ON
	Signal is pulsed and result length is longer than pulse	Decrease Result Length.
	Signal is too noisy	Try selecting the Low SNR Enhancement checkbox (Advanced Tab of Demod Properties). Must be in a modulation format that supports Low SNR Signal-to-Noise Ratio Enhancement.
	Wrong symbol rate	Enter correct Symbol Rate (not bit rate)
	Wrong filter selections	Select proper measurement and reference filters and correct setting for Alpha / BT.
	Wrong modulation format	Select correct modulation format
	I/Q offset too large	Reduce LO feedthrough (or DC offset for I+jQ receiver)
	I/Q misaligned using I+jQ receiver	Ensure that ch1 and ch2 trigger delays are correct

Spectrum Disappears	Spectrum not present or digital demodulation not configured correctly.	View the Spectrum trace (Trace > Data > Spectrum). If Pulse Search is on also examine the time and search time traces (Trace > Data > Time or Trace > Data > Search Time). Use these traces to determine if the spectrum is present or if the digital demodulator isn't configured properly.

Eye diagrams show incorrect symbol timing	Signal path has phase distortion	Adjust clock delay to minimize 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.
	Clock delay is incorrect	Adjust clock delay to minimize EVM
	I and Q are misaligned using I+jQ receiver	Check trigger delay and cable length Adjust for delay using external calibration
	Signal path limits amplitude fluctuations for non-MSK Minimum Shift Keying signals	Remove non-linearity, or compensate with clock delay

SYNC NOT FOUND	Search length too short or search offset incorrect	Increase Search Length, adjust search offset
	Incorrect sync word	Check and correct sync word
	Incorrect state definitions	Change state definitions or sync word
	Wrong MSK demodulator	Select correct MSK demodulator

PULSE NOT FOUND	Signal not pulsed	Turn off Pulse Search
	Search length too short	Increase Search Length or adjust triggering to ensure that leading and trailing edges of pulse are fully within search length
	Signal length not optimized	Try the following method to determine Search Length when performing a Pulse Search: SL = (2*MaxOn) + MaxOff Where:SL is search length in seconds. MaxOn is the maximum on time of a pulse in seconds. MaxOff is the maximum off time between pulses in seconds.

Pulse found but incorrectly aligned	Poor signal-to noise ratio or on/off ratio	Check input Range setting Reduce Span to eliminate excess noise and interfering signals.
	Pulse is asymmetrical	Use Constellation Sync Search to align result length
	Span is too narrow	Increase Span to prevent degradation of pulse edges

Error Vector Time shows large error between symbols	Wrong filter alpha	Adjust filter's Alpha

Error Vector Time shows regions of large errors	Distortion or a poor signal to-noise ratio causing bit errors	Check input Range
	Large frequency errors interfering with carrier lock	Adjust Center frequency
	Large I/Q offsets causing bit errors	Reduce LO feedthrough (or DC offset if using I+jQ receiver)

EVM larger than expected and Error Vector Time shows errors that increase towards beginning and end of result length	Wrong symbol rate	Make fine adjustments to the Symbol Rate
	Result length larger than pulse	Decrease Result Length

EVM larger than expected and Error Vector Time shows a uniform distribution of errors	Frequency span too narrow	Increase Span
	Input is overloaded or under ranged	Adjust input Range
	Result length is too short	Increase result length
	Wrong filter selections	Check that filter selections allow the filtered measured signal to match the reference signal
	Channel conditions have biased symbol timing recovery	Observe eye diagrams and adjust clock delay to compensate symbol timing recovery.
	Time corrections disabled	Enable time corrections
	Signal too close to DC	Increase carrier frequency
	I and Q misaligned using I+jQ receiver	Check trigger delay settings and cable lengths
	AC coupling when using I+jQ receiver	Select DC coupling on both input channels
	I and Q imbalance	Ensure impedance is identical for both inputs
	Sync enabled and sync word not unique. Result incorrectly positioned with respect to signal	Ensure that sync word is unique
	For MSK type 1, signal contains I/Q offset	Select MSK Type 2 to remove I/Q offset term from result

The trace data constellation symbol location shape, such as dots, is not properly displayed.	The display driver is not functioning correctly.	Contact your PC manufacturer to see if a newer version of the display driver is available. If not, try decreasing the hardware acceleration. See the Windows documentation for instructions.