IQ Noise Correction

Menu Path: Input > IQ Noise Correction

IQNC is currently under BETA support. This means that the programming API 1) Access Preamble Indicator, or 2) Application Programming Interface or measurement results may be improved in a future release.

IQ Noise Correction (89601EVMC), or IQ-NC, is a feature that can be used to compensate the acquired signal such that the measured 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. will not include the contribution from the analysis hardware. This is done by acquiring multiple repetitions of the same signal and using averaging to separate the noise from the signal, scaling the noise, and then recombining.

Enabled - enables IQ Noise Correction. If the Enabled checkbox is grayed out, hover over the "(Not Supported)" message to see a tool tip with possible reasons. See IQ-NC Tips and Assumptions below for more information.

Average Count - number of repetitions of the signal to acquire and use during IQ Noise Correction. Increased Average Count will improve the accuracy of the resulting EVM at the expense of speed and memory. You can increase the count and see if the EVM changes significantly. At some point, there will be diminishing returns and the EVM should stabilize.

(Max) Repetition Period - (Signal Mode = Bursted)

Auto-detect = unchecked: specifies the exact time between repetitions of the burst.
Auto-detect = checked: specifies the maximum time between repetitions of the burst. This will not be as fast as when Auto-detect is unchecked and the exact period is specified.

Signal Parameters

Auto-configure from Measurement - When enabled, the following parameters are auto-configured by the measurement:

Signal Mode

(when Signal Mode = Continuous) Repetition Period and the corresponding Auto-detect parameter

Note: Changing any of the auto-configured parameters will cause the Auto-configure from Measurement check box to be disabled.

Signal Mode - indicates the type of signal being corrected and influences the search for repetitions within the acquisition.

Bursted - signal consists of bursts, with idle time between. The amount of idle time is allowed to vary when Auto-detect is checked. Otherwise it is expected to be constantly repeating with the specified Repetition Period.

Continuous - signal is transmitted continuously without gaps. If the repetition period is known, Auto-detect may be unchecked and the period specified. Otherwise, Auto-detect should be checked.

Repetition Period - (Signal Mode = Continuous and Auto-detect is unchecked) specifies the expected time between repetitions of the signal.

Repetition Period Auto-detect - (Signal Mode = Continuous) when checked, the exact period of the signal will be autodetected. This will not be as fast as when Auto-detect is unchecked and the exact period specified in Repetition Period.

IQ-NC Tips and Assumptions

There are several assumptions that must be followed to ensure correct measurement results:

the signal must be gap-free in both time and frequency. While it is fine that a signal is bursted, within the burst there should not be any "off" portions.
the signal must be the same for each repetition
the noise profile can be measured accurately at the current DUT Device under Test: An acronym used to describe some type of electrical apparatus connected to test instrumentation. The apparatus can range from a single component to a complex subsystem such as a mobile phone, base station or MSC. power level. Some analysis hardware can internally terminate the input, but others can only attenuate. When enough attenuation is not present to reduce the signal sufficiently, the analyzer noise will be estimated as larger than it actually is and the EVM value may be over-compensated (lower than it should be). To check whether this may be happening, check the Noise Correction Spectrum trace for any signal leakage. If present, you can reduce the DUT's transmit level.
the signal's occupied bandwidth should fill the VSA's Measurement Span (the DUT+analyzer noise is computed over the Measurement Span)

When the noise ratio (analyzer / (DUT+analyzer)) approaches 1.0, the accuracy of the IQNC measurement decreases. This is typically the case for low DUT powers where the analyzer is using significant amplification. In these cases, it is possible to see an IQNC EVM that is several dB below the theoretical kTB EVM limit. This is due to the need for increasingly better accuracy of the noise profile measurement as the ratio approaches 1.0.

IQ Noise Correction is only able to compensate the analysis hardware's uncorrelated noise component. Contribution of noise from other sources (correlated with the data, such as distortion) cannot be reduced using this method.

While it is possible to look at results such as IQ constellations and Error Vector Spectrum traces, these traces will still show the effect of the analyzer noise contribution, but scaled such that the overall EVM metrics do not include the uncorrelated noise power of the analyzer.

To improve measurement speed, follow these recommendations:

For bursted signals, reduce the idle time between repetitions
Configure the Repetition Period and de-select Auto-detect. This will reduce the amount of data to acquire and also reduce the searching required to find the repetitions.

IQ Noise Correction can be used with any repeating signal and any measurement. However certain measurements provide some level of auto configuration of the IQ Noise Correction parameters.

If IQ Noise Correction cannot be performed, the Search Time (or Main Time) trace will show an IQNC? trace LED. If you click on this LED, you can see the reason why IQ Noise Corrections cannot be performed. The typical case is that not enough repetitions could be found.

To use IQ Noise Corrections, first configure your measurement to successfully measure EVM. Then make sure the signal is repeating and configure the Average Count you wish to use. If the signal is a bursted signal, configure the Repetition Period parameter. Then you can click the Enabled check box to perform IQ Noise Corrections.

Cases where IQ Noise Corrections is not supported:

Logical I+jQ input channel operation
Not licensed (requires 89601EVMC license)
Power Spectrum measurement (SEM/ACP Adjacent Channel Power: The power from a modulated communications channel that leaks into an adjacent channel. This leakage is usually specified as a ratio to the power in the main channel, but is sometimes an absolute power./Power Spectrum)
Segmented Capture enabled
Noise profile is not available from the current hardware (or recording)
Other reasons supplied by the current measurement

To see whether your analysis hardware supports IQ Noise Corrections, see your hardware's About topic (under Measurement Platforms > Available Measurement Hardware) and look for the IQ Noise Correction section. If present, it will indicate which hardware models are supported.

When Signal Mode = Bursted, the output of IQ Noise Corrections (typically in a Search Time trace) will contain the corrected burst, but the points outside the burst will be set to a constant small value. This is due to several reasons, but basically to avoid compensating IQ samples that do not contain a signal.