This topic covers configuring the TDECQ operator when it is a secondary to a Reference Receiver operator.

Low-SNR Measurement Mode

The Low-SNR Measurement Mode setting allows the TDECQ operator to operate when large amounts of noise is present on the signal. However, it is recommended that you instead use System Impulse Response Correction configured with the Intrinsic Noise/Jitter Reduction setting turned on (default setting). To locate this setting, in the Channel dialog click Configure to open the System Impulse Response Correction Setup dialog.

Starting with FlexDCA revision A.08.30, the TDECQ operator's Low-SNR Measurement Mode setting is turned off. Low-SNR Measurement Mode requires the L-RND or L-MFG package licenses.

Specify Pattern Symbol Sequence

Use these settings to specify input waveform's pattern symbol sequence (NRZ or PAM4). You can select to have the pattern automatically detected (default), select from a know standard Known Pattern, or a BERT Pattern File (*.ptrn). Available known patterns are listed here. If you select to import a Pattern File, you can specify if the pattern uses Gray Coding. Gray coding, or reflected binary code, is a coding pattern where successive symbols differ by one binary bit. For example in the case of PAM4, binary bit sequences 00, 01,10, and 11 represent levels 0, 1, 2, and 3.

Aliased Noise Processing

The TDECQ equalizer's Aliased Noise Processing compensates for the filter function's side effect of reducing a signals noise for adjacent measurement points. Processing noise causes the operator to quantify the noise on each point on the operator's input waveform and restore this noise on the corresponding data point on the output waveform. The processing noise setting (Process Spectrum) setting is permanently turned on and you cannot turn it off.

With the TDECQ operator, the Process Spectrum selection is the only option and is always selected. Although the other Noise Processing setting are unavailable, the following paragraphs describe the way FlexDCA generally handles noise processing with other operators.

None Selection

The option None does not make any special considerations for aliased noise. This is an appropriate choice when the waveform is known to have no aliased components, such as from a real-time or simulated source. This also is the preferred choice when dealing with averaged waveforms where the desired outcome is to remove noise and interference components that are not correlated to the trigger signal.

Process Spectrum Selection

The option Process Spectrum works by applying the transfer function of the filter to the power spectrum of the noise to determine the appropriate magnitude of the noise on the output signal.

The default behavior of the Process Spectrum noise processing option is to use the noise power spectrum of the input signal. If the input signal is a sampling scope channel with SIRC active, this spectrum will be established by the measured hardware response of the channel. For other channels, the response will be assumed Gaussian with a 3 dB frequency corresponding to the nominal channel bandwidth. This behavior can be overridden by clearing the Track Input Response checkbox and manually entering a bandwidth. If this option is utilized the response will be presumed Gaussian with the selected 3 dB bandwidth.

Preserve RMS Selection

The option Preserve RMS is appropriate when the noise bandwidth is very low relative to the channel and filter bandwidth. It is also appropriate when the aliased components are known to have most of the power within a range of in-band frequencies. This could be from laser RIN, for example, or from intentionally modulated interference/crosstalk. When Preserve RMS is selected, the RMS magnitude of the aliased components will be scaled by the DC gain (the sum of the taps) of the filter. For the low-pass filters (Bessel, Butterworth, Gaussian, Sinc), the DC gain is unity. For the equalizers and embedding/de-embedding operators, the DC gain depends upon the settings of the operator.

By tracking the accumulated effects of the filtering operations, accurate noise processing can be done even when chaining operations as illustrated in the following figure. In addition to the sampled waveform, information about the acquisition channel and noise power spectrum are maintained in each signal and appropriately processed by each filter. The complete set of auxiliary information is also included when storing FlexDCA waveforms in the *.wfmx file format.