Receiver Modes

The new Phase III receivers (including the 16 SHAO receivers and 2 NI receivers), are capable of operating in either critically sampled or oversampling modes.

• Critically Sampled mode implements the same internal critically sampled Polyphase Filter Bank (PFB) as the legacy RRI receivers. This means they can be operated in conjunction with the legacy RRI receivers, thus expanding the array beyond 128T.

• Oversampling mode implements a new oversampling PFB within the Phase III receivers in addition to some special handling of the data within MWAX. The new oversampled PFB provides the capability of capturing a continuous spectrum across the full 30 MHz bandwidth without the need to flag channel edges that would have otherwise been corrupted due to aliasing. This mode is exclusive to Phase III receivers, and is therefore limited to the compact array (because the Phase III receivers are attached to the compact array).

Critically Sampled Mode

The MWA samples raw voltages at a rate of 655.36 Msps before the Polyphase Filter Bank (PFB) subsequently channelizes the spectrum into 256 individual “coarse” channels between 0 and the Nyquist frequency. In critically sampled mode, each channel is 1.28 MHz wide and spaced 1.28 MHz apart. The filter response of a single coarse channel can be seen in Figure 1, which also highlights one of the shortfalls of the critically sampled PFB, i.e. aliasing. In the figure, the pale blue traces show how the aliased components of the filter response wrap back into the channel, highlighting how the filter does not sufficiently attenuate aliased signals near the channel edges, thus contaminating the data.

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Figure 1: Critically Sample PFB Filter Response

Figure 2 shows an otherwise flat input spectrum as measured within a single coarse channel of the critically sampled PFB. After the flat spectrum has been shaped by the filter and then contaminated by aliased signals, the resulting power response at the channel edges fall to only -3dB compared to the center of the channel. According to the ideal filter response in Figure 1 (see more details here), it should be -6dB, however the power from the aliased components has increased it to -3dB.

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Figure 2: Critically Sampled PFB - The measured power after aliasing for an otherwise flat input spectrum.

Oversampling Mode

The Oversampling Polyphase Filter Bank (PFB) also channelizes the spectrum into 256 “coarse” channels resulting in the same channel spacing of 1.28 MHz. However, in this case, it oversamples each channel by a factor of 32/25 resulting in a channel width of 1.6384 MHz. This results in coarse channels that overlap, and importantly, these overlapping regions are redundant and can be discarded without any loss of data. The filter coefficients of the new oversampled PFB have been designed such that aliasing only occurs within the redundant region, and is therefore discarded. Figure 3 shows how the increased bandwidth of the oversampled channel (blue) pushes the filter transition region outside of the 1.28 MHz wide band of interest (denoted by vertical black lines). The regions highlighted in blue are redundant, and are discarded within the MWAX correlator, and aliased signals within the band of interest are attenuated by at least 70 dB.

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Figure 3: (black) Critically Sampled PFB Response. (blue) Oversampled PFB Response.

Once the overlapping regions have been discarded, each coarse channel can be concatenated with adjacent channels to form a flat spectrum as shown in Figure 4. The remaining pass-band ripple (shown within the expanded box) is bounded by +- 0.25 dB, and can be digitally equalized within the correlator at the ultra-fine channel level, if ‘deripple’ is enabled.

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Figure 4: Constructing a flat spectrum from adjacent oversampled coarse channels.

In oversampling mode, the MWAX correlator handles the job of fine-channelising the oversampled coarse channels and discarding the redundant regions prior to correlation. The width of a coarse channel at the output of the MWAX correlator is therefore still 1.28 MHz.

Observations that use the Voltage Capture System (VCS) contain the raw output from the receivers and therefore still have the full 1.6384 MHz bandwidth. This has implications on existing data processing pipelines and data storage requirements.