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[<< Home](/home#5-digital-back-end-design-panel-charge-4)
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[<< Section 4.4](/4-signal-transport/4.1)
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## 5.1 Digital Back End Capabilities and Specifications
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### 5.1.1. Functional Description
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The digital beamformer back end subsystem receives 138 L-band RF signals (plus up to 6 auxiliaries) over optical fiber analog downlinks from the antennas on the RF front end phased array feed (PAF) cryostat at GBT prime focus. It linearly combines the large number of antenna outputs into proper illumination patterns on the dish to produce 40 separately steered beams on the sky. The digital beamformer performs nine principal real-time signal processing functions:
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1. Direct RF sampling (with no analog mix down) with 14-bit ADCs
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2. Digital down-mixing and sample decimation to complex baseband representation
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3. Frequency coarse channelization of all antenna data streams using an oversampled polyphase filter bank (OSPFB, which is oversampled to eliminate spectral gain scalloping and aliasing of conventional filter banks)
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4. A corner turn operation to reorder data streams from the channelizer, which are ordered by antenna index, into frequency channel ordered streams which can be partitioned out to the beamformer processors, each responsible for a range of channels across all antennas.
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5. Digital beamformer computation using separate complex antenna weights for each frequency channel and each of 40 dual polarized beams.
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6. Fast dump integrating spectrometer over all coarse channels, for use in Pulsar and FRB searches and mapping
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7. A second-stage "zoom" fine-channelized spectrometer for very narrowband resolution HI and spectral line searches and mapping
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8. Full array antenna coarse channelized cross correlator for beamformer weight calibration
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9. Data product formatting and writing to high speed distributed file storage
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The figure below illustrates the principal hardware components and signal interconnections for the digital beamformer back end. The F-Engine implements functions 1 through 3 using 12 RF System on a Chip (RFSoC) boards, as discussed in [Section 5.2](./5.2). Not shown in the block diagram, but connected to RFSoC boards and housed in the rack mount chassis for the RFSoC are the RFoF receiver downlink boards as described in [Section 4.3](../4-signal-transport/4.3).
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The XB-Engine (correlator/beamformer) implements functions 5 through 9 using 25 high performance blade servers hosting 50 data center class Ampere series graphical processor units (GPUs) as described in [Section 5.3](./5.3). Finally, the corner turn of function 5 is implemented by the 100 Gigabit ethernet network interfaces on the RFSoCs and in the servers, and the 60-port 100 GbE ethernet switch which re-routes the data packets.
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<div align="center">
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<img src="../uploads/70f52dc741b8afaf2c3545b19dd02d32/Design_Specifications_4_slide_8.png">
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</div>
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#### 5.1.2. Operational Modes
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The XB-Engine subsystem of the digital beamformer back end may be configured by the instrument operator/observer to function in three distinct signal processing modes. This is accomplished in the command and control software by loading and running one of three different software images in the graphical processor units (GPUs). The signal processing modes are described below
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**Pulsar/Transient Coarse Spectrometer Mode**
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This mode is used primarily for pulsar or transient FRB surveys. As shown in the figure below of the GPU signal processing functional blocks, internet data packets with frequency channelized signals from the F-Engine are first linearly combined in the beamformer using distinct weights for each antenna, frequency channel, and beam number. This forms 40 dual polarized beams on the sky for every coarse frequency channel (1250 channels interspaced by 244.1 kHz). Next, a full-Stokes integrating power spectral density estimate is computed for each beam, with coarse (244.1 kHz channel separation) frequency resolution. The operator/observer may select integration dump intervals as short as 64 microseconds. These spectrometer data products are saved to the distributed file storage system through the InfiniBand switch.
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Pulsar detection processing is accomplished with a user-provided post-processing software pipeline operating on the coarse spectrometer data products to perform a dispersion measure search.
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<div align="center">
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<img src="../uploads/a5077920a6f2ad5237aad551b3474064/Design_Specifications_4_slide_19.png">
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</div>
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**HI/Spectral Line Fine Spectrometer Mode**
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This mode is primarily for narrowband spectral line surveys, such as HI or OH detection. As in Pulsar/Transient mode, the coarse channelized data is first beamformed, then a second stage "zoom" spectrometer breaks up each coarse channel into 192 fine channels (1.27 kHz fine channel bandwidth resolution) using a conventional polyphase filter bank (PFB) implemented on the GPUs. Since the F-Engine FPGA channelizer is an oversampled polyphase filter bank (OSPFB) implementation, the second stage zoom spectrometer does not suffer from the spectral gain scalloping and coarse band edge signal aliasing inherent in many 2-stage spectrometers.
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Following the zoom PFB, a fine resolution, a full-Stokes integrating power spectral density estimate is computed for each beam, with fine (1.27 kHz channel separation) frequency resolution for up to 96,000 channels spanning 122.1 Mhz. These spectrometer data products are saved to the distributed file storage system through the InfiniBand switch.
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<div align="center">
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<img src="../uploads/f546e80666563edf42508607ceb520d8/Design_Specifications_4_slide_18.png">
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</div>
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**Beamformer Weight Calibration Mode**
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The beamformer requires periodically updated (order of a few weeks) beamformer weight values for every beam and frequency channel. This is necessary to preserve beampattern shape as the electronic signal chains, array antenna element electromagnetic interactions, and dish optics drift over time. The weights are recomputed by collecting array statistical covariance data between all element pairs while steering the dish so that the beam direction being calibrated points toward a bright on-sky calibration source. This is repeated in a grid pattern until all 40 beam directions have produced calibration covariance matrix data. The figure below depicts the signal processing algorithm implemented in the GPUs to compute antenna array covariance matrices for beamformer calibration.
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<div align="center">
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<img src="../uploads/822c4c88aec2904db434ddaa2676c526/Design_Specifications_4_slide_20.png">
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</div>
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#### 5.1.3. Beamformer Specifications
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The following three figures present key performance parameters for the digital beamformer back end.
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<div align="center">
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<img src="../uploads/481ac968c30bb93fce87d89f963db7f6/Design_Specifications_4_slide_2.png">
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<img src="../uploads/1e3b6220dd8287d1ff0e259e83b0fdbd/Design_Specifications_4_slide_3.png">
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<img src="../uploads/63629236e9e84de3e18cb012356d2866/Design_Specifications_4_slide_4.png">
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</div>
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[Section 5.2 >>](./5.2) |