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[<< Home](/home#4-signal-transport-panel-charges-3-and-4)
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[<< Section 4.2](./4.2)
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## 4.3 Receiver Design
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Prior to signal processing on the digital backend, the analog signal on the single-mode optical fiber must be converted back to an electrical signal, amplified, filtered, and sent into the ADCs of the RFSoC on the Xilinx ZCU216. This is the role of the RF over Fiber receiver.
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### 4.3.1 Receiver Design and Construction
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The first component of the receiver is the photodiode, which acts as a high impedance current source supplying current that is proportional to light intensity on the optical fiber. As this light is modulated by an RF signal on the TX card, the output current is proportionally modulated, and the RF signal can be isolated using a DC blocking capacitor.
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Junction capacitance of the photodiode is 0.6 pF, which introduces about 177 Ohms of reactance to the signal at the center of our band and causes a declining slope in gain with increasing fequency. To compensate for the parasitic capacitance, along with gain ripple due to mismatch, we placed an inductor virtually in parallel with the photodiode to create a resonance at the center of our band, making the gain curve more symmetric. At the cost of 6 dB of midband gain at the resonant peak, we then flattened response by placing a 50 $`\Omega`$ resistor in parallel with the photodiode to match the source.
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The RF signal output from the photodiode is amplified by 15 dB before passing through a custom 1300-1720 MHz bandpass antialiasing filter. A directional coupler follows this filter to inject a reference signal for calibrating ADC relative phase. The 50 $`\Omega`$ single-ended signal line is then converted with a balun to a 100 $`\Omega`$ differential pair compatible with the ADC inputs of the Xilinx ZCU216 RFSoC board.
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#### Single-Channel Receiver and Balun Board
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Single-channel RFoF receiver boards were fabricated to test performance of our circuit deigns and board layout up to the filter. The coupler and balun were not included for these test boards. A simplified schematic diagram and image of our most recent single-channel RX test board are shown below:
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<div align="center">
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<img src="../uploads/d6e1f2215c3dd280cbceffa325b2fb36/1_ch_RX_Schematic.PNG" width = "470"/>
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<img src="../uploads/56008a263b7cccf835381c18328a7bbe/IMG_0853.jpg" width = "470"/>
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Figure 1: Left: Simplified single-channel RFoF receiver schematic, Right: Single-channel RFoF receiver.
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</div>
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To interface with the ZCU216 RFSoC we built a 16-channel balun board, with couplers for phase calibration signal injection included on each channel (see Fig. 2). With exception of the SMA connectors and T-lines to the couplers, the layout of this board is practically identical to post-bandpass-filter signal chain of the 16-channel RX boards we are developing. This allows us to characterize performance of all post-RX test board components and T-lines of the RFoF receiver sub-system. Using the single-channel RX test board in conjunction with the 16-channel balun board, we can characterize the full RX system performance.
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<div align="center">
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<img src="../uploads/2cf45c4ef5652f1a53f05bb30bf54398/IMG_0867.jpg" width = "600"/>
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Figure 2: 16-channel balun board.
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</div>
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#### 16-Channel Receiver
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As seen in Figure 3, the final version of the RFoF receiver sub-system is a 16-channel RX board. This board can support up to 16 receiver channels, but only 12 channels will be populated, which corresponds to the 12 ADC inputs per RFSOC which will be used for ALPACA. The 16 channel RFoF receiver uses both top and bottom surfaces to mount components, with 8 channels on the top layer and 8 channels on the bottom. A partially populated version of this board used for testing is shown in Figure 3.
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<div align="center">
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<img src="../uploads/b79c01413575357e96a1ff73d3962d96/IMG_0850.jpg" width = "500"/>
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Figure 3: Partially-populated 16-channel receiver test board.
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</div>
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### 4.3.2 Receiver Performance
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To test receiver performance, we must include a laser in the test setup. We have a standalone laser circuit board that we use for testing with a flat response over frequency but 3.5 dB lower response than the anticipated final matched laser transmitter design. A Vector Network Analyzer was used to find the gain of three single-channel RX boards operating with an unmatched laser transmitter board (see Fig. 4). With a broadband matched laser, the gain would take the same shape but would be 3.5 dB higher. This test did not include couplers, baluns, and differential signal pairs into the high density connector on the ZCU216.
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<div align="center">
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<img src="../uploads/caffe61f776c09162b5cca3ecdc46490/RX_module_gain_with_L1_50ohms_in_par_w_PD.png" width = "600"/>
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Figure 4: Gain of unmatched laser connected to single-channel receiver boards.
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</div>
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Figure 5 shows our physical setup of devices for testing the noise and linearity characteristics of the full receiver system with a laser circuit board (at the bottom right of the image) as the transmitting device. Since the laser introduces more noise and non-linearities than the rest of the devices in the signal path, we could not isolate the noise and linearity of the receiver system. As the system dynamic range is determined primarily by the laser and ADCs, we have demonstrated that the receiver board meets specification in terms of linearity.
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<div align="center">
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<img src="../uploads/7ec4ced5c07631e4a26fc71cc2a94de4/IMG_0872.jpg" width = "500"/>
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Figure 5: Physical setup of the single-channel receiver connected to the 16-channel balun board on the Xilinx ZCU216.
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</div>
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[Section 4.4 >>](./4.4) |
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\ No newline at end of file |