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[<< Home](/home#3-front-end-design-panel-charge-3)
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[<< Section 3.7](/3-front-end-design/3.7)
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## 3.8 Adapting the ALPACA Cryostat for GBT Prime Focus
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In the original Arecibo based design, the second stage amplifiers, filters and fiber optic transmitters for the 138 RF channels were on 36 Printed Circuit Boards (PCBs), 4 channels to a board. Pairs of boards, with RF fed from an 8-wire stripline exiting through the top plate of the cryostat, were housed in 18 vertical boxes arranged radially around the outer area of the top plate of the cryostat.
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With the potential move to the GBT, it was found that this ring of PCB boards and boxes had a diameter larger than could accommodated by the prime focus cage on the GBT when the dipole array was located at the approximate focal plane. The solution adopted was to make the PCB pairs horizontal instead of vertical lowering the vertical height of the cards and enclosure to about 3 inches. The 18 pairs of PCBs just fit around the circumference of the top plate of the cryostat and removable covers allow the PCBs to be removed and replaced in case of a failure.
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<div align="center">
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<img src="../uploads/641d4c52272c529b37516c50d0a26d88/image.png" width="480"></div>
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<kbd>
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Figure 1: The two 18-sided rings attached to the cryostat top plate (pink). Rails attached to the green side plates are used to insert the PCBs, pale green and blank in this image. The four connectors can be seen on the foreground part of the image, left to tight: 4 fiber connectors, 4 strip line RF connectors, 4 bias lines plus power lines and a single calibration signal connector. The bias current is carried on the RF signal lines from the LNAs
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</kbd>
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### Cart for transport
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<div align="center">
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<img src="../uploads/032d4b6de554158feac724b6db421cf1/cryostat_in_cart.png" width="480"></div>
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<kbd>
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Figure 2: The complete cryostat assembly as currently designed. It is shown mounted on the transport cart (already built). From the bottom: White, the HDPE cover over the foam layers; Green, the dewar itself and the height adjustable red lifting lugs needed for installation in the GBT prime focus cage; Red, the openings for the two second stage removable PCBs. Weather proof covers will go over the openings. Blue, a cylinder extension that will attach to the rotatable Sterling mount attached to the prime focus cage. Helium lines, fiber optic lines, bias supply lines, etc will come through this cylinder to the equipment box above (also blue). All dimensions are in inches.
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</kbd>
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### Cryostat-Sterling Mount Interface
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One major difference between the Arecibo and GBT installations is that ALPACA’s location inside the temperature controlled Gregorian dome receiver room of the Arecibo telescope shielded it from the elements. At the GBT prime focus, ALPACA will be open to the elements including large temperature changes. Auxiliary equipment such as the bias supply are housed in an equipment box that has to be weather proofed and temperature controlled (Figure 2). The precise design of this box has not yet been completed, it is waiting on more input from GBO technical staff.
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### Blind-mate Connection from Stripline to RFoF PCB
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<div align="center">
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<img src="../uploads/3db16cadba6b0d51f096b196ce7bb509/Rfof_Blindmate.jpg" width="480"></div>
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<kbd>Figure 3: All the connectors (Fiber, SMP and Bias) on the RFoF PCB (green) and the interface plate (grey) at the cryostat have been carefully chosen and the their placement modelled in CAD to enable a blind-mate connection.
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</kbd>
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- The blind-mate configuration will help in installation and any maintenance and servicing operations.
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- Custom ganged connectors were designed by us and are being built by Delta RF for the RFoF PCB and the flexible striplines. These implement guide pins to guide the SMP bullets to the connectors. The fiber connectors are held in place on the PCB using a custom 3-D printed bracket.
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- Please refer to [Section 4.1](../4-signal-transport/4.1) and [Section 4.2](../4-signal-transport/4.2) for more details on the RFoF PCB.
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[Section 3.9 >>](/3-front-end-design/3.9) |
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\ No newline at end of file |