Woody Underwood
About Me
My name is Woody Underwood. I'm a junior entering my seventh semester in the physics program at UConn. Since the summer of 2008, I've been working in the lab of Dr. Richard Jones designing electronics for the US Department of Energy's GlueX experiment. My assignment is to develop three circuit boards that work together to tag photons coming from the diamond radiator. My circuitry essentially measures (indirectly) the energy of these photons in order to determine if they are of interest to GlueX. The three boards I am designing consist of a digital board, an analog board, and a connecting backplane.
About My Circuitry
Electrons leaving the diamond radiator are deflected by a magnetic field into an array of scintillating fibres, producing photons. Wave guides carry these photons to SiPMs (silicon photomultipliers) mounted on the analog circuit board. The analog board contains transimpedance amplifiers and summing circuitry to condition the signals for digitization.
The sensitivity of the SiPMs and the gain of the amplifiers on the analog board are controlled both by the power supply VCC and bias voltages supplied from the digital board. The digital board receives commands from a computer via ethernet, and uses a 32-channel DAC to output appropriate bias voltages to the SiPMs on the analog board. The digital and analog boards are connected by means of a backplane, which is also responsible for providing power and grounds to both boards.
Summary of Fall 2008 Work
All circuitry design work was done using Altium Designer. The digital board was the first to be designed. The first step in designing the digital board was to review the list of key components that had already been selected by Igor and Dr. Jones. These components included such things as the Xilinx Spartan-3A FPGA, and the Analog Devices AD5535 DAC. I began by looking through datasheets for these components to find out their needs, including power and decoupling requirements. I reviewed the pinout diagrams, and then looked through Altium’s standard libraries to find components that matched (in many cases the particular component I was looking for was not in the library, but a similar footprint or schematic symbol was). For components without matching schematic symbols, I entered pinout information from the datasheets into Microsoft Excel, using a layout compatible with Altium’s Smart Grid Insert function. Then, I was able to literally copy and paste pin information from Excel into Altium to generate the schematic symbols I needed.
Once I had appropriate schematic symbols available for all parts, I began making appropriate connections in the schematic view in Altium. Though tedious, this task was not exceedingly difficult. I finished the schematics in several days, and then moved on to PCB design. I switched into Altium's PCB view. The footprints corresponding to the components I used in the schematics were automatically inserted by Altium. My job was then to position these components in logical places on the board and make all of the connections corresponding to the nets defined in the schematics.
Due to the large number of components being placed in the limited space available on the digital board, Altium's auto-router proved completely useless. Therefore, I routed the board manually. Despite Altium's revolutionary convergence of schematic and PCB design into a single program, this was no easy task. During the routing process, I had to take into account not only the connections that had to be made, but also things such as avoiding crosstalk and minimizing trace length for sensitive components. I was able to complete routing after several weeks of work. The digital board design has since been completed and the board has been printed. It is currently awaiting assembly.
The analog board provided a host of new challenges. The basic schematic for the transimpedance amplifier on the analog board was completed by Igor and Dr. Jones before the semester. Inputting the schematic into Altium was not very difficult. However, one problem I encountered was that the analog board contains 32 copies of this amplifier circuitry. After failing to find any way to insert multiple copies of both the schematic and its corresponding PCB layout, I decided to insert only single copies of each schematic page, and copy and paste the PCB layout to produce 32 copies of the amplifier circuitry. At first this seemed like a quick and easy way to get all of the necessary circuitry onto the PCB. However, I eventually discovered that this procedure would lead to major problems with the board assembly process (due to duplicate component designators, and for other reasons). Fortunately, this revelation came around the same time that Igor and Dr. Jones found a problem with the performance of the amplifier circuit. Making any changes to the amplifier circuit at this point will require a major reroute of all the traces on the analog board. Since the board needs to be completely redesigned anyway, this will give me another chance to find a way to match schematics with all 32 copies of the amplifier circuitry.
The backplane design is currently in progress. It should be relatively easy to complete. All that remains to be done is to add the LEMO connectors and power inputs. The board is simple enough that it can be routed completely by the auto-router, though a quick hand routing will probably be superior. I anticipate that I can complete the backplane with a few days of concentrated work over break.
Included below are links to the files I have been working on. Included in the files for each board is a "SmartPDF," viewable in Adobe Reader. For those without Altium Designer, these may be the best files to look at. They include complete schematics and PCB layout, and are also indexed by component.
Any questions about the tagger circuitry can be directed to me at mitchell.underwood@uconn.edu
Related Files
- DigitalBoard.zip:
- Altium Project File (SiPM Control Board.PrjPcb)
- Altium PCB Layout File (Prototype1.PcbDoc)
- Altium Schematic Files (*.SchDoc)
- Altium Annotation Document (SiPM Control Board.Annotation)
- Not used, but generated by Altium when opening the project
- Altium PRJPCBSTRUCTURE File (SiPM Control Board.PRJPCBSTRUCTURE)
- Not used, but generated by Altium when opening the project)
- “SmartPDF” of the board and schematics (SiPM Control Board.pdf)
- Can be used to explore the PCB layout and schematics without needing Altium
- Pick and Place File for board population (Pick Place for Prototype1.txt)
- Used by board assembler
- NC Drill Files (Prototype1.txt, Prototype1.DRR, Prototype1.DRL)
- Used by board printer
- Gerber Files for all layers (in folder Gerbers)
- Used by board printer
- Altium CAMtastic file (CAMtastic4 FINAL.Cam)
- Basically a composite of all the Gerbers
- Photos and 3D rendering of populated board
- In folder “Photos”
- EMF Files showing different layers
- In folder “EMF Renderings”
- AutoCad File of PCB (Prototype1 Autocad.DWG)
- Altium Library of Custom Footprints for Digital Board (GlueX IC Library.SchLib)
- Current as of completion of digital board
- This library has since been updated for the backplane
- Analog Board 20081211.zip:
- Altium Project File (AnalogBoard.PrjPcb)
- Altium PCB Layout File (AnalogBoardPCB.PcbDoc)
- Altium Schematic Files (Amplifer1.SchDoc, Summer.SchDoc)
- “SmartPDF” of the board and schematics (AnalogBoard.pdf)
- Altium Component Definition for SiPM (SiPM Library.PcbLib)
- Contains part footprint and pin information for the SiPM component
- Backplane 20081211.zip
- Altium Project File (Backplane.PrjPcb)
- Alitum PCB Layout File (Backplane.PcbDoc)
- Altium Schematic Files (Analog Connector.SchDoc, Digital Connector.SchDoc)
- Analog Connector = Eurocard to analog board
- Digital Connector = Eurocard to digital board, +3.3V voltage regulator, and location identifier jumper
- LEMO connections not yet included in these schematics
- “SmartPDF” of the board and schematics (Backplane.pdf)
- Pin layout files used to define pinouts for custom components (Pin Layout, 96 pin connector.xlsx, Pinouts.xlsx)
- Pin Layout, 96 pin connector = pinout definition for 96 pin Eurocard connector
- Pinouts.xlsx = pinout definitions for digital board, which were reused for the 48 pin digital Eurocard receptacle on backplane
- Altium Library of Custom Components (GlueX IC Library.SchLib)
- UPDATED to include new backplane components
- An older version of this library was used for the digital board
Summary of Spring 2009 Work
At the end of the fall semester, Dr. Jones, Igor, and I determined that the amplifier/summing circuit we had designed simply didn’t have sufficient performance characteristics to be useful for GlueX. During the beginning of the spring semester, Igor came up with a new design, utilizing more transistors, to provide the high gain, fain response, picoseconds resolution amplifier that we needed. The design performed flawlessly both in MatLab simulations and in a handmade single channel prototype. The first of my goals for the spring 2009 semester was to capture the schematic for this new amplifier into Altium designer, and layout a new amplifier board. The second of my goals, of course, was to complete production of the digital control board prototypes.
Since the fall semester left us with three unpopulated digital control board PCBs, getting those PCBs assembled with their components was the first priority. I began the semester by tracking down all of the components we needed (some of which were selected at the end of the fall semester), and making appropriate substitutions for components whose availability had changed since the fall. In the process of selecting these components, I noticed several places where it seemed like power consumption on the board may be somewhat high. To fix this, I developed a spreadsheet in Microsoft Excel that calculates optimal resistor values to use for to obtain a specified voltage divider stiffness. With this tool, I was able to optimize power consumption across the board, and select appropriate components. Once all components had been selected, ordered, and received, we sent the order out to Screamin’ Circuits for assembly. The boards came back several weeks later, and are currently awaiting testing by some undergraduates who will be in the lab this summer.
While Igor was finalizing his amplifier/summing circuit, I worked briefly on design of the backplane. A number of details regarding trace impedance and board dimensions were ironed out. Nonetheless, many problems still remain which I will need to tackle over the summer. The first of these problems is that we have yet to find an appropriate low cost coaxial connector to route signals off the backplane. In addition, screws with which to mount the backplane to the tagger must be selected so that appropriate holes can be created for them on the PCB.
Once we were satisfied that the amplifier/summing circuit performed as required, I shifted work from the backplane to the amplifier board. Around the same time, I began working on a poster to present my work at the Frontiers in Undergraduate Research Exhibition held during Open House Weekend here at UConn. If you’re interested in my poster, check it out here.
To start the amplifier/summing circuit project, I searched high and low for information about how to handle multi-channel designs in Altium. Not surprisingly, the first Google result on the query “multichannel design Altium” had everything I was missing during the fall when I was trying to lay out the original amplifier circuit. Using my new knowledge of Altium’s multichannel capabilities, I captured Igor’s new design into the schematics editor of Altium Designer. With proper nested schematic sheets, the entire 30 channel amplifier/6 channel summer design was compressed into just 4 schematic sheets, vs. the ~40 or so that would have been required had I laid out the complete schematics of the old design. I spent about a week and a half trying to figure out how to handle nesting independent nets from a repeated subsheet into another repeated subsheet which also produces independent nets from the nets of the first sheet. This sounds somewhat complicated, and I suppose perhaps it is a somewhat unique situation, since none of the ~5 sample multichannel projects included with Altium had such a construction in them. Basically, each summing circuit has five amplifier subcircuits, each of which puts out its own signal independent of the other four. From the perspective of the entire board, there are 6 summers, each of which has five independent signals coming from the amplifiers, and one summed signal. Determining how to get Altium to realize the proper connections from each individual amplifier, through that amplifier’s summer, to the main schematic was a complicated mess of naming conventions, but eventually I was able to make Altium reflect all of the appropriate connections in the PCB view. Though there are a few net naming issues still to be resolved, this problem has mostly been ironed out.
The final few weeks of the semester were spent laying out components in the PCB view of Altium. As of right now, a compact design for an amplifier measuring 0.183”x~1.3” has been completed. Making use of 0201 size components, this amplifier is approximately .2” shorter than the old design, despite incorporating an extra transistor. The amplifier design features an isolating ground trace running the length of the amplifier to prevent crosstalk between channels. A prototype layout of the summing circuit has also been completed, though some layout issues there remain to be resolved over the summer.