Hi Element 14 Community,
Welcome to The Ulster Society of Student Engineers second project; A radio frequency (RF) telescope.
1. What is the Need?
A team of society members noted that Northern Ireland does not have an RF telescope. This presents both a great opportunity for members to learn about practical RF and also the opportunity to present the design to local children so they can learn more about both Space, Electronics and Control systems.
The idea behind the project is to create a small RF telescope that can be easily transported between venues to show to young people and engage them in STEM (Science, Technology, Engineering and Maths) learning. By learning from building the first transportable telescope it is then hoped that the project can be further improved before moving to build a much larger permanent installation that will act as a citizen science project.
Figure 1 - A CAD model of the final mechanical design - Created in Solid Edge ST10 and rendered in KeyShot 360 (Free for students)
2. The Project Specification
The Society intends to create a prototype of an RF telescope. The final design must;
1. Maintain separate X and Y axis movement
2. Maintain self control
3. Be portable
4. Be easy for children to understand
5. Collect RF intensity data
6. Be ethical
It should be noted that this first prototype will look for signals in the Ku band (about 12-18 GHz), the frequency that domestic satellite TV is broadcast at. This will allow our prototype to serve as a proof-of-concept design before moving onto different frequencies, requiring more expensive and complicated equipment
3. The Circuit Design
With the design requirements laid out in section two a flow chart was created as shown in Figure 2.
Figure 2 - A simplified flow chart of the project
The team decided on several of key factors from this exercise:
1. To ensure that the device is portable it should be mounted on some form of trolley
2. To ensure that the device is portable it should be powered by an external power supply. Due to current draws in excess of 8 Amps it was decided to use a standard car battery
3. To ensure that the device is open to be used by the largest number of makers an arduino would be utilised as the main controller, with any custom circuit designs being incorporated into a shield and open sourced
4. The device must be able to data log
From this the team was able to create the following circuit design as shown in Figure 3.
Figure 3 - An overview of the shield schematic design
3.1 Under Voltage Lock Out (UVLO)
With the specification set in section 3 the subsystems were designed to meet this criteria. The following section breaks down the schematics and explains the teams thoughts and ideas about part selection.
After selecting some outlined components for certain parts of the project, such as an SD card reader to allow for data logging, it became apparent that we would need multiple voltage sources. In order to satisfy this and keep the project mobile, the team decided to settle on powering the system via one lead-acid car battery (12.6 V) which could be cascaded through a series of regulators and converters to meet all the power supply requirements. The main concern from this was the inherent danger of over-discharging a lead acid battery. By having unregulated charging and discharging the cells that make up the battery can be damaged, thus increasing the internal resistance of the battery. This can cause difficulties re-charging and in the worst case scenario causes a short between multiple cells, not to mention shortening the batteries overall life.
In order to solve this problem it was decided to implement Under Voltage Lock Out (UVLO) protection. This will disconnect the battery from the circuit when discharged below 10.7 V.
The first of these circuits is shown in Figure 4. This circuit is a Under Voltage Lock Out (UVLO), that means that if the sense input (pin 4) is less than 400 mV (i.e. the Lead Acid car battery voltage is 10.7 V), the whole system will shut off to protect the battery from over-discharge. The voltage divider values can easily be calculated from the data sheet. Using equation one;
Vmon = (1 + R1/R2) * Vit
During normal operation conditions (Vin > 10.7V), the OUT (pin 6) will be open-drain (floating), this will 'turn on' MOSFET Q2 via a pull-up resistor (R8), which pulls the gate pin of Q1 to 0 V and turning on the P-MOSFET (Q1). A 9.1 V zener diode prevents a Vgs of less than -8 V across Q1, limited by a 270R resistor (R9)
If under voltage conditions are met (Vin < 10.7V), the OUT pin will be pulled to 0 V. This will 'turn off' Q2 and therefore the Q1 will be switched off. This cuts-off the output to the rest of the circuit and turns on a red LED (D2) as a warning.
Figure 4 - Schematic for the first power stage - A simple Under Voltage Lock Out circuit
3.2 Arduino Regulated Supply
With the design being planned to run an arduino from a 12.6V car battery, it was necessary to create a lower voltage regulated supply for the arduino. The team decided to use a DC-DC switching buck step down regulator. The component was chosen as it met the required input and output voltage range (11-16 V) and was fairly efficient.
The supply works in a similar manner to other buck converters, that is the input DC input is switched on and off at a high frequency, this is used to charge the inductor and stabilize the output. The ratio of R14 and R15 defines the voltage output as seen from the data sheet. Capacitors C2 and C3 help to further stabilize the input and output voltage through the smoothing effects of capacitors.
An important point to mention the team feels has been left out by a lot of electronics guides is capacitor de-rating. In industry, it is common practice to increase the voltage rating of a capacitor to at least double the maximum voltage it will be exposed to. This explains why C2 and C3 ratings are much larger than the expected voltage. Capacitors are derated for two main reasons;
1. As a capacitor fills to its maximum rated value the capacitance value will significantly reduce.
2. The operating temperature of a capacitor is unlikely to remain constant, this will affect its capacitance and alter the maximum voltage, hence why all capacitors have a maximum operating temperature.
3. Capacitors like to blow up
Figure 5 - A 5V DC-DC switching buck step down regulator to power the arduino remotely.
3.3 SD Card Reader Regulated Supply
Although the design will be portable the team decided against integrating a laptop or computer into the system so that it could be left unattended for prolonged periods of time to gather data. Instead the information would be gathered by saving a '.txt' file to a memory card allowing the data to be extracted and saved at a later date. The data could then also be processed with MATLAB.
The design is a very simple as shown in Figure 5. Five Volts feeds in from the previous stage and is regulated down to 3V3. Capacitors C4 and C5 act as smoothing capacitors to ensure a constant and spike free output. The bypass capacitor C6 reduces output noise.
Figure 6 - A simple 5V to 3V3 linear regulator
3.4 RF Detector Supply
The final section of the circuitry that required power was the radio frequency detector. For this initial prototype the team decided to make use of an off-the-shelf satellite finder to allow for rapid prototyping and testing before creating a custom designed Low Noise Block downconverter (LNB). In order to test the current draw of the LNB, the team connected it to a current limited supply and slowly increased the current limit until it leveled off at 106 mA. The team elected to upscale this to 200 mA to ensure smooth operation.
A was used again this circuitry as with most power supplies follows the Typical Application Schematic as set out in the data sheet with values recalculated to suit the projects needs.
Figure 7 - A Boost Regulator to power the LNB.
3.5 SD Card Reader Support Circuitry
Originally the team acquired a standard SD card reader, the idea behind this was that a modular plug and play approach would enable quick prototyping and progress. After arrival it quickly became apparent that the module required some form of driving circuitry.
Figure 8 - The SD Card reader module
After some reverse engineering the driver schematic in Figure 9 was implemented. This makes use of the SPI communication. U5A-U5D are a series of buffers integrated onto one Integrated Circuit (IC); this level-shifts between the 5 V Arduino logic to the 3V3 SD logic. It should be noted that pins 1,4,10 and 13 act as enable pins and only allow data to pass when held low. This buffer is needed to load data onto the SD card.
Note U5D does not have any connection on Pin 11 or 12 and effectively acts as a pass through for 3V3 volts. There are also a series of smoothing capacitors in this design to eliminate any noise, although the circuit is digital and therefore less susceptible to noise than its analog counterpart, excessive noise can still cause problems.
Figure 9 - SD Card reader schematic
3.6 Power Indication
One final touch that the team wished to add was a series of power indicator lights these are useful when testing as they allow for feedback of each sub-circuit. The are low power (4 mW) and provide good indication. One point to note should the Under-Voltage Lock Out (UVLO) trigger (See figure 4), only green LED D3 will light, with red LED D2 (See Figure 4) lighting to alert the user that the battery has a low charge and should be disconnected. This LED will discharge the battery at a negligible rate and will therefore not damage the battery.
Figure 10 - Power Status Indicator LED's
With the sub-circuits designed the team then created a mock up of how this would be located on the shield before designing and routing the PCB as shown in Figure 11.
Figure 11 - an outline of the layout of the final arduino uno shield
With all the circuitry planned, any experimental or unfamiliar circuitry was tested on breadboard; a good example of this was the motor drivers. As seen in Figure 12 the team was able to test the stepper motor drivers interface with an arduino, this process was completed using the drivers DIP switches to limit the current to 1/8th of its maximum current (500mA).
Figure 12 - Breadboard testing of the stepper motors in its final location
5. PCB Design
The PCB design was also created and reviewed with Altium Designer (Its free for UK students). The red side indicated the 'top' side of the board whist the blue side is the 'bottom' of the board as shown in Figure 13 and 14.
An important point to note for anyone wanting to create their own shield is to make sure that all the components fit without snagging on the arduino itself. Figure 16 shows how the team exported the 3D shield model generated in altium designer and used Solid Edge (Again free for students) and a pre-existing arduino CAD file to ensure that the boards would fit together without damaging each other.
The design itself had ample space to fit onto the shield footprint. If additional circuitry was required the SD card reader could be changed to a micro SD card reader. There is also plenty of space on the back of the board. Pads surrounded by ground plane have been partially separated from the ground plane with smaller channels (reliefs); this ensures that when soldered the ground plane can not sink away heat, making it easier to solder. A series of test points have also been incorporated into the design to allow for quicker fault-finding.
Figure 13 - PCB shield design - Top side
Figure 14 - PCB shield design - bottom side
Figure 15 - PCB design - 3D model of the shield created in Altium Designer
Figure 16 - PCB Design - Altium 3D model file exported and connected to an arduino model in Solid Edge to test fit clearances
This is the current state of play for the project, the team are about to order the components and parts needed as seen in the table below and will keep you all up to date when the team have the PCB's manufactured. Be sure to keep an eye out for us!
Have a great day!
|#||Designator||Description||Manufacturer 1||Manufacturer Part Number||Supplier||Supplier Part Number||Quantity|
|4||C4, C5, C7||Capacitor||Taiyo Yuden||LMK107B7105KA-T||Farnell||1845766||3|
|9||D1||Zener Single Diode, 9.1 V, 500 mW, SOD-80C, 5 %, 2 Pins, 200 °C||NXP Semiconductors||BZV55-C9V1,115||Farnell||1097208||1|
|10||D2||SMD LED RED||Kingbright||KPT-2012EC||Farnell||2099236||1|
|11||D3, D4, D5, D6||SMD LED GREEN||Kingbright||KP-2012LSGC||Farnell||2463991||4|
|12||D7||Schottky Rectifier, Barrier, 60 V, 1 A, Single, SOD-123F, 2 Pins, 660 mV||Nexperia||Farnell||1510694||1|
|13||F1||Fuse, Surface Mount, Micro Chip Fuse Series, 1 A, 32 VDC, Fast Acting, 0402||Panasonic||ERB-RD1R00X||Farnell||1897005||1|
|14||J1||Header, 10-Pin, 2.54mm Pitch||Harwin||Farnell||1022261||1|
|15||J2, J3||Header, 8-Pin, 2.54mm Pitch||Harwin||Farnell||1022257||2|
|16||J4||Header, 6-Pin, 2.54mm Pitch||Harwin||Farnell||1022255||1|
|17||J5||Terminal Block, 5 mm, 8 Ways, 26-14 AWG, Push In||Phoenix Contact||Farnell||2072383||1|
|18||J6||SD Card, 9 Way Right Angle Memory Card Connector||Hirose||DM1B-DSF-PEJ(82)||RSComponents||685-0788||1|
|21||Q1||MOSFET Transistor, P Channel, -3.1 A, -20 V, 0.062 ohm, -4.5 V, -1.01 V||Diodes||DMP2170U-7||Farnell||2709531||1|
|22||Q2||N-Channel MOSFET - 190mA, 1.6V||Nexperia||Farnell||2191746||1|
|33||R11, R12, TP1, TP2, TP3, TP4||Resistor, DNF, Test Point, 1mm Diameter||6|
|39||U1||Voltage Detector, 1.8V-18Vsupp., 150 µs delay, Active-Low, Open-Drain reset, WSON-6||Texas Instruments||Farnell||2507096||1|
|40||U2||DC-DC Switching Buck Step Down Regulator, Adjustable, 4.75V-28Vin, 1V-6Vout, 0.5Aout, WSON-10||Texas Instruments||TPS62175DQCT||Farnell||2255013||1|
|41||U3||Fixed LDO Voltage Regulator, 2.7V to 5.5V, 210mV Dropout, 3.3Vout, 150mAout, SOT-23-5||Microchip||Farnell||2510305||1|
|42||U4||DC-DC Switching Boost Regulator, Adjustable, 1.2 MHz, 3V-18Vin, 3V-38V/1.2 A out, WSON-6||Texas Instruments||TPS61170DRVR||Farnell||2764809||1|
|43||U5||Buffer, 2 V to 5.5 V, TSSOP-14||Texas Instruments||Farnell||1470744||1|
Created By Christopher McCausland
Edited By Michael Jennings