Lecture
Это окончание невероятной информации про детектор частиц.
...
Radiation Detectors, Scintillation Counters" >
And this is the rest of the system board's circuit.
The heart of the system is the MCU STM32L151CBT6A (unlike the variant without the A suffix, it has twice the RAM — 32 kB). Almost all 48 of its pins are used. The exceptions are PA9 and PA10, which are RxD and TxD of the first USART; just in case, I made test pads for them, to which it will be easy to solder in the future. Among the notable features here is a somewhat tricky arrangement for sensing the state of DA1's CHRG pin, with a pull-up from PB14 switched in when it is necessary to determine whether the battery is charging or has already finished charging, plus the anti-phase connection of the buzzer through inverter DD2. The circuit reflects a mistake of mine: when the MCU is put into STANDBY mode, this inverter's input is left floating, which leads to significant additional current consumption and even oscillation. Here this input needs to be pulled to ground through a 100 kilohm resistor. Attention should be paid to the quality of the crystal resonators, especially ZQ1. With standard 12 pF clock crystals the controller's clock will not run properly; a hard-to-find crystal with a 7 pF load capacitance will have to be sourced. The MCU is more tolerant of ZQ2, but even there, with the first Chinese crystal that comes to hand, one can run into a failure to start or operation at the wrong frequency. Unfortunately, the STM32Lxx line (this one specifically) is very demanding about crystal quality.
The keyboard is connected in a fairly standard way — port lines are pulled up to the supply by external resistors R17-R21 and are pulled to ground by the buttons. On the keyboard board, RC networks are soldered in parallel with the buttons to suppress bounce. When the On button is pressed, inverter DD3 generates a high-level signal, which is fed to the WKUP input and wakes the MCU if it is in the STANDBY state. To prevent accidental turn-on from interference, the network R22C23 is installed. The keyboard lines are routed to consecutive port lines, which allows it to be read with a single port-read instruction.
The microSD card is connected in SPI mode, since this MCU lacks an SDIO controller. The USB port is connected in the simplest way, through two resistors on the DP and DM lines. The STM32 MCUs themselves are fairly “rugged” as regards static electricity, and no other external communications (excluding the programmer for flashing over SWD) will be connected to the radiometer, so there is no need for serious overvoltage protection on the port.
The MCU's power circuit is taken from the datasheet and has nothing special about it. During tuning I added a 100 Ω resistor in series with inductor L6, which greatly reduced the voltage ripple on VDDA. The capacitance of C30 can be increased to 1 µF by soldering another 0.01 µF capacitor in parallel with it (onto the same pads).
When power is applied, the 3.3 V source, which supplies the MCU, starts working immediately. The other power sources are off. After the MCU starts up and initializes the peripherals, it raises the POWER_ON line (port PA15), starting the 5 V source and supplying power to the analogue section and the high-voltage source. To turn on power to the display, the DISP_ON line (PA8) must be raised; similarly, to turn on the satellite navigation module, the GPS_EN line (PA1) is raised, but unlike the display there is no dedicated power switch for it — the receiver's own power-control input is used instead (it must have one). The display is turned off while the instrument is running precisely by removing its power.
The counting pulses (TRIG) from the analogue unit arrive at line PB0, triggering an interrupt on the falling edge. The pulses carrying information about the particle's energy (SP) arrive at ADC channel 21. The duration of this pulse's “plateau”, during which the level stays unchanged, is almost 10 µs, which allows several ADC conversions to be made after the MCU “wakes up” and enters the interrupt. Resistor R34 removes the “ringing”.
The system board measures 64x80 mm and is designed for mounting most resistors and capacitors of the 0603 package size. Most of the components are mounted on one side, except for three capacitors in the MCU's power decoupling network and two resistors forming the battery voltage divider for measuring it.
The analogue board starts working right away if it is assembled without errors. In the initial version the board lacked elements R8 and C9; without them the circuit worked flawlessly from a generator, but the amplitude of the pulses at the SP output was random and did not depend on the amplitude of the pulses at the input. Introducing these elements fixed the situation.
If the analogue section of the instrument oscillates, there is one way to fight it — shielding the TRIG line and minimizing the length of the connecting harness. The connection to the detector should also be as short as possible and shielded.
R5 should be selected for reliable passage of the pulses given by the detector with an americium-241 source, while ensuring the comparator does not respond to the dark noise of the Si-PMT at the maximum operating temperature.
The analogue board must be carefully and thoroughly cleaned of every trace of flux, especially around the peak detector, then dried well and baked at 150°C. Insulation resistance of even a gigaohm will sharply degrade its operation and make it unstable.
Start assembling the system board with the power supplies. First assemble the 3.3 V source and verify that it works. Then the 5 V source (not forgetting to temporarily connect POWER_ON to the +3.3V rail), and after that the high-voltage source. Before the first power-up, set R13 to the upper position (as drawn) and do not solder in C20 until the voltage has been set. Set the high-voltage source's output to 28 V. Verify that the charger works correctly, monitoring the voltage on the battery during charging. After verifying correct operation and the correct voltage on all supply rails, the MCU and all its associated components can be soldered in. To test the MCU it is advisable to write and flash some kind of Blink program. Don't forget to flash the display too.
Besides scientific experiments, particle detectors also find application in applied tasks — in medicine (low-dose X-ray machines, tomographs, radiation therapy), in materials science (flaw detection), and for pre-flight screening of passengers and baggage at airports.
Часть 1 Particle Detectors: Elementary Particle Detectors, Ionizing Radiation Detectors, Scintillation Counters
Часть 2 Applied Uses - Particle Detectors: Elementary Particle Detectors, Ionizing Radiation
Comments