Further to the previous post where we asked if our installers and users wanted to see nano SIMs we have made a start on the electronics design of the new board. Let’s see how quickly we can get this into production! The prototypes should be here in about 2 weeks I reckon which gives us a tight deadline of at least 2014! Hopefully mid-December for full production.
We have used a nano SIM holder and also a new module by Quectel; the M66. This gives us a lot more space on the PCB so we can have the two outputs and 1 input. It also keeps the SIM holder up to date with current treands. One thing this module does feature is Bluetooth. I don’t think we’ll see a Bluetooth app integrated this year but for 2015 this could be the answer to adding several numbers at once without using the dial up modem software.
We’re at the stage with the PCB where we’ve got a definitive layout for the components. When designing a PCB you have to juggle with conflicting requirements as the where the components go. Some things have no compromise. The red circles in the picture show the areas where no traces can be placed in order to meet electrical clearance standards for safety. Beyond this the distance between certain components must be minimised to keep the PCB traces as small as possible for Electro-Magnetic Compatibility (EMC) compliance. This is no more important in our design than in the power supply section.
We use a DC-DC converter to efficiently step the voltage of the supply (12-24V) down to the 4v required by the GSM chipset. Without this you would need at least a 2A power supply and the board would get very hot! The top priority here is the minimise the current loops of this power supply and to do this we have to move the leads of the critical components as close to each other as possible.
On the charging loop these are D1, C28 (which stores all the switched energy), the switch (the 8-pin IC). If we look at the picture the current loop here is drawn in yellow. The area of the yellow loop is magnetised when charging the circuit. We need to minimise this area by keeping the components as close as possible to each other. We’ve even angled the capacitor C28!
On the output side of the power supply the current loop is shown in pink. The main decoupling filter capacitor is the MLCC C19 and not the large electrolytic C11. The frequency is such that a 100uF cap is enough for filtering this stage. Current flows from C19 through the ground plane and L1 (a ferrite bead circled in brown) to C11 which stores the energy required for a GSM burst. This arrangement provides a low impedance filter connection for the module to the power supply. We’ve found this arrangement works better than another linear regulator stage and reduces the cost of the system. The blue loop shows the current loop for the GSM module which is powered by C11. The other 3 MLCC capacitors provide a low inductance current source for the 1-2GHz power amplifier.