The Tech Blog

Tech Simplified: SIM vs eSIM vs iSIM

Posted under Educational, Technology on April 2nd, 2023 by Amaldev

Folks who have been using phones for the last 10+yrs would have probably seen the evolution of SIM “Cards”. The Physical Plastic card(known as universal integrated circuit card (UICC)) SIMs had a large Credit card size, called 1FF(1st Form factor) in the very early 90s. The smaller one 2FF(Mini) would be one most are familiar with. The SIMs just continued to drop in size to 3FF(Micro) and 4FF (Nano) as phones wanted to pack more and more. The inherent technology used is more or less the same in these generations. Usually, a telecom operator hard-codes their details onto this plastic SIM and then the moment you put this Physical SIM in your phone, it activates and you can communicate with your mobile tower. It contains a small microchip that stores the info.

eSIMs(Embedded SIM) have been in the market since 2016 and have started to gain traction with higher-end phones starting to support them. It’s actually a software stack(embedded-UICC standard) that enables a user to virtually load their network profiles via an application. They “usually” come as a small chip soldered directly onto your phone’s motherboard. The form factor size is called MFF2(Machine to Machine form factor). Formfactor has nothing to do with it being an eSIM or not. There are a lot of articles wrongly mentioning that since it’s a soldered chip in MFF2 form, it’s an eSIM. eSIMs are possible with physical cards too. The benefits of eSIMs are that you can rewrite the memory and load any operator on top of it with a configuration file. Multiple profiles can be loaded on a single chip too for ease of swapping profiles. It’s not hardcoded anymore. It’s tiny and usually resistant to mechanical motion abuse as the previous generation cards. These MFF2 chips can be beefy microcontrollers(ARM Cortex M3 similar) with encryption cores.

iSIMs(Integrated SIM) are the extension of eSIMs wherein there is no external chip anymore. It’s all integrated into your chipset/SoC. It has all the benefits of eSIM but just doesn’t take up any more PCB area as it’s implemented inside the chip. In a few years’ time, pretty sure this will be the way ahead for the industry.

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Back to Basics: Skin Depth

Posted under Uncategorized on March 26th, 2023 by Amaldev

Skin effect is a phenomenon that occurs in conductors carrying alternating current (AC), in which the current density is highest near the surface of the conductor and decreases towards the centre. It’s measured as the distance from the outer surface of a conductor to the point where the current falls to 37%(1/e) of the max current value. It depends on the frequency of the signal travelling in the cable. Higher the frequency, lower the depth from the surface it travels. For a DC signal(f=0Hz), it travels through the entire cross-section of the signal.

So Why does it occur? When an alternating current flows through a conductor, it generates a changing magnetic field around the conductor. This magnetic field, in turn, induces an opposing electric field within the conductor, which creates eddy currents that flow in a circular path around the conductor. These eddy currents produce a secondary magnetic field that opposes the original magnetic field. This field is strongest in the centre and hence it pushes the current towards the outer surface of the conductor.

This means that to carry a high-frequency signal, it’s relatively wasteful to have a very thick conductor as most of the current is on the outside surface. Hence you would see the usage of multi-stranded wires than solid core wires to carry these types of signals. Induction cooktops you use to cook food in your kitchen also rely on high-frequency switching coils. Due to its high freq, the skin depth kicks in and heating only happens in a thin region of the bottom surface of your pan. Skin depth depends inversely on the conductivity of the material and magnetic permeability. Better a conductor, lower the skin depth. Similarly, more magnetic a material, lower the skin depth. For PCB designs in very high frequencies, Skin depth usually would need to be factored in the field solver as impedance will get affected.

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