Back to Basics: Bit Rate vs Baud Rate

Posted under Back to Basics, Educational, Technology on October 16th, 2022 by

This is one of the most confusing questions out there. Folks use these interchangeably without really knowing what is what. Definitions first.

Bit Rate means the number of bits transmitted per sec.

Baud Rate means the number of symbols transmitted per sec.

Now, what the heck is a symbol? The symbol is just a way to wrap information. One signal block can be represented by let’s say 2bits, 5 bits, 6 bits or 32bits depending on how the receiver and transmitter agree. Think of this like a secret communication protocol which you and your friend use to communicate.

Electrically let’s say you want to send 4 voltage levels from 0-3V. I will use 0V – 00, 1V – 01, 2V – 10, 3V – 11. Here each information symbol is encoded by 2 bits. So now if I transmit 1 symbol per second, my baud rate is 1 but my bit rate is 2 bits per second as each symbol contains 2 bits.

Now, where does all the confusion arise from? In let’s say serial communication between embedded devices, you use binary signalling, meaning 1 symbol = 1 bit(either 0 or 1). In this case, the baud rate is always the same as the bit rate. This is one of the reasons you see baud rates and bit rates used interchangeably. Take UART, usually on Windows, the default bit rate is kept as 9600, which is equivalent to 9600 bits per sec. This means 1 bit has a time length of 1/9600 = 104us.

But now to confuse this further, this is not the actual data transfer because UART needs a start bit and a stop bit for every 8bits of actual data. For every 8bit data takes 104us *10 = 1.04ms to transfer. This is the UART protocol overhead. So actual meaningful information bit rate(avoiding protocols) = 8bits every 1.04ms = 7692 bits per second.

Hoping this can clear up most doubts out there on the topic.

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Flipped Capacitors

Posted under Back to Basics, Educational, Product Design, Technology on September 11th, 2022 by
Different Capacitor Shapes

Most electronic design engineers know SMD capacitor parts come in the “standard” sizes of let’s say 1206, 0805, 0603,0402, 0201 etc. The common element of these parts is that their size is longer along terminals(between 2 pads you actually solder) than along the width of the part. Most times width of the part is half the length. There is a different class of capacitors which are wider than long. These are called flipped or reverse geometry capacitors. So the sizes will be 0612, 0508, 0306, 0204 etc. (To be honest, you might not find a flipped capacitor for every “standard” size capacitor though)

Why do we want to use a flipped capacitor? In one of my older posts, I explained that a capacitor in real life is not a capacitor by itself. It has an equivalent series resistance(ESR) and inductance(ESL). For an ideal capacitor, we would want to have ESRs and ESLs to be zero. Since there is an ESL in a capacitor, you usually see a V-shaped impedance curve with frequency in capacitor datasheets. The impedance value keeps getting low till a point and then it increases due to ESL. Here is where flipped capacitors come into the picture. Flipped capacitors are designed specifically to reduce ESL. When you flip the width and length dimensions of a capacitor, it reduces the soldering pad distances(or the length). Lower travel length means lower inductance and lower conductor length(Not to complicate it more, loop area also reduces, which is a good thing). So current needs to travel a shorter distance. A wider flipped capacitor effectively can have an impedance curve which is similar to 3-4 same standard-size capacitors. That means overall lower BOM count and layout space is lesser. For these reasons, these wider capacitors are starting to gain acceptance in the PCB design space.

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