RF Signal Chain and Link Budget Basics for IoT
Did you ever play the game Telephone as a kid? One person comes up with a message and whispers it into the ear of the next person. Messages get passed from one person to another, until the final message is announced and compared to the original. Sometimes, there’s a minor difference between the two messages, and other times, the message is lost completely.
When sending a signal from an Internet of Things (IoT) device to a base station (or master database), numerous components and environmental factors can affect transmission. Each element is like a kid in the game of Telephone, receiving information and transferring it to the next portion of the chain. Like those kids, elements vary in reliability, capriciousness, and how well they work with each other.
Link Budgets and Signal Chains for IoT
A signal chain isn’t exclusive to IoT, or even RF, applications. The broadest definition is a series of signal-conditioning elements that provide their processed input as an output to the next element in the signal chain. So, those kids playing telephone could loosely be considered a signal chain.
Concerns about signal chain are in acoustic or audio processing, DSP, and other RF applications. You may also hear the terms “circuit topology” or “signal flow“ used to describe the signal chain; however, those can be application-specific usages with alternate meanings.
Regarding your specific IoT application, the signal chain encompasses every component and transmission medium that your signal passes through, from the initial transmitter to the final receiver. This could be as simple as a transmitting antenna, air, and the receiving antenna. It might also include transmitting through walls, forests, or rain, along with on-board signal processing and antenna connections.
A link budget quantifies your signal chain.
A link budget calculates the attenuation and gain through each component and medium that the signal travels through. You start with the transmission power, and ensure that you have enough to meet the sensitivity requirements of each receiver, especially the final receiver at your base station.
What Affects Signal Chain Performance?
While component quality is the most obvious factor in RF signal chain performance, it is only one of several important considerations:
- Surrounding material: When planning your signal chain, you should give ample thought to the material surrounding your sensor system. This includes casing material as well as environmental obstructions like concrete walls, thick vegetation, and pipes. Think of it like trying to whisper a message through a scarf; you have to whisper extra loud to get your message through.
- Distance: The distance your signal needs to travel affects your link budget. Even in air or free space, attenuation inevitably occurs as the signal spreads.
- Lossy connections: Bad connections almost destroyed the first signal chain I worked on. When we used custom cables, huge amounts of insertion loss appeared. This was never accounted for in our calculations. The signal power simply wasn’t high enough to make it through without better terminations in the cables. In Telephone, if a kid spits when he whispers, no one will get close enough to hear what he says, and the message won’t make it through.
- Environmental noise: If you have lots of sensors transmitting on the same band, or other signals competing for bandwidth, there is a possibility of interference. Good receiver and transmitter hardware should be able to distinguish messages. You should find out how signal collisions are handled by your hardware to make sure simultaneous transmissions can still get through.
How to Determine Your Link Budget
I’m not sure how to calculate a link budget for kids playing Telephone, but RF is a bit more straightforward.
A link budget for kids is much harder to calculate than for your components.
The basic equation for a link budget is:
Received Power (dB) = Transmitted Power (dB) + Gains (dB) − Losses (dB)
Start with the transmitted power at the source and add in the gain from antennas and repeaters. Then, subtract the losses for your connectors and anything your signal transmits through, whether it’s air or a transmission line. Remember that decibels are a log unit, so you’re effectively multiplying and dividing instead of adding and subtracting.
Optimize RF Signal Chain and Link Budget
Keeping track of various within your signal chain, along with their performance specifications, can take a lot of time (and spreadsheets) to manage. Using comprehensive PCB design software that can manage your list and specs can make the entire design process much easier.
You’ll want to use design software which can keep track of design rules, mark potential errors, make it easy to track component placement and rout, and thankfully Altium Designer® has all of these capabilities and more. If you’re looking for more help on understanding your link budget, ask an expert at Altium.