In almost every RF design, you're going to see Baluns. Baluns (Balanced-Unbalanced) are ways to convert between two different forms of the same RF signal. Specifically, they convert between a balanced and unbalanced signal (hence the name).
When our signals come in from an antenna, they're single-ended and very noisy (hence, unbalanced). Each of the incoming signals is also referenced to ground. A balun takes these single-ended signals and converts them to differential signals. Typically, baluns are only placed on the receive side of a transceiver chip.
If you’re unsure of what a differential signal is, I’ll explain below, but I recommend researching it further if it interests you.
Before understanding why we use a differential signal, we must first understand the difference between a single-ended and a differential signal.
Single-Ended signals are what we are typically used to and what you’d imagine. It’s a single connection between a transmitter and receiver (1 trace). So lets say I had a direct connection from the antenna to the receive pin on an IC, that would be a single-ended signal.
On the other hand, differential signals express a signal two connections/traces; one signal carries the “positive” (non-inverting) signal, while the other carries the “negative” (inverting) signal.
Once the signal arrives at the receiver, it can extract the data by taking the potential difference (volts) between the two signals. Here’s an example with a digital signal. By using the difference between the two signals, the IC can determine which is considered a high voltage and which is considered a low voltage.
Now, you might be wondering, why use a differential signal since they require more traces (and thus space) on a PCB? Well, there are a number of advantages, like reduced voltages in signals and less electromagnetic interference (you can imagine how that's important in our RF). I won’t explain each one, so if you’re interested I recommend searching more.
So, as explained before, our signals usually come in from the antenna as single-ended signals referenced from ground. However, for some of the reasons above, it’s better for our ICs to process them as differential signals. The lower voltage helps with keeping the overall system at lower power, and the reduced EMI interference helps keep our signals clear and reduce SNR.
Baluns can also perform impedance matching. When dealing with RF signals, we want to make sure that all our components are running at the same impedance. If there is a difference, the system can experience significant signal losses. We have an entire page in the comms onboarding about this, so read that if you're interested in learning more about it.
That sounds simple enough, but to actually design and implement one is a lot harder. Typically, you can buy ICs to do this for you, and you can consider a variety of factors when selecting one. However, with RF designs, you generally want to limit the number of ICs and amplifiers when dealing with high frequencies. Thus, we try to make a functioning one with RLC passives (resistors, capacitors, inductors).
There are two main types of Baluns: wide band and narrow band. As the name suggests, wide bands operator over a wider range of frequencies, while narrow bands are much smaller. Wide bands are typically made from multiple coils wrapped around a ferrite material. They’re extremely similar to transformers. Narrow band baluns on the other hand, can be made entirely with passive components (inductors and capacitors).
The design in our CC1120 custom transceiver is called an LC lumped Balun. As the name suggests, they use Inductors and Capacitors to create balanced signals. This can be seen below.
