How does PSK or Phase Shift Keying actually work?

Foundations of Amateur Radio

Previously I've explained how Radio-teletype or RTTY works from a technical perspective. If you recall, it uses a technique called Frequency Shift Keying, or FSK to encode digital information. It does this by transmitting a carrier across two alternating frequencies, allocating one as a SPACE and the other as a MARK, or as a binary 0 and a binary 1.

There are several other ways of encoding information and today I'm going to look at Phase Shift Keying, or PSK, which I find humorous, because Phase is spelled with a P, but it sounds like an F, which links the FSK and PSK together, but then I've always had a strange sense of humour.

Imagine if you will a sine wave. It's the one you learned in high-school, nothing sophisticated about it, just keeps going up and down over time. Now imagine another one. Also going up and down over time. If these two sine waves are synchronised, going up and down at the same time, the difference between them is 0.

If one of the sine waves is going up, while the other is going down, then the difference between them is 1.

That is enough to give you a binary 0 and a binary 1. One of the sine waves is a carrier, so it's transmitted continuously, and the other is changed depending on whether you're sending a 0 or a 1.

These two sine waves are said to be "in-phase" when they're both going up and down at the same time, and "out of phase" when they're going in opposite directions. This is how Phase Shift Keying works. And the simple example I gave is known as BPSK, or Binary Phase-Shift Keying.

There are countless variations on this. For example, you don't need to have them going in completely 180 degrees opposite directions, you could go only 90 degrees, or even 45 degrees, which would allow you to encode more information across a shorter time span at the cost of less accurate decoding at the other end.

You could play with the carrier and instead have the signal be compared to itself, making it more robust in some circumstances, or you could have multiple of these signals happening at the same time.

You could change the amplitude of the carrier and allocate specific byte values to each combination. For example, one variation, an encoding method called "16-QAM" allows you to create 16 different signals, which equates to sending 4 bits at a time.

Each of these have different advantages and disadvantages, trade-offs between speed, reliability, error detection, impact of polarisation changes in the ionosphere, energy efficiency, etc.

You might be surprised to learn that these techniques are not only used inside Amateur Radio and PSK31, they're also used in Wi-Fi, Bluetooth, Ethernet, RFID and countless other places, like remote controls, hard-drives, tape recorders, satellite communications, mobile telephony, etc.

If you get hooked, there's lots of maths that you can associate with all of this - if that floats your boat, but you don't need any maths to grasp how it works.

Phase Shift Keying, one of the many Digital Modes that make our world go round.

I'm Onno VK6FLAB