Brief Introduction to FSK Modulation

FSK is different than ASK or OOK since its data modulation method is through frequency change or shifting instead of transmitting signal on and off, which means in FSK transmitter, it will always transmit a complete wave, the only difference is transmitting frequency change. FSK transmitter is more widely used in circumstances where requires a higher bit-rate, such as in cellphone and wifi hotspot.

Although vast majority garage and gate transmitters are OOK encoder, there are several garage and gate makers who adapt FSK technology in their transmitters, such as Sommer in 4020 and 4026 transmitter, ATA in PTX-5 trio-code transmitters and Merlin in M842 transmitters etc, so we think it might be necessary to write a brief introduction to FSK modulation method.


Original photo is taken on Aug 1st 2013 which shows a typical SR-M1 Merlin compatible FSK garage gate transmitter working at 433.92MHz – as shown on spectrum analyzer, updated on May 11st 2016 with new FSK spectrum capture which shows waveform more clearly.

You can easily point out that the biggest difference between OOK and FSK modulation is latter has two wave curves instead of one, the two wave curves shows respective frequency, in this case as above photo, the center frequency is 433.92MHz, and frequency deviation between two is 60KHz, so the lower frequency would be 433.89MHz while the higher frequency would be 433.95MHz. 60KHz is considered a common value regarding FSK frequency deviation.

Basically the data 01 stream equals to different wave position in FSK modulation, if the data stream is 00000.. then you can only see a continuous wave curve centered at lower frequency, while 11111… means a continuous wave centered at higher frequency, thus 0101.. will cause the wave jump from left to right and then back.

FSK modulation also has different system of coding 0 and 1, sometimes the higher frequency means 1 and lower means 0, but sometimes it would be just the opposite.

Unlike OOK transmitter, which has a simplified circuit design, can use 2SC3356 as RF transistor, FSK transmitter requires much more complicated circuit, and often transmitter IC is involved in design, the commonly used are Infineon TDK5110 and SiLabs SI4010, the latter is highly recommended as it’s a SoC chip that integrates microcontroller, crystal and all components required in transmitter, requires only several surrounding components to build a working high performance transmitter.