RC Spread Spectrum Demystified. I want to personally thank David for sharing his RC Spread Spectrum Demystified article with me and better yet, allowing me to post it on my web site for the greater RC community to benefit from. David is an electrical engineer at Tektronix which Im sure many of you electronics folks know, build some of the best electrical test measurement equipment on the market. There he works with oscilloscopes, spectrum analyzers, logic analyzers, video monitors and generators. Broad spectrum experience with electronics hardware and software. Experience First Person Viewing flight from the cockpit with the Air Hogs Official DR1 FPV Race Drone. Take to the skies viewing your flight from the camera on the. Here you can find out what is happening in the world of Phoenix, including new models, flying sites and program updates. Rc Helicopter Indoor Flight Simulation' title='Rc Helicopter Indoor Flight Simulation' />In other words, he knows his stuff and Im so grateful for his contribution to my web site. Onto the article. Go to a regional fun fly or national event to watch all the airplanes and helicopters flying without need of a frequency board. You have to wonder how it works. Is it true that the full 8. MHz wide 2. 4 GHz band can support as many as 2. If you are like me, you are not willing to accept that several radios can share the same frequency until you understand how its done. This article has evolved considerably, woven with popular theories and explanations that each in their turn were found to be incorrect. All too often the correct information is buried in theoretical formulas that few of you would care to understand. The objective of this article is to present the theory of how our RC radios work in a format that each of you can understand. GHz RC Spread Spectrum. The. following are three important technologies used by our 2. GHz band. radios. Rc Helicopter Indoor Flight Simulation TrainingA frequency band that is increasingly popular for a growing. FHSS. Frequency Hopping Spread Spectrum Transmitter and receiver hop in. One. or more packets of data are transmitted before each hop. DSSS. Direct Sequence Spread Spectrum Digital sequence modulation applied. Looks like noise on a spectrum analyzer. The. recommended form of modulation used by FHSS for transmitting data. CRC. Cyclical Redundancy Code A key premise is that data sometimes gets. The transmitters and receivers CRC calculation wont match. RC receivers ignore flawed packets, Wi Fi asks. Futabas. first 2. GHz RC radios started out with 1. Hz modulated FHSS and later. DSSS which was introduced as FASST. Spektrums DSSS only. DSM which was later enhanced with. Rc Helicopter Indoor Flight Simulation' title='Rc Helicopter Indoor Flight Simulation' />DSM2, more recently followed with 2. FHSS. frequencies called DSMX. Figure 1 to the left illustrates how DSSS modulation is used for each of 3. FHSS frequencies. The following table illustrates with a few examples that DSSS and FHSS are both being used. All of the currently marketed 2. GHz RC radios that I know of are now using both. Code Division Multiple Access CDMACDMA is the DSSS technology used by our RC radios. For us, Multiple Access means it is designed for several radios on the same frequency. Figure 2 here to the right illustrates how each data. Note that the encoding. The higher the ratio of coding bits for each data bit the better the DSSS process can extract data from noise. The ratio is referred to as Process Gain which will be illustrated and explained later. RC radios use a much higher process gain than Wi Fi. 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Here are three process gain examples that help to explain a key benefit of DSSS Most perhaps all Wi Fi uses a process gain of 1. Range is about 8. Spektrum and other radios use 6. CDMA codes. Range is 2 to 3 miles or more, a mile or less for Park Fliers. GPS not 2. 4 GHz has a process gain of 1. Satellite altitude is about 1. GPS satellites use a common frequency for the 2. US satellites which use different DSSS codes without any frequency hopping. Communicating Edges. It is important to understand that CDMA communicates at the edges of each coding bit and not the high or low levels. This section will help you understand how it works. Those of you with an electronics background understand the relationship between rise time and bandwidth. The faster a signal shifts from one level to another, the wider the spread of harmonics and the wider the bandwidth. The same thing happens if a signal shifts between two frequencies or shifts between two phases e. The faster the shift the wider the band width of the spread spectrum. There are spread spectrum Shift Key SK technologies that accomplish CDMA edge modulation by shifting between two frequencies, two or four phases or amplitudes. Figure 3 to the right illustrates Shift Keying. Each. rapid shift results in a narrow pulse of spread spectrum energy that. Most of our RC radios use Gaussian Frequency Shift Keying. GFSK. Gaussian specifies the type of shaping filter used to. The Gaussian filter. In this case less. Receiver Decoding. The CDMA illustration to the left is applicable for the different forms of shift keying. A multiplier circuit is fed with a pattern of 0s, 1s and 1s timed by the DSSS code series used by the transmitter. Most of the time the incoming signal is being ignored because it is being multiplied by 0. Two radio systems out of sync with each other will not see each other. A Phase Lock Loop PLL plays a vital roll in keeping your receivers 2. GHz clock in sync with your transmitters clock. The PLL acts like a flywheel to keep the receivers clock on track until the next edge is detected. The PLL speeds up or slows down to accommodate Doppler shift. The following Figure 5 uses an illustratively short DSSS code. The output of the multiplier circuit connects to an integrator that accumulates the sum of each edge. The longer the series of coding bits, the higher the staircase will rise or descend for a final determination at the end of each data bit series. A successful decode is accomplished as long as the final summation is correctly positive or negative. The following crude simulation uses a 6. DSSS code and noise that is more than 2. As mentioned already, the number of bits in the DSSS code pattern is referred to as process gain. Increasing the gain increases your receivers ability to lift your transmitters faint signals out of the noise at a considerable distance. It also helps to separate your radios signal from that of other radios that may have hopped to your radios cur rent frequency. CDMA timing and process gain make it possible for several radios to share the same frequency. Explains why 2. 4 GHz microwave ovens and basic Wi Fi are not a threat. Why FHSS Frequency hopping is primarily needed in order for our aircraft to survive the 2. GHz commercial products that can menace our radios. One of the most threatening examples is a high powered 2. GHz non spread video link using a high gain i. High gain antennas for outdoor Wi Fi are also a concern. Lookout for these antennas and dont fly through their beams. Geek Stuff. This section, with its unexplained alphabet soup, is for those who like to dig much deeper than this article can go. These are the Integrated Circuits ICs that our radios use Micro Linear ML2. Futaba FASST. Cypress CRYF6. JRSpektrums DSM, DSM2, DSMX. Texas Instruments CC2. Hi. Tec, Co rona, Fr. Sky, Tactic, Futaba S FHSS, Wi Spys USB spectrum analyzer. Texas Instruments CC2. JRs DMSS. These are ICs designed for commercial applications. The radio manufacturers patented features are coded into the CPU and MCU. The first three ICs listed above use GFSK modulation and are used in popular consumer products e. Several model aviation enthusiasts use these ICs to build and program their own radios. Some participate in the Fr. Sky open source project.