Spektrum DX6 Spread Spectrum Radio Control System

Since Horizon Hobby introduced the Spektrum DX6 spread spectrum radio control system in mid-2005, it has been a spectacular hit everywhere it is seen. Every review extols the virtues of a system that is totally immune to interference from both inband channels and from electrical noise.

Disclaimer: This website is not in any way affiliated with Horizon Hobby, the distributor for the Spektrum DX6 radio control system. This website's sole purpose is to promote the exceptional ingenuity and out-of-the-box thinking that went into creating this system. We owe a debt of gratitude to the folks at Horizon Hobby for their initiative. They are also responsible for the magnificent Blade CP helicopter, which I also own.

Attn: There is a newly written section below on the operation of the Cypress Wireless USB ICs below. Be sure to read it to clear up a lot of questions people have about the operation of this system!

New Additions

Horizon Hobby sells a capacitor to the receiver that prevent a possible lock-up condition when high current draw servos are used (as with for a sail for a boat). Part number SPM1600, $6.

Click here for a very cool method for greatly extending the transmitter operation time by making a simple replacement of the linear regulator with a switching supply that is a direct solder-in replacement. Compliments of Dimension Engineering.

Click here to download the user's manual. It contains all the programming information.

If you happen to be an RF engineer or hobbyist, please visit my RFCafe.com website.

What makes the Spektrum DX6 radio control system so unique is that it is the first commercially available system that, number one, it operates in the unlicensed 2.4 GHz frequency band and, that number two, it employs spread spectrum digital encoding to effect the control signaling. The 2.4 GHz unlicensed band is one of several ranges of frequencies that the U.S. Federal Communications Commission (FCC) has reserved for operation of a class of devices designated as Industrial, Scientific, and Medical (ISM). It happens to be the band on which the vast majority of computer wireless LANs (WLANs) operate (IEEE802.11b/c/g/n), as well as all of the available Bluetooth devices. Microwave ovens, wireless security systems, and a host of other products also run in the 2.4 GHz band.

So, with all those systems competing for usage in the same bandwidth, why are there no serious interference problems? The answer is in the spread spectrum encoding-modulation and demodulation-decoding schemes. Radio systems like the traditional 72 MHz band versions modulate servo positional information onto carrier tones tuned to discrete frequencies (pulse-position modulation, PPM, and pulse-coded modulation, PCM). While the intelligence is digitally encoded, the effect is to cause only a relatively small deviation in the carrier frequency off of its center frequency. There are a total of 50 ranging from 72.010 MHz (ch 11) to 72.990 MHz (ch 60), with 20 kHz (0.020 MHz) spacing between channels. Each transmitter must be careful not to modulate its signal far enough in to an adjacent channel to the point that interference is experienced.

In contrast, spread spectrum systems occupy a wide bandwidth (often 1 MHz or more) and digitally "spread" the encoded signal out across the entire band through modulation. The digital information is encrypted with a sort of security code that makes it unique, and the receiver must decrypt the message using the same code. Depending upon the instantaneous digital code value (millions of possible codes), the transmitter frequency is tuned (modulated) to a specific frequency within the allocated band.

Accordingly, there are as many possible frequencies to tune to in the band as there are digital codes (millions). Since each data bit makes up only a small portion of the total message, the transmitter broadcasts on each frequency for a short period of time.

This pseudorandom assignment of frequencies accounts for the "spread" part of spread spectrum since it spreads the signal over a wide spectrum of frequencies. In the receiver, the information is demodulated and decoded and the individual bits are reassembled into a recognizable string of data that is sent to the appropriate servos (or motor electronic speed controller, ESC).

Additional digital bits are added to the data stream, and are used to enable the automatic correction of a certain number of missing bits that arrive at the receiver – this is called error detection and correction. Bits can be missed in the receiver for many reasons, chief of which are low signal level (separation between transmitter and receiver too great) and interference from another radio system. So, if a data stream that is a couple thousand bits long is missing a few valid bits, the microprocessor can determine what the correct value ("1" or "0") should be. This is where the spread spectrum scheme provides its immunity to interference. The probability of two or more transmitters broadcasting a single bit at exactly the same frequency within the broad bandwidth is extremely low. However, if they do, then each affected system's receiver is capable of "filling in" the missing information and the signal gets processed as if no problem existed. Your servos never get issued incorrect commands.

These Spektrum DX6 transmitter spectrum analyzer plots were made on a Rohde & Schwarz instrument. As can be seen, there are two signals, one at 2433 MHz and the other at 2473 MHz, that are separated by 40 MHz. The signals are sent alternately. Zooming in on the 2433 MHz signal reveal that it has about a 830 kHz bandwidth. Carrier suppression is not the greatest as it can be seen protruding above the spread spectrum waveform in the center. The 2473 MHz signal is similar to the one shown here.

The FCC describes two fundamental types of spread spectrum methods: frequency-hopping spread spectrum (FHSS) and direct-sequence spread spectrum (DSSS). The Spektrum DX6 radio control system employs DSSS. The Federal Communications Commission (FCC) requires all manufacturers of equipment that is capable of generating radio frequency (RF) energy, whether intentional (a valid signal) or unintentional ("noise"), to submit products to an approved testing laboratory to validate that they conform to FCC guidelines. Such products carry an FCC approval identification sticker, and on the sticker is an FCC ID number.

The FCC maintains a public database of all submissions, with great detail as to the test methods and results, as well as photographs of the inside and outside of the products. The inside photos are of particular interest, since they usually provide a pretty good image of the printed circuit boards (PCBs) and in some cases, enough clarity to make out component part numbers.

Binding

The binding process has been a mystery since it was first introduced. Here are excerpts from the Spektrum DX6 user's manual:

"Transmitter - When the transmitter is turned on, the system scans the 2.4 GHz band, finds an open channel and locks on that channel. Next the transmitter scans for a second open channel and, when found, transmits on that second open channel. The system is now transmitting simultaneously on two 2.4 GHz channels, giving two paths of security.

Receiver

Each AR6000 receiver features patented DuaLink (pat pend) technology and is actually two receivers in one, hence the dual antennas. When turned on, the first receiver scans the 2.4 GHz band until it finds the specific transmitter's code (called GUID) that it has been programmed to recognize (see binding page 24) and locks on that signal. Then the second receiver scans the 2.4 GHz band, finds the second transmitted code that it's been programmed to recognize and locks on that signal. This whole process takes less than 5 seconds. The receiver is then locked to that transmitter via two independent channels, and is virtually immune to model generated or outside interference."

Keep that in mind while reading the next section.

Cypress CYWUSB6953 & CYWUSB6935 Transceivers

Cypress pulled the full datasheet for the CYWUSB6953 transceiver, which is the one used in both the transmitter and the receiver, and replaced it with a partial version. The original had an extensive functional description of the entire chip. Now, it refers the reader to the datasheet for the CYWUSB6935 transceiver for a functional description of the RF section. I have included links to both IC datasheets below (under the photos).

A lot of discussion has taken place about the frequency and operation of the transmitter. Here is a quote from the datasheet, "The CYWUSB6935 contains a 2.4 GHz radio transceiver, a GFSK modem, and a dual DSSS reconfigurable baseband. The radio and baseband are both code- and frequency-agile. Forty-nine spreading codes selected for optimal performance (Gold codes) are supported across 78, 1 MHz channels yielding a theoretical spectral capacity of 3822 channels." I guess that settles it.

The datasheet specifies a maximum output power of 0 dBm (1 mW). However, the RF PCB of the transmitter has a SiGe SE2526A front-end module, which is described as follows, "SiGe Semiconductor's SE2526A is a complete RF front end integrating a high-performance power amplifier, power detector, transmit/receive switch, diversity switch and harmonic filtering into a compact chip-scale device that is fully matched to 50ohm. The SE2526A replaces up to 40 components typically required for the RF front-end, thereby simplifying the design of wireless systems. Unigen's Juno-LPA modules, based on Cypress Semiconductor Corp's WirelessUSB LR (CYWUSB6935) radio-system-on-a-chip device and SiGe's SE2526A, are able to transmit and receive wireless signals over a range up to 1 km." ...and that settles that.

So, what we have here is a basic short-range wireless SoC (System-on-Chip) mated to a front-end module that has a power amplifier (PA) to boost the output power for longer ranges, and a built-in antenna switch to route the RF energy either from the CYWUSB6953 RF Out port through the PA and to the antenna, or from the antenna to the RF In port on the CYWUSB6953. In doing so, it allows the transmitter to "listen" for active transmitter channels via its receiver RSS (received signal strength) function and assign transmit channels on only clear frequencies.

That leaves open the question about exactly how the receiver, which contains two identical CYWUSB6953 circuits, goes about binding with the transmitter and detecting the operating frequencies. Does the receiver transmit a low-level signal out to the transmitter so that the two can arbitrate clear channels of operation? That possibility was suggested by a person who wrote to me recently (thanks to Mr. Rod W.). The answer is that I still do not know for certain, but an alternate solution is proposed next.

From the description given in the user's manual (see excerpt above), it seems that during the binding process the receiver listens for a special signal from the transmitter that tells it what the GUID (basically an encoding sequence) of that transmitter is. The binding operation is separate from the receiver power-up process. My guess is that all transmitters use a certain channel to send their GUIDs so the receivers know to always go there to learn them when binding. Then, during power-up, the two receiver channels use that GUID code when searching for which two frequencies the transmitter has chosen. In that manner, it is never necessary for the receiver to broadcast any information back to the transmitter.

OK, so where do the 4.2 billion possible codes come from? 4.2 billion is roughly the number of unique codes available in a 32-bit binary system, which is likely the code length used by the Spektrum DX6, compliments of the Wireless USB IC. Recall that the IC was originally designed to operate in a very dense RF environment with data security being a high priority. The more bits that are used to encode data, the more difficult it is to hack (decode, intercept, pirate, etc.). A lot of data intercepting schemes rely on brute force processing to basically try every possible code sequence possible until the correct one is stumbled upon. It takes a long time to try 4.2 billion codes, even at modern computing speeds. Still, stories have appeared recently about the relative ease with which the standard 40-bit (1.1 trillion combinations) encryption on RFID devices can be cracked using just a laptop computer. For another reference point, the encryption scheme used for the https:// secure websites uses 128 bits, which represents 3.4x10^38 (that's 34 followed by 37 zeros) possible unique codes.

Transmitter

The Spektrum DX6 transmitter carries an FCC ID number of R8KUGWR2USXXXX. The hyperlink attached to the FCC ID number is the page on the FCC website that contains links to all the available documents on the transmitter. The internal photographs document shows a wireless spread spectrum module manufactured by Uniden (Fremont, CA). This is somewhat odd, because it appears that only the module itself has been qualified, and not the entire transmitter - which itself has digital circuitry that could radiate unintentionally in excess of the allowable limits. This module and radio system operates in the 2.4 GHz Industrial-Scientific-Medicine (ISM) band, which is unlicensed and is the same band as the wireless LAN (WLAN) products that are used for personal computers.

The pictures shown here of the RF PCB removed from my transmitter and the one submitted to the FCC for the qualification tests are essentially the same. The main differences are the much smaller crystal oscillator unit and the elimination of one of the coaxial connectors.

Spektrum AR6000 Receiver

The receiver case is held together with double-sided foam tape attached to the upper and lower PC boards, and then a little silicon glue at the two antenna exit points to provide strain relief. Inside, there are two printed circuit boards (PCBs). One has two duplicate circuits for the spread spectrum receivers and a "motherboard" that contains the signal decoder and servo drivers. Cypress Semiconductor model CYWUSB6953 "WirelessUSB™ PRoC™ Flash Programmable MCU Radio" integrated circuits (ICs) are used (see online datasheet), along with 13 MHz oscillators. The Cypress IC is capable of performing both the transmit and the receive function, making it extremely versatile.

There does not appear to be a preamplifier used on the receiver front-end, so the exceptional range of 1,500 feet or more is provided pretty spectacular. Even though these types of direct sequence spread spectrum (DSSS) systems in the ISM band (2.4 GHz), like wireless local networks (WLANs) and Bluetooth systems are typically advertised for operation over relatively short ranges (300 feet or so), the range is for a given minimum data rate.

At greater ranges, the error rates increase and more packets of data are lost and must be resent, or the system data rate must be slowed down. Radio control system data rates need only be in the tens of kilohertz realm and not in the tens of megahertz realm, so the operational range is extended. There is a Factoid that I wrote for my RF Cafe website that describes the large number of WLAN networks I was able to detect in a Podunk town in West Virginia, where the houses hosting the WLAN routers were 100-300 feet away.

According to the datasheet, receive mode current is 61.3 mA. I measured the current draw of the external Electronic Speed Control (ElectricFly ESC-10) AR6000 receiver (motor off, servos idle) and got 80.3 mA @ 4.88 V. The current was measured between the NiMH battery pack (9.6 V, 650 >mAh) and the ESC, and the voltage was measured across the positive and negative pins of the receiver channel 6 pins. At some point, I will measure it without the ESC, but for this measurement all I had on-hand was a 7.2 V Li-Poly battery pack in my Li'l Poke airplane.

One interesting specification is the electrostatic discharge (ESD) ratings, which are only 500 V on the RF (antenna) pins and 1,500 V on all other (power and control) pins. The de facto industry standard for silicon devices is 2,000 V minimum, with 4,000 V being typically seen. A static charge of 500 V can easily accumulate on the human body simply by walking across a carpeted floor or even combing the hair with a plastic comb. If such a charge finds a discharge path through one of the two antennas, the even could (and likely would) cause a failure in the IC. Accordingly, extreme care should be exercised to avoid contact with the ends of the antenna wires. Placing a small dollop of silicon (e.g., tub caulk) on the ends of the antennas will actually provide a little extra protection since it will prevent direct contact with the copper wire inside the insulation.

Gimbal Centering

Someone wrote asking about how to convert the left gimbal throttle stick to be self-centering like the other three axis. Unfortunately, the conversion requires the installation of both a spring and a plastic lever that rests against two pins on the stick axis. From what I have read on the forums, getting Horizon Hobby to provide replacement parts for the Spektrum DX6 system is like trying to get blood out of a turnip, as the saying goes. It seems everyone has had to come up with a work-around for broken antennas, cables, and anything else. About the only thing to do is to remove one of the existing levels and attempts to replicate it in plastic or metal. It probably would not be really hard to do, but would be time-consuming. The pictures below are close-ups of the gimbal centering assembly. The top one is with the stick centered, and the bottom one is with the stick in a maximum deflection position.

Posted March, 2006