Spektrum 2.4 Technical FAQ's
Spektrum radios are very popular and since I've made my own DSM2 compatible receivers I have some technical observations that people might find useful below.
Spektrum/JR rx's all use an RF layout recommended by their chip manufacturer and I would expect them to also use components that meet their specification. Where I've followed the same approach with my rx's I get excellent results. 2.4 rx's are 'synthesized' and derive their frequency from a fixed value crystal. The accuracy of the crystal affects range and interference resilience. The crystal in the Spektrum rx's I've tested have been well within spec.
Here is an example of the performance a good AR6100 copy. This is a receiver with single RF board and a single active aerial receiving 96.5% of all signals to 1700 feet. It never lost as much as half the signals in any 1 second. In other words, this would not have come close to triggering failsafe on a Spektrum rx. Seems pretty good to me. I'd expect all Spektrum rx's to be at least as good. Although Spektrum classify the AR6100 as a parkfly rx, in practice I find this level of performance to be good enough for me as a full range 'sport' rx. Extra features which enhance this performance are discussed below.
MULTIPLE AERIALS & RECEIVERS
Antenna Diversity: Signal strength is greatest when the Rx and Tx aerials are parallel. Multiple aerials exploit this when orientated at 90' to each other because the model is constantly changing atitude and direction. Quantifying the benefit is difficult but expecting 40% greater range would be reasonable from tests I've done. Note that only the last 30mm is the active part of long aerials (eg: AR500).
RF boards: The simpler Rx's have one set of receiving components; they use an electronic switch to toggle between their aerials when they have two. The multiple board Rx's have a set of receiving components for each aerial (ie: multiple receivers). This allows every transmission to be received on all aerials simultaneously thus maximising reception, reducing latency and improving servo accuracy (fewer 'holds').
Multi-path: Satellites are receivers in their own right. The extra advantage they provide is that they can be positioned away from objects which may block signals (eg: engines, fuel tanks, batteries, etc). Spektrum call this MultiLink. Receivers with long aerials (eg: AR500) are also useful in this regard. Please note that humans block signals so don't stand immediately behind anyone when flying.
A special point is needed on aerials for some receivers. The AR6100, AR6200, AR7000 and Satellites have two aerial wires each. These act as one aerial (a dipole design). As such they are orientated in a straight line (180') in Spektrum marketing photos which is how they are intended to be used. The aerial pairs in the base AR6200 and AR7000 receivers should be orientated at 90' relative to the pairs in their satellites to optimise the antenna diversity.
||1 active aerial
||2 active aerials
||3+ active aerials
|1 RF board
|2 RF boards
|3+ RF boards
Failsafe vs Hold: Spektrum Rx's memorise stick positions during bind and may use these for failsafe. On low end Rx's, only Channel 1 (marked Throttle) is driven to this position on failsafe; the other channels hold the last position before signal loss. Spektrum call this 'SmartSafe'.
High end Rx's use SmartSafe by default but all channels can be driven to their failsafe positions on signal loss. To activate this feature insert bind plug, switch Rx on and then remove bind plug. Now switch Tx on to bind.
The term 'hold' can have two meanings in a Spektrum context. It's normal to miss occassional signals during flight as shown in the chart above. On an old FM based system some of these might have been glitches. In 2.4 based systems, the Rx only responds to perfect signals and replays the previous good one until a new good one is received. This is normally called 'hold' and it is common for there to be hundreds and even thousands of this type of hold in a single flight.
Should the signal be lost for an extended period of time (about 1 second), 'failsafe' will eventually be invoked and servos will be moved to those positions instead of holding the last good one. New Spektrum Rx's have a second Led. Spektrum says the number of flashes indicates the number of 'holds' but what they mean is the number of 'failsafe' events experienced (ie: the number of times signal was lost for 1s or more).
Should an Rx ever stop responding in flight this is called a 'lockout'. 2.4 Rx's are controlled by software programs. Lockouts should never happen and are usually caused by a software bug.
Resolution: Tx's convert stick movements into numeric values. In the DX5/DX6i/DX7 there are 1024 positions at max throws including trims. 1024 may seem a strange number but it is derived from the binary numbering system and 8 bits = 256, 10 bits = 1024 and 11 bits = 2048.
On 1024 systems, each trim click represents a movement of '4'. 1024/4 = 256. So if you consider a single trim click to be a fine movement, a '256' based system would be enough for you and a '1024' system 4x more accurate. Certainly good enough for most modellers.
The DX7SE/DX8/DSX9 and other high end tx's convert stick movements into 2048 positions. Only certain rx's (eg: AR6255/AR7600/AR9000) can benefit from this increased resolution; the Tx will use 1024 with receivers that only can accommodate 1024. Certain tx's allow you to change the trim click resolution (eg: DSX9) like the Futaba FF9 allows but few people realise exists.
Brownouts: Spektrum receivers require a little over 3v to operate. Their manuals say more to provide a sensible buffer. 4.8v batteries and 5v BEC are therefore normally fine. However, what is not well known is that servos, when they work hard, draw fairly large currents which can reduce battery voltage to 3v and less. These dips are usually so short they can only be seen on an oscilloscope. To help prevent the rx from seeing them, each rx has a capacitor to store some charge. Spektrum seem to use the same size capacitor on every rx (220uF). So an AR9000 with three RF boards will use that up three times quicker than an AR500 with only one RF board.
While it's worth knowing about brownouts, you should not fret unduly about them. The main Led on Spektrum Rx's flashes when a brownout has occured. So you can see if you are experiencing this by wiggling your sticks wildly on the ground. This makes the servos draw heavy current. If the Led stays solid with a flat battery then you don't have a problem. If you experience resets (flashes), you should use a larger/higher quality battery. Load test the battery before every flying session or flight. Spektrum also sell an addon capacitor to provide more short term storage. DIY buffs can make their own or replace the internal cap with a higher value.
Should you experient a brownout, the rx will reset itself during which time you have no control. This should be quick and may appear as a glitch. The Rx has to rescan for the frequencies your Tx is using. To accelerate this process, Spektrum save the frequencies that were previously in use which allows a 'quick reconnect' capability. Some receivers were initially released without this so check that yours works.
DuaLink: Spektrum transmitters always use two frequencies and hop between them. Both carry the same data so their system has a 100% redundancy.
Current consumption: Spektrum RF boards draw about 18mA on average and the Led another 5 or 6mA. So an AR6100 draws about 24mA, the AR500/6110/6250 about 28mA, and the multi-board Rx's proportionately more. This is vastly more than FM Rx's.
More information on 2.4GHz can be found here:
* Overview of 2.4GHz
* 2.4GHz scanner
* 2.4GHz range tests
* My 2.4 GHz Receivers
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