Radio Systems – how to select a radio system - BASIC

[Armortek]

Modern radio control systems suffer from an alphabet soup of acronyms and obscure gobbledegook. Navigating through these to a good choice of radio for your Armortek model can seem daunting at first sight. This topic is in two parts, the first a guide to help make sense of the terminology (which is for guidance and can be skipped) and the second, which has the vital knowledge to avoid the very few systems which won’t work.

First the techie bit.

Modern RC Systems allow lots of users to operate on the same frequency band at the same time – a good thing. They also have good resistance to loss of signal from interference. Another good thing.

They do this by using a variety of proprietary waveforms based on spread spectrum technology (ie they spread the carrier signal over a wide band of frequencies and do clever stuff with it).

The most sophisticated systems combine spread spectrum (the wide band bit) with one or more frequency hopping modes (the clever stuff bit) to give added protection from interference and signal loss. You can think of this as providing a moving target which makes it difficult for interference to hit.

The spread spectrum bit is standard for all modern radio systems. The clever bit is the way they change the frequency of the carrier. There are two approaches which aren’t mutually exclusive (ie they can be (and are) sometimes combined).

Frequency Hopping Spread Spectrum (FHSS) hops over a range of channels and is frequency agile but can be vulnerable to interference from overlapping or simultaneous signals. Direct Sequence Spread Spectrum (DSSS) alters the phase of the signal and has the opposite advantages/limitations. Later hybrid systems use a combination of FHSS and DSSS. OK so far?

The “take away” is: RC radios are resistant to interference. They can hop around on a pogo stick or hop from one foot to another or – clever boys – both at the same time.

Now for the alphabet soup (which is all about marketing hype and technobabble). The most common brands are:

The FHSS group:

Futaba: FASST – Futaba Advanced Spread Spectrum Technology (Futaba brand name),
T-FHSS (Futaba) – Futaba FHSS system with Telemetry capability (Futaba brand name)
S-FHSS (Futaba) – Futaba FHSS system with S-Bus capability (Futaba brand name)
Hitec: AFHSS Advanced FHSS – brand name for Hitec FHSS
Carson RC Systems: budget FHSS

The DSSS group:

Spektrum: DSM2 – early system using DSSS and a second signal channel. Spektrum brand name.

The hybrids with both FHSS and DSSS:

Spektrum: DSMX – hybrid combining DSSS with a proprietary frequency hopping mode. Spektrum brand name.
Spektrum: DSMR – hybrid combining DSSS with proprietary frequency hopping over a narrower band. Intended for RC vehicles and boats. Spektrum brand name.
JR: DMSS – hybrid combining FHSS and DSSS. JR brand name.

What about the older stuff (sometimes called “legacy kit”)? The modern systems operate in a broad frequency band around 2.4GHz. Earlier systems used discrete frequencies generated by crystal in a narrow band around 27 or 35 MHz, are prone to mutual interference and are not recommended. 40 MHz systems for ground use are also available in UK and are also not recommended (and you’d look uncool if you used them).

The final variable of interest is the rate at which positional data is sent over the carrier from Tx to Rx and the rate at which that data is sent from the Rx to the servos. This is called Frame Rate. The “standard” frame length is 22ms (= frame rate of aobut 45Hz or frames/sec) but newer systems can also use shorter/faster pulses of 11ms or even less. (Note: manufacturers use frame length/rate interchangeably, so will refer to a “22ms frame rate”).

If you really want to read up on this, I’d recommend the page below (and the next three) for a (relatively) simple explanation:

http://www.rcmodelreviews.com/spreadspectrum04.shtml

The bit you need to know

Now the bit you need to know before you can choose a radio system compatible with Armortek:

Armortek Motion Systems

Armortek Motions Systems are now in their third generation:
- Up to 2009: system based on 4QD commercial speed controllers (no longer supported)

- 2009 to 2016: proprietary modules:

- From 2016 (SdKfz 251, Tiger 131, Morris Quad onwards): simplified and improved system based on two modules. Compatible with all known existing transmissions systems:

Here’s the beef:

The Armortek system measures signal frame rate to generate the appropriate motor response to drive the model. There are a very few systems which are incompatible. Here’s why.

The second generation “2009” modules can accept frame rates above 14ms. Known incompatibility issues include:

Spektrum radios allow selection of frame rates (usually either 22 or 11 ms). As long as 22ms is selected, the system will be compatible.

Futaba DX7se (note the “se”): budget system with 11ms frame rate only. Not compatible. (DX7 without the se is compatible).

Futaba J series: S-FHSS mode uses 6.8ms fast frame rates incompatible with Speed Controller. FHSS mode uses 13.8ms and is not compatible. It also restricts Tx to four channels only (ie not much use for RC tanks).

The third generation (ie latest) modules can accept faster frame rates.
In our factory we use the following for testing:
- Spektrum DX6 Transmitter and AR610 Receiver
- Futaba T6K Transmitter and R2006GS Receiver

There are known compatibility issues with T-FHSS receivers.