Review: FrSky/FriSky 2.4GHz FHSS (Getting Technical)

THIS SYSTEM STARTS TO SHINE

Dated: 20 Jan 2010

FrSky claims that their system uses "Advanced Continuous Channel Shifting Technology" but that's just marketing-speak for frequency-hopping.

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Looking at the system on a spectrum analyzer it is apparent that it makes good use of the 2.4GHz band, using a total of about 50 different frequencies that appear to be switched between on a pseudo-random basis.

More technical detail can be had by looking at the documents held by the FCC as part of the certification process -- yes, it has FCC certification if that's important to you.

Also, because of the power levels and modulation type, this system should also be legal in many parts of the EU (suitable approvals notwithstanding).

The Transmitter Modules

The Futaba/Hitec module is designed to work either-way up, since Hitec and Futaba orient their modules differently. The use of a rear-mounted antenna means the FrSky module is just as happy in a Futaba transmitter as a Hitec one although the trade-off is that when either transmitter is laid-down, the antenna itself will take some of the weight.

Other manufacturers such as WFly have opted to have a top-mount antenna on their Futaba modules but that makes them very awkward to use in a Hitec transmitter, since the antenna then faces directly downwards. FrSky's choice (like Corona's) is a reasonable compromise.

I had some difficulty inserting the Futaba/Hitec module into a Futaba 9C radio, although it fell smoothly into my Hitec radios -- go figure. A little adjustment of the pins and some relief with sandpaper fixed the 9C situation though.

The JR module fits very nicely in my 9XII/9303 radio and because there's only one way up for JR modules, the antenna is mounted on a plastic platform so that it fits nicely between the handle and the case. This means that when the radio is laid down it's the handle that carries the weight, not the antenna. This is good -- Assan could learn from this setup.

When using the JR-compatible module with my early iMax 9X radio I found that the pins in the back of the iMax were not long enough to ensure a reliable connection with the module. FrSky assure me they are fixing this occasional issue and I know of several more recent Turnigy/FlySky radios that are using the FrSky system without issues.

Inside the modules there are two PC boards, one containing a bit of logic and a power regulator, the other carrying the 2.4GHz circuitry. Pretty standard stuff here, not much to complain about. Assembly and design is to a good standard (unlike some early Chinese-made transmitter modules from other companies such as FlySky).

Because the module is happy to run from as little as 6V, the JR version is an ideal candidate for use in the FlySky/iMax/Turnigy 9X with my $8 lithium battery modification.

The Receivers

The 8-channel receiver has two antennas which feed into a front end that allows switching between the two.

Does this mean that there is true diversity?

Yes. On inspection it is clear that the receiver is rapidly switching back and forth between the two antennas, allowing the strength of the signal from each to be compared and the strongest one used for maximum range and reliability. This comes as a pleasant contrast to some other Chinese-made RC brands which either include the second antenna "just for show" or simply tie the two together thus providing a system that is no better than (and sometimes actually inferior to) a single-antenna.

And speaking of range...

The 8-channel unit claims a range of 1.2Kms which would appear to be somewhat conservative, based on my own testing.

Placing the transmitter approximately 1 meter above the ground, I walked the length of our local airfield (around 1,000 meters) with receiver in-hand. At the end of the tarmac, everything was still working as it should and the receiver still had a solid lock -- even when I placed my body between the distant transmitter and the receiver clutched in my hand.

Suffice to say that range ought not to be an issue with this system. The RF decks being used have long-proven themselves to be more than up to the task of creating a strong link between transmitter and receiver.

The tiny 4-channel receiver is supposed to have somewhat less range but, since it's designed obviously for smaller models, the claim of 500m would seem more than adequate.

Low-voltage performance and reboot

I threw the 8-channel receiver on the bench and wound down the voltage until it stopped working. Like many of its peers, this Chinese-made 2.4GHz system performed very well -- far better than some of the more popular Spektrum systems.

It wasn't until the operating voltage fell below 2.8 V that the receiver's LED started blinking and servo movement stopped.

When the voltage was slowly restored, the receiver instantly burst back into life with an almost non-existent reboot time.

Similarly, turning off the transmitter causes the failsafe to kick-in and turning it back on results in a near-instant restoration of control (the only delay really being the time the radio itself takes to boot up).

What's more, this system is very quick to link-up when you first turn it on and there's no tendency to drive the servos to their stops before link-up occurs, as has been observed on some other Chinese-made 2.4GHz receivers.

I'm actually very impressed with the reboot characteristics of the FrSky.

How good is it, really?

One thing that impressed me with the FrSky system was the smoothness of the servos when using this system.

I was appalled at the way the Corona FHSS system made the servo response so incredibly jerky due to some kind of random latency it introduced and I've heard rumors that some folk have noticed a similar (albeit not so severe) effect with their DSSS system.

Fortunately, no such effects were noticed, either on the JR or Hitec/Futaba modules.

I've been flying the test system in my new 46-sized 3D profile plane for a few weeks and it's both responsive and rock-solid in performance. Why do I test these systems in a highly aerobatic profile? Simple -- the massive control surfaces and huge throws soon show any glitches or momentary lock-outs that might occur and would otherwise go unnoticed on a regular sport-type model.

Since this review was originally written, the FrSky system has been flown in two of my pulsejet powered models (Youtube Video of my pulse-jet Tamecat)) with great success.

Another factor that has really impressed me about the FrSky gear is that the manufacturer listens and acts on feedback. During the review process I made several suggestions to FrSky about things that could be improved -- they've taken all those points onboard and the systems now being shipped include those changes.

What's more -- FrSky aren't sitting on their laurels -- they have a bi-directional telemetry-enabled system on the way (I'm about to start playing with a pre-release sample). This is very cool stuff and I'll be publishing a whole lot more about this in coming weeks -- including details of a DIY telemetry hub and some DIY sensors for things such as air/ground-speed, altitude, etc.

FrSky certainly seem to have set a new standard for Chinese-made 2.4GHz module/receiver systems, let's see if the others can catch up.

Pro's:

• Good antenna placement on JR module
• Frequency hopping
• 8-channel and lightweight 4-channel receivers
• Very good low-voltage performance
• Excellent reboot times
• Easy to bind and fast link-up
• Very smooth servo movement
• Very effective, easy to set failsafe
• FCC Certified
• A low-power range-test mode
• Made by a company that listens to customers

Cons:

• Futaba module too tight in 9C transmitter
• Only 8-channels when used in a JR 9-channel radio

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