Independent tests prove lack of frequency hopping with XPS

[klhoard]

I've read a few blog and forum postings from guys that are **really** proud of their cell phone jammers and love bragging about how they enjoy going into crowded settings and shutting down folks cell phones. I can only imagine what these jerks will do if they find out they can go to a fly-in and make model airplanes crash.

[Foamaholic]

Is the Maxstream module supposed to hop, or is it an XPS modification that is supposed to make it hop? If the Maxstream hops, then it looks like XPS did something to it to make it not hop.

[XJet]

	Quote: Originally Posted by Foamaholic
	Is the Maxstream module supposed to hop, or is it an XPS modification that is supposed to make it hop? If the Maxstream hops, then it looks like XPS did something to it to make it not hop.

No, the XbeePro modules are not designed to frequency-hop, nor are they particularly frequency agile when driven by user software that attempts to make them hop.

That is why any significant change to the XPS frequency agility will require a return-to-base upgrade, probably involving reflashing the XPS and MaxStream firmware (assuming MaxStream is willing to write the new code), or perhaps even a total RF module replacement. This will *not* be a low-cost operation for XPS so I suggest that it will never happen.

If JD has shipped the "tens of thousands" of units he claims (which I strongly doubt) then it could be an even bigger nightmare than the Futaba ZGUID problem -- given the relative resources of each company).

I repeat my belief that JD will end up trying to resolve this situation by saying "XPS 2.4GHz has only limited hopping abilities" and those who want true frequency agility should upgrade to our 900MHz gear which is a FHSS system"

This will leave the 2.4GHz users with systems that either won't hop or will only hop when Mars is in the seventh house with Jupiter ascending and a 5knot wind blowing exactly from the Southeast (ie: not often if at all).

It's pretty obvious that JD has always known he'd have hopping issues with the XBeePro modules, hence the *very* careful wording of his frequency hopping claims "only hops when potential interference is encountered". If you examine the very precise semantics of that claim, the system may well be working as advertised (who can really define "potential interference"?) -- however, when it really matters (ie: when *actual* interference is encountered) XPS appears unable to change frequencies... but he never claimed it would hop in response to *actual* interference so how can you complain?

Even the ideas that JD has mooted so far for "improving" the XPS frequency hopping are marginal in their practicality and if he can pull off a practical implementation that actually works effectively in the field then I'll take my hat off to him.

[Foamaholic]

Thanks. That's what I wondered about. Lends credence to the tests that show it doesn't hop, and I don't think JD should have claimed that it did, unless he has done something to make it hop, which it doesn't look like he did.

[sweetpea]

great testing Kiwi. It will be interesting to see how the other systems respond to the exact same test setup.

[1bwana1]

I don't expect the other systems to Hop in response to interference.

By design the Spektrum/JR picks and sits on two frequencies. It may be interesting to design a test to see how intelligently it picks these frequencies.

The Futaba FAAST system will hop continuously regardless of how busy an individual frequency is. It depends on being on a given frequency for so short a time that the interference doesn't matter.

The only system that depends on hopping to cope with a changing environment is XPS.

[sweetpea]

I don't expect that either.

I just want to see how each system reacts with the same type of interference.

I want to see Spektrum work on one freq
I want to see FASST hop through the interference

And have both with no ill effects.

[Eflyer_75214]

Hey Kiwi, You buy very expensive servos eh. Per the web site, the Wi Spy 2.4 is $399 and the v1 is $199. Course, when comparing that to a large size model, it is not an expensive item I would guess.

For us guys that go with smaller models, would be a good club purchase.

Really appreciate the time you are putting in to testing the radios. Will you be able to obtain one of the new Airtronics RDS8000 units to add to the test group at some point?

Thanks again,

Ron

[XJet]

On the subject of 2.4GHz spectrum analyzers (SA), I'm just waiting for some parts to arrive before I complete my next version of a two-part SA (and the test-rig sig-gen of course) which can be flown aloft in a model (where it logs the data to memory) or used in hand-held form at ground-level.

My first one used a plain 128x64 LCD display to graph the data in realtime or playback the logged data from the airborne unit but was somewhat limited in spectral resolution. I'm just waiting on the 2.4GHz chipset and an OLED display that would allow a color display to be created.

The new 2.4GHz chipset I'm using also allows me to scan the entire band in 1MHz increments which will carry an overlay that indicates the JR/Spektrum, XPS and FASST channel assignments.

I think these would be a useful pre-flight or airborne black-box device for those who are planning on committing to 2.4GHz.

Once the bits arrive I'll post a video showing the next prototype in action.

I don't plan to manufacture these (where would I find the time?) but if there's a demand I'll probably knock out a few (pretty much at cost) for those who really want them. Price-wise I'd expect them to be less than the WiSpy 2.4GHz USB device and have the benefits of being fully self-contained (no PC necessary), as well as providing in-air data-logging to memory.

[zoomer260]

Double post.

[zoomer260]

Where did all the hecklers go ?

I for one am glad that XJet has shown renewed interest in the thread he started as all the info we as a community can get out of this can only be a good thing !

[JEFFRO503]

I can't tell Xjet and kiwi thanks enough for all the work they have put into this , but i'll try....Thanks guys!!

And onto another couple questions.....

1.) How many actual channels are there for lets say futaba , to be able to hop 200 times per second? What i mean is , how many 2.4 channels is it searching for?

2.) XJET : i'm not sure about the exact wording JD used in his description of the XPS stuff , but he mentioned the words "only hops when potential interference is encountered"........***?? The key word in this sentence is "hops"........that's just it........IT DOESN'T HOP , no matter what. Either "potenial" or "actual" is completely non-existant in this phrase i believe ....isn't it?

3.) when you guys test the futaba , Spektrum or JR stuff , will you test on other channels in the 2.4 range that are close to the freq's these run on , and see if some of the interferance overlaps into the channels the radio is putting out?

The reason for this last question is for something that happened to me just a few years ago. When myself and my dad we're driving a couple of our RC cars together , i was running FM channel 4 ( 75 MHZ) , and he was running AM channel 4 ( 27 MHZ ). I had already been driving for a good 5 minutes , and when he turned his transmitter on , and gave it throttle , my truck went wide open! I had full Brakes on my transmitter , and his car was sitting there with the brakes on.......my truck hit my dad at full throttle at about 35 MPH. It was a T-maxx at the time and it completely sent him to the ground in alot of pain. I have never been able to figure out how that happened and i felt terrible seeing my dad just laying there. I didn't know that FM and AM channels could overlap like that....but they did!

hence my question about overlapping 2.4 signals......could get pretty hairy with two big gassers up there flying around!!

[sweetpea]

Actually you are incorrect........AM 4 and FM 4 are the same channel. One is not 75mhz.....they are both 27mhz. FM in this instance would be less prone to intereference from AM but it will still be affected. Some of the upper end radios were able to reject the AM signal a little more than others but it would still cause unwanted inputs to FM. AM on the other hand was just plain screwed.

This was a major misconception when I was racing R/C cars years ago. Lots of guys thought AM and FM on the same channel couldn't interefere. The channels were the same its just how the signal is sent that is different.

[XJet]

	Quote: Originally Posted by JEFFRO503
	1.) How many actual channels are there for lets say futaba , to be able to hop 200 times per second? What i mean is , how many 2.4 channels is it searching for?

There are no fixed number of channels on 2.4GHz -- it's just a range of frequencies that can be divided up into as many (or as few) smaller chunks of frequency (channels) as you want.

In theory, you could use the whole 2.4GHz band as a single channel, or divide it up into hundreds of much narrower channels -- it's really up to each manufacturer to decide what's the best way to achieve the desired results.

Spektrum/JR have divided it up into 80 channels and use two at a time, allowing up to 40 simultaneous users.

Futaba has divided the band up into 32 (or thereabouts) channels.

XPS has divided the band up into just 13 channels -- which means even if it can hop, its options are limited to one of just 12 other segments of the band.

	Quote:
	2.) XJET : i'm not sure about the exact wording JD used in his description of the XPS stuff , but he mentioned the words "only hops when potential interference is encountered"........***?? The key word in this sentence is "hops"........that's just it........IT DOESN'T HOP , no matter what. Either "potenial" or "actual" is completely non-existant in this phrase i believe ....isn't it?

I strongly suspect that JD has done his best to implement a hopping system within the constraints of the XBeePro modules he's using. Unfortunately these modules just aren't designed to be frequency agile so although he may have been able to get XPS to hop under carefully controlled circumstances, the practical effectiveness of that hopping seems to be virtually zero in the real world -- which is where it counts.

	Quote:
	3.) when you guys test the futaba , Spektrum or JR stuff , will you test on other channels in the 2.4 range that are close to the freq's these run on , and see if some of the interferance overlaps into the channels the radio is putting out?

My tests will include using signals that:

1. closely match the operating frequency and spread of each system
2. intrude into just part of the frequency spread used by each system
3. cover and extend beyond the frequency spread used by each system

I'll also try different duty-cycles (ie: turn the interference off and on rather than simply provide a continuous signal) to see how well the various systems actually recover from a bad burst of noise. Bad noise will eventually knock out *all* of the systems (they're not bullet-proof) and the speed at which they recover and restore control is a very important (IMHO) aspect of their operation.

	Quote:
	hence my question about overlapping 2.4 signals......could get pretty hairy with two big gassers up there flying around!!

The interesting thing about Spread Spectrum, especially the way most of the RC systems are implemented, is that you can actually have more than one system sharing a single "channel" without causing problems.

That's because they're not on all the time like the old 72MHz systems are. All of the SS systems I've seen to date use some form of multiplexing which basically means they can take turns using a single chunk of frequency without causing too many problems. The way the signal is also distributed across the channel as part of the spread-spectrum technique also means that even when two dissimilar systems are transmitting at the same time it can still be possible for them not to interfere) In fact this is an essential aspect of this RC gear and explains why the constantly hopping FASST system doesn't knock out XPS or Spektrum when it periodically transmits on the same frequencies that either of the others may be using.

I'll have to get my finger out and put my 2.4GHz information pages up on my website. These hopefully explain many of the small (but important) details with more clarity than I can provide here.

[JEFFRO503]

	Quote: Originally Posted by sweetpea
	Actually you are incorrect........AM 4 and FM 4 are the same channel. One is not 75mhz.....they are both 27mhz. FM in this instance would be less prone to intereference from AM but it will still be affected. Some of the upper end radios were able to reject the AM signal a little more than others but it would still cause unwanted inputs to FM. AM on the other hand was just plain screwed. This was a major misconception when I was racing R/C cars years ago. Lots of guys thought AM and FM on the same channel couldn't interefere. The channels were the same its just how the signal is sent that is different.

Sweatpea..what b do you mean they are the same? All older radios for ground control we're 27.something before the big switch to 75mhz. My M8 ( which is one of the top of the line radio's for surface) is 75mhz . Are you saying that 27mhz and 75 mhz is the same? i don't get it.