Independent tests prove lack of frequency hopping with XPS

[jonkoppisch]

Oh Please.. FB's bring it up all of the time to justify the hop..

Obviously, that's where it came from..

[Woketman]

Instead of using the word "Boeing", why not just substitute "Fantasyland" from now on when describing that test? It should result in the very same credibility!!!

[GBR2]

The main problem with the hopping issue isn't if it hops (which it probably does but only under those super specific XPS mystery conditions) but whether it hops in a useful manner to provide frequency redundancy. Spectrum does that through using 2 channels and FASST by hopping all the time. I've talked to numerous members in our club and given them this JD quote:

“We are the ONLY 2.4GHz system that has the ability to monitor the available frequencies in the 2.4GHz range and change as necessary, in real time. This means that the plane as well as the transmitter can make the change occur. If you flew your plane over a school with a huge 14dbi external 802.xx antenna, that would likely be cause for switching to a new frequency (away from whatever channel that 802.xx was on.”

They all assumed that if interfering noise were to show up on the "channel" in use and there were clearer ones, that the system would just move to it. That is the kind of frequency redundancy hopping that people expected based on all the marketing talk JD did about the system. The videos by Tychoc and Kiwi show that this type of frequency redundancy hopping is not currently present in XPS hardware. Whether it hops under those very specific XPS noise hopping conditions really is immaterial as it isn't very useful.

I know one of the main reasons I purchased a small amount of equipment early on was I assumed it had the type of frequency redundancy that the JD marketing campaign was promoting. I also liked the fact that it was made in the US but in the end I feel I was basically deceived about an important aspect of any 2.4 system I would consider purchasing. Currently I'm using Spektrum equipment.

[Kiwi]

I often wonder if the continued denial of what I found and what others have shown as fact is in fact an attempt to have this thread locked down so it disappears.

It makes sense if you keep nagging at the denial syndrome people get bored or sick of it and close it.

Well that is not going to happen as I'm waiting for the Mk2 version to be released from XPS so we can do some field testing on it as well.

Just for the record I have three other brands in the gunsight and pre orders are placed for a sample. They too will be put through the same sequence of tests.

In the time being if any of you are interested in delving into the 2.4Ghz quagmire dont forget the guys who helped us do this.

www.nutsaboutnets.com

www.metageek.com

Take a look at the websites and see what new data they keep coming up with.

[Gordito Volador]

	Quote: Originally Posted by capgains
	Yep this is a whole new level of obsession.

Absolutely correct! The Fan Boys get their marching orders and here they come to re-educate the unclean. Just watched another XPS at our field lock out on a big gasser doing 3D. Still waiting for the post mortem, but I'll bet in the end the grass was wet.

Many thanks to Kiwi for his efforts to visually document this issue!

Regards, Bill

[adjonym]

	Quote: Originally Posted by skubacb
	As JD has proven himself? <img> humour, humour, humour. This besides being funny may even be delusional. I don't claim to have read everything he has said but this is certainly not correct. One last time; for the intellectually challenged: The XPS system will only hop under specific conditions. Apparently, Kiwi's 'test' did NOT provide those conditions.

Once again, for the intellectually challanged: Extreme Power System is required by law to provide the evidence needed to back up their claims in marketing.
Now, point me to the evidence XPS has provided about their frequency switching technology claims.
And again, for the intellectually challenged ones, that means that the burden of proof lies with the manufacturer, not with the customers, or possible customers. Those laws are in place to protect both customers AND possible customers, so that they can base their purchase on the features a product actually have, not what the manufacturer CLAIMS that they have.

Now, if all comanies acted as Extreme Power Systems, you'd have cancer medicine with 100% success rate, all it would take would be a 1-800 number and a credit card. You'd have cars with 10000mpg rating, just with the addition of the X capacitor, and so on and on. These might be bad examples, since they are easy to spot, and turn away as bs, but you get the idea.

I think this is the base of all this fuzz. We, as consumers should always act upon company's that refuses to follow those laws, and even base their marketing upon it(JD claims he won't provide proof since it helps sales). Those laws are there to keep scammers out of the loop.

No matter how you turn things around, the fact still remains; Extreme Power Systems are breaking the consumers law, and continue to do so well aware of the fact that they are indeed breaking those laws.

[wrightme]

	Quote: Originally Posted by Gordito Volador
	Absolutely correct! The Fan Boys get their marching orders and here they come to re-educate the unclean. Just watched another XPS at our field lock out on a big gasser doing 3D. Still waiting for the post mortem, but I'll bet in the end the grass was wet. Many thanks to Kiwi for his efforts to visually document this issue! Regards, Bill

"Marching Orders?" That is rich.

Any of you who believe that all XPS users are at JD's beck and call are fooling yourselves.

[zoomer260]

	Quote: Originally Posted by adjonym
	Once again, for the intellectually challanged: Extreme Power System is required by law to provide the evidence needed to back up their claims in marketing. Now, point me to the evidence XPS has provided about their frequency switching technology claims. And again, for the intellectually challenged ones, that means that the burden of proof lies with the manufacturer, not with the customers, or possible customers. Those laws are in place to protect both customers AND possible customers, so that they can base their purchase on the features a product actually have, not what the manufacturer CLAIMS that they have. Now, if all comanies acted as Extreme Power Systems, you'd have cancer medicine with 100% success rate, all it would take would be a 1-800 number and a credit card. You'd have cars with 10000mpg rating, just with the addition of the X capacitor, and so on and on. These might be bad examples, since they are easy to spot, and turn away as bs, but you get the idea. I think this is the base of all this fuzz. We, as consumers should always act upon company's that refuses to follow those laws, and even base their marketing upon it(JD claims he won't provide proof since it helps sales). Those laws are there to keep scammers out of the loop. No matter how you turn things around, the fact still remains; Extreme Power Systems are breaking the consumers law, and continue to do so well aware of the fact that they are indeed breaking those laws.

My bad. I thought for a minute that you were saying JD has proven the "hop". Just the fact he won't show it hopping "in real world testing" was enough evidence for me that it would not. Kiwi did a lot of work attempting to get it to and the results are in.

[tadawson]

	Quote:
	Nothing was saturated,

That has not, and cannot be proven with any of the test gear in that video. The saturation being discussed here is RX front end - not band, which cannot be seen on any external test equipment - you have to monitor the internals of the RX to see it. The net effect of an RX front end saturation would be that it would desensitize the RX across the band, and render it's ability to scan moot. Not gonna be able to negotiate much of anything that way. Not to mention that the noise source was also close to the TX, which also has a receiver in it, used to negotiate changes. How much might the close range noise source have affected that? Unknown at this point . . . . A more sensitive RX will typically encounter this type of problem at lower levels than a lesser sensitive unit, so it is not impossible that this test simply sat in that window.

This is why I continue to suggest that this test should have been run (or be rerun) at realistic signal levels to what would be seen in flight. With an interference source and TX unit sitting basically right on top of the RX, the chance of this happening is far to great to ignore. The apparent success of other systems under these conditions is basically meaningless, since it may simply mean that they have less sensitive receivers in the first place (as well as other possibilities).

So, for me, it comes down to this:

ONLY TESTS DONE AT IN-FLIGHT SIGNAL LEVELS HAVE ANY CREDIBILITY. THE REST ARE FAR TOO PRONE TO OTHER ISSUES TO BE DEFINITIVE IN THEIR CONCLUSIONS.

And, as stated before, band saturation and RX front end saturation have very little if anything in common, other than the word "saturation". It appears that there may be a number of folks here who do not understand those definitions, as evidenced by claims like my beginning quote, where there was no possible way of knowing that . . .

- Tim

[jonkoppisch]

	Quote: Originally Posted by tadawson
	... So, for me, it comes down to this: ONLY TESTS DONE AT IN-FLIGHT SIGNAL LEVELS HAVE ANY CREDIBILITY. THE REST ARE FAR TOO PRONE TO OTHER ISSUES TO BE DEFINITIVE IN THEIR CONCLUSIONS.... - Tim

Hey Gordito Volador,
Get ahold of the guy that just lost his plane and tell him that the tests have no credibility...


http://www.rcgroups.com/forums/showt...18#post8770718

	Quote: Originally Posted by JimDrew
	We have the only system that communicates noise in the environment and in fact WILL hop to a cleaner frequency if the noise floor rises. That hop may not even be due to the aircraft experiencing a problem, it could also be from the transmitter side (like other 2.4GHz systems on the flight line). ......

[Toumal]

	Quote: Originally Posted by tadawson
	That has not, and cannot be proven with any of the test gear in that video. The saturation being discussed here is RX front end - not band, which cannot be seen on any external test equipment - you have to monitor the internals of the RX to see it.

That is incorrect. If the transmitter's power levels were high enough to saturate the receivers front-end, it would not matter whether it was transmitting on the right channel or an adjacent one. Since the video clearly shows XPS stopping to work the moment the interfering TX occupies the exact same channel, saturation is highly unlikely.
EDIT: Unless the receiver has a super-selective front-end... Like, oh, some military gear.

And again, for the record, you don't "saturate the band", saturation is a property of the receiver.

Besides, FASST and Spektrum were also tested and were not "saturated" either. They kept working, and that despite JD claiming that XPS is less susceptible to saturation (-well, due to the fact that he held the receiver further away from the TX...) in his own video. So regardless of whether you believe me that saturation does its fatal work even on adjacent channels or not, there's still the fact that the other two systems were not experiencing any difficulty whatsoever.



Furthermore, I agree with Kiwi that the discussions here have become an exercise in futility, and apparently that's the plan. The amount of half-knowledge is astonishing, and it's disturbing to such statements to be put out into the wild, contradicting the laws of physics, contradicting logic, and contradicting facts upheld by video evidence and controlled tests. I know there's room for debate on a variety of things, but when people make unfounded statements as to whether or not a groundplane monopole antenna has a significant blind spot or not, talk about saturating a frequency band, and make illogical statements as to whether or not hopping on interference happens under super-special circumstances only... that's not a debate, that's a waste of time.

Here's the deal: I don't tell you how to set up a gasoline or glow engine, because that's something I have little clue about, and you don't tell me that the stuff I studied for years and the things I work with all the time are wrong and all my experience with RF tech just an illusion. If you're telling someone that flying with a single 2.4GHz groundplane monopole antenna in a model with a big lump of an engine block is no problem and that the antenna's "dimple" is insignificant, you're potentially endangering the lives of other people. If you're telling someone that frequency hopping under "special circumstances" is enough while confusing interference with saturation, that's irresponsible. This is not a "no, you're wrong"-contest. A lack of reliability, especially when the manufacturer is in the phase of denial, can cost lives.

I hope the moderators do the right thing and prevent the "everything's fine with XPS"-messiahs from turning this forum into an Irrlicht for people who seek factual information. I don't want this thread closed either, but I think an IP check would be a good thing to do.

[tadawson]

	Quote: Originally Posted by Toumal
	Here's the deal: I don't tell you how to set up a gasoline or glow engine, because that's something I have little clue about, and you don't tell me that the stuff I studied for years and the things I work with all the time are wrong and all my experience with RF tech just an illusion.

Well, since you brought it up . . . My background is as a degreed engineer in communications and solid state electronics. Not to be a prick, but everywhere I have been, engineer > technician when it comes to understanding this stuff. So, I think I have a pretty darn good grip, and no amount of waffling that I have yet heard has in any way invalidated my concerns with the shortcomings of these tests . . .

- Tim

[XJet]

	Quote: Originally Posted by tadawson
	Well, since you brought it up . . . My background is as a degreed engineer in communications and solid state electronics. Not to be a prick, but everywhere I have been, engineer > technician when it comes to understanding this stuff. So, I think I have a pretty darn good grip, and no amount of waffling that I have yet heard has in any way invalidated my concerns with the shortcomings of these tests . . . Tim

Like yourself Tim, I'm a EE who's been in this game (design, development, manufacturing, maintenance) for far more years than I care to admit. I started my training in RF back in the mid 1970s and just a few years later became strongly involved in digital control/telemetry systems.

Even "experts" can disagree occasionally :-)

In Kiwi's tests, the fact that the system continue to operate when the interfering signal was outside the receiver's operating frequencies shows that the receiver's front end was not overloaded to the point where it could not have performed a scan.

Typical front-end overload causes receiver sensitivity to plumet *and* often results in intermod products that cause an off-frequency signal to block an on-frequency signal due to those IM artifacts. If there's AGC involved, it can also kick in and simply wind down the entire sensitivity of the RF chain.

You know that but others might not.

The fact that XPS continued to operate *until* the interfering signal transgressed onto the portion of the band that the receiver was using indicates that there was no significant front-end saturation.

Remember also that the front end of these 2.4GHz systems is tuned *very* broadly (80Mhz) so there's no in-band, off-frequency rejection there.

If the XPS receiver was still able to "hear" the desired signal when subjected to an in-band/off-channel interfering signal then the front end wasn't saturated or significantly desensitized by that interfering signal. This means that *if* JD's hopping worked, he could have done a band-scan, located a free portion and jumped to it.

The reality is that without the ability to negotiate new frequency, it's just not possible to reliably hop. Until the Tx and Rx can agree where to hop, there'd be no point in trying. This is an intrinsic limitation of any system built around a single frequency RF link with a single receiver.

That's why I have always disputed JD's claims right from the start -- it simply CAN NOT be done with the XBeePro modules.

Spectrum has already negotiated a fall-back frequency and FASST has an already negotiated hopping schedule so they can handle interference.

By the time XPS realizes that it's been hit by interference - it's already too late to hop because neither end can inform the other where to go. Any system that relies on negotiating a hop before the link is lost on the current frequency runs the risk of losing sync and thus any hope of restoring control in less than several seconds (ie: crash).

Note also that when this whole issue blew up, JD promised he'd upload a video proving that XPS would hop.

What happened to that?

Instead, he tried to change the subject by uploading a clearly "rigged" video showing how the competition would stop working due to saturation when held right beside the antenna of an interfering source -- while XPS continued to work (when it was held at a distance that would have produced a field around ten times *LOWER* than the others were subjected to.

If it really did hop, why didn't JD just spend his time uploading the video evidence?

The answer: because the damned thing will *only* hop when his very stringent (and totally unrealistic) scenario of a slowly rising noise floor over 15 seconds is experienced -- a scenario that even he was clearly unable to duplicate on the bench.

Now as I said before, if JD has gotten DigiChip to rewrite the XBeePro firmware to support true satellite receivers and perhaps he opts for a dual-frequency multiplexed transmission then XPS may turn into a very trustworthy and useful system.

But that won't recover the huge loss of trust that has been produced by his simple refusal to admit that he's grossly overstated the frequency agility of this system.

[tadawson]

Not much to argue with there, but what has not really been considered, is what is the effect of the interference signal on the receive side of the RX module? Once again, with everything in such close proximity, I don't think that is something that can be ignored. Also, the intermod products will be different being right on top vs. being on an adjacent channel, and not knowing the details of this receivers internals, do we know where those will fall, since we are dealing with pretty wideband signals, but close in freq? Would that also not tend to make the IM products fall much closer to the main signal than with a different channel of the interference signal? Nothing certain here, but I still have to wonder . . . Do you not agree, at the very least, that the test would have been better done at closer to in-flight RF field strengths? If nothing else, it would remove what (at least in my mind) I see as a potential variable to the result . . . it may not change anything, but then again . . . we can be sure unless that is tried as well, no?

- Tim

[Toumal]

	Quote: Originally Posted by tadawson
	Well, since you brought it up . . . My background is as a degreed engineer in communications and solid state electronics. Not to be a prick, but everywhere I have been, engineer > technician when it comes to understanding this stuff. So, I think I have a pretty darn good grip,

Tell you the truth, I'm an engineer in communication ("Nachrichtentechnik") but that doesn't really make me an expert in these things. I'd say that building your own 2.4GHz "Pringles"-antenna can be a very helpful learning experience, but then again they also forced me to do field calculations and stuff. Stuff like "How much dB loss do you get if an antenna with so and so many inches emitting a x MHz frequency points straight at you, compared to a 90 degree elevation angle" was actually a typical test question, and I've experienced the effects of such things first-hand. But hey, don't listen to me, listen to XJet and Kiwi, who probably built their own 800W CB amps when I was still playing with Legos.

	Quote: Originally Posted by tadawson
	and no amount of waffling that I have yet heard has in any way invalidated my concerns with the shortcomings of these tests . . . - Tim

I think we can agree on two things:

1) It would be great if a future test were held at larger (and potentially more realistic) distances - i.e. 50 meters between RX and TX, interfering TX also 50 meters away. Alternative test setups should be explored also, to simulate the aircraft being far away from the pilot but relatively close to an interfering transmitter, for example.

2) The distances as well as a top-down layout should be recorded so that the test is repeatable by others under the same circumstances. Transmitter power should be measured as well.

	Quote: Originally Posted by XJet
	That's why I have always disputed JD's claims right from the start -- it simply CAN NOT be done with the XBeePro modules.

I don't know if that's really the case! Think about it: TX and RX could negotiate a hopping scheme right after powerup. That scheme could cover all channels and could then just be run through in a sequence that's also pre-negotiated. XPS could stay on a channel until there's no ACK from the receiver (TX side) or no more new frames from the transmitter (RX side). In either of these cases, the transmitter and the receiver could proceed to the next frequence in a fixed time window fashion - that way you would avoid having the RX start the hop sooner than the TX, and so forth. Or, the system could hop all the time! That would make things a lot easier too.

...or is there some limitation in XBee that I'm not aware of?