Independent tests prove lack of frequency hopping with XPS

[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"........WTF?? 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"........WTF?? 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.

[Kiwi]

	Quote: Originally Posted by 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

Ron, sorry for the false info there mate. I was referring to the www.nutsaboutnets.com scanner and software. Thats about $90 but I'm not using it as the driver seems to have issues. Monday I hope to fix that with some help from them and we will be on the road again.

Apologies mate.

[ricoalonso]

Kiwi / Xjet,
Just FYI: you may consider to do additional testing for XPS with different power settings in the Tx module and/or different country settings in the Rx. Someone seems to show that it hopped. Pls check this thread in RCG and read posts specifically starting on post #6 http://www.rcgroups.com/forums/showthread.php?t=815913

Unfortunately, the same person has another post later about his crashed plane with XPS, but one interesting item to note is that his unit seems to be set for another country and the Tx is transmitting at lower power. Here's the other thread: http://www.rcgroups.com/forums/showthread.php?t=816378

Thanks
^R^

[Kiwi]

OK guys,

A day of tinkering and coming to grips with the scanner software and systems.

Sunday is my fly day so I launched myself out of bed early and off to the field I went. Trusty laptop in hand and no plane. Had a prop fail last week and new one wont be here till Wednesday plus I had a new guy to train so it was still an enjoyable day.

OK one of the things I want to do this year is profile the flying fields I visit. Just for my own interests sake and nothing else so the first place was home of course.

Well the field is about 2 miles from an active runway with heavies coming in and out two or three an hour. Military base, loaded with new equipment including F16's etc. So I expected some pretty reasonable traffic as those guys own the airwaves pretty much.

The following screen shots show what the day had in store. Pretty much clear skies. No 2.4Ghz traffic to speak of. We look pretty good here for any system.

What that shot is showing is:
1. the lower graph shows the average traffic in solid green with the peaks or spikes in blue. The yellow line is the live action.

2. the upper graph shows the duration and intensity. Now you can see some faint smokey lines that show some far off traffic but nothing that would would allow you see what it is or suffer any events from. Again this is at ground level + 9 ft so what your airplane will see could and should be a little more. Where this is there are no trees and the ground slopes away for miles. There's not much chance of anything being hidden from the scanner here.

[Kiwi]

OK

I was getting ready to come home when a friend and very adventurous young guy turned up with his powered sail plane and on board video system. Yes you guessed it. 2.4 Ghz. Perfect for another test.

I already had the clean scan so this would show what an on board video setup can do to ruin your day.

This system is on WiFI channel 8 and as I said before this channel thing is really just a rough way of saying where about the signal might appear, its definitely not a precise science with the things I have tested so far.

So lets explain what the scanner is showing.

Lets start in the upper window. Near the center but the highest image (red, green, yellow) line shows the receiver being turned on. You can see it sends a very strong signal when its switched. Its close to channel 8 but more a 7 or a 6. OK the receiver is on but it only fired that burst of energy and then waited.

Twelve seconds later the video transmitter is turned on and you can see what it done. It hit three channels and then locked in. That smokey line above channel 9 shows the spectrum and strength it is using.

Going to the lower graph you can how strong and how spread out that initial burst of energy was. It took up four bands but only for a millisecond. Not enough to knock over a healthy 2.4Ghz RC system.

Something that is new to me and one of the reasons I find playing with this stuff so interesting is that a flat graph like this does not really tell the whole story. 2.4 Ghz appears to be three dimensional. That is the frequency also has depth. So you can have a lot of traffic on one channel so long as one of the transmitters does not hog that frequency with a 100% duty cycle.

I have some new software that I hope will show you how that works. It scans the X axis and Y axis and also the third access (D for me) but you techies will have a better name for it.

So here's what happens when someone arrives at the field and turns on a 2.4 Ghz video system.

[Kiwi]

Gents the reason I have not posted the test on the XPS yet is that now that I understand a little more about this I need that 3D software to see that I am in fact flooding the XPS system. Example: If my Airhorn is only using up 50% of the cycle time that would leave XPS with the other 50% and likely not enough to force the hop.

In the test carried out on RCG the entire frequency was swamped, slowly and quickly. XPS never moved. I somehow think I wont make it move either but what I have found that even hitting the same frequency (say Ch 1) that XPS is running on it does not always go into fail safe. That tells me there may be holes in the signal I am generating and XPS uses those holes.

So far this is all speculation but the new software should be running tomorrow and the information will be a whole lot more complete.

At least we are on it guys and the results will follow I promise.

[KrisW]

	Quote: Originally Posted by Kiwi
	OK I was getting ready to come home when a friend and very adventurous young guy turned up with his powered sail plane and on board video system. Yes you guessed it. 2.4 Ghz. Perfect for another test. I already had the clean scan so this would show what an on board video setup can do to ruin your day. This system is on WiFI channel 8 and as I said before this channel thing is really just a rough way of saying where about the signal might appear, its definitely not a precise science with the things I have tested so far. So lets explain what the scanner is showing. Lets start in the upper window. Near the center but the highest image (red, green, yellow) line shows the receiver being turned on. You can see it sends a very strong signal when its switched. Its close to channel 8 but more a 7 or a 6. OK the receiver is on but it only fired that burst of energy and then waited. Twelve seconds later the video transmitter is turned on and you can see what it done. It hit three channels and then locked in. That smokey line above channel 9 shows the spectrum and strength it is using. Going to the lower graph you can how strong and how spread out that initial burst of energy was. It took up four bands but only for a millisecond. Not enough to knock over a healthy 2.4Ghz RC system. Something that is new to me and one of the reasons I find playing with this stuff so interesting is that a flat graph like this does not really tell the whole story. 2.4 Ghz appears to be three dimensional. That is the frequency also has depth. So you can have a lot of traffic on one channel so long as one of the transmitters does not hog that frequency with a 100% duty cycle. I have some new software that I hope will show you how that works. It scans the X axis and Y axis and also the third access (D for me) but you techies will have a better name for it. So here's what happens when someone arrives at the field and turns on a 2.4 Ghz video system.

Spiffy-kean. . neato. . ultra coolio. . . . .keep it coming Kiwi