Kiwi's XPS test results are up.

[madmax]

Easytiger, good to see you again, and Snoopy, welcome. Your points have been made. Move on and help keep this thread opened. Any more bickerin' we'll shut her down until Kiwi is back from his big weekend coming up in Chilie.. Thanks. MD

[easytiger]

	Quote: Originally Posted by madmax
	Easytiger, good to see you again, and Snoopy, welcome. Your points have been made. Move on and help keep this thread opened. Any more bickerin' we'll shut her down until Kiwi is back from his big weekend coming up in Chilie.. Thanks. MD

Yup, my point is made, I'm reverting back to lurkin' status, rather than getting sucked in again. Watch my back, though, I don't want to be called an "obvious idiot" or anything like that.

Best o' luck to all.

[50%plane]

Easytiger, I too must be obviously stupid as I agree about 2.4 systems not hoping as being dangerous.


50%

[tadawson]

	Quote:
	And once one turns on to the same channel as you are using on XPS, your goose is cooked.

Not true in the majority of cases. Devices such as XPS in the ISM band do not transmit constantly, and listen for "clear space" on the channel before transmitting. Due to this, numerous devices can occupy a single channel at the same time with no degradation or interference. The oddball video system is the notable exception, but is a very distant minority in terms of number of devices in the band. So, no, XPS is **not** like 72 in that if another radio turns on and selects the same channel, **nothing** happens, other than they both work with XPS . . . quite unlike 72 . . .

So, I vote with the folks that make the statement that XPS is superior in that regard, for the reasons detailed above . . .

- Tim

[USN_POPS]

I do not claim to be an expert, and hope you guys won't beat up on me too bad. I thought I'd take a shot using the basic XPD analyzer and my laptop and see what happened. I just monitored a supposedly better system at the field today with my XPD. And the results were suprising.

The noise floor at the field before any system was turned on was pertty stable at -83 to -78DB. As soon as the other system was turned on, the noise floor began to jump around at about -63 to -48 DB and then after a few seconds, the 2 channels appeared at about -42 to -43 DB. Which was just barely above the noise floor, and occasionally one of the channels would drop into the noise floor. The signals hung pretty close to the -42 to -43 range during the entire time. Transmitter was about 6-8 ft from the XDP.

My XPS system 6-8 ft from the XDP holds one channel at about -25 to -28 DB depending on orientation. As I move the TX away from the XDP at that distance, it never goes below -38 DB, which is higher than the other system on the same analyzer, and always aove the noise floor.

At home with all the wireless gear, wireless phones etc, the noise floor runs at -68 to -83 DB, and the XPS system still holds from -38 DB in another room to -23 at a range of 4-5 ft.

This is the first time I tried this at the field, but I plan on doing some more testing. Not even sure it proves anything yet. But in the past a stronger signal under any circumstances is better than a weaker one.

[BZFrank]

	Quote: Originally Posted by tadawson
	Devices such as XPS in the ISM band do not transmit constantly, and listen for "clear space" on the channel before transmitting.

The first statement is true but not the second. XPS does not seem to use the IEEE 802.15.4 CSMA/CA (p-Persistance) feature, it just sends its data frames out regardless what else happens on the same channel.

Frank

[ss40]

Just another happy XPS user stating my opinion. It works for me and has for a season. 2nd season about to start. As one who has been a victim on 72Mhz (proven and documented and witnessed, luckily minor incident) I would like to say that in my flying conditions and location any of the 2.4Ghz systems are far superior to the 72Mhz system and when I started RC about 8 years ago I could not understand why we were playing with pins and crystals as I had left that far behind me many years prior in full-size flying of sailplanes. So I welcome the new world and it is anything but more dangerous. At the current time 2.4Ghz has added a level of safety at the majority of sites that has never existed before.
I am a technogeek and retired computer nut who welcomes new tech when it works.
My .02, and in the interest of fairness here I expect no rebuttal to my simple honest opinion.
Thank you for listening/reading.

[XJet]

	Quote: Originally Posted by RichardCorby
	I do not claim to be an expert, and hope you guys won't beat up on me too bad. I thought I'd take a shot using the basic XPD analyzer and my laptop and see what happened. I just monitored a supposedly better system at the field today with my XPD. And the results were suprising. The noise floor at the field before any system was turned on was pertty stable at -83 to -78DB. As soon as the other system was turned on, the noise floor began to jump around at about -63 to -48 DB and then after a few seconds, the 2 channels appeared at about -42 to -43 DB. Which was just barely above the noise floor, and occasionally one of the channels would drop into the noise floor. The signals hung pretty close to the -42 to -43 range during the entire time. Transmitter was about 6-8 ft from the XDP. My XPS system 6-8 ft from the XDP holds one channel at about -25 to -28 DB depending on orientation. As I move the TX away from the XDP at that distance, it never goes below -38 DB, which is higher than the other system on the same analyzer, and always aove the noise floor. At home with all the wireless gear, wireless phones etc, the noise floor runs at -68 to -83 DB, and the XPS system still holds from -38 DB in another room to -23 at a range of 4-5 ft. This is the first time I tried this at the field, but I plan on doing some more testing. Not even sure it proves anything yet. But in the past a stronger signal under any circumstances is better than a weaker one.

What you're seeing is inaccurate reporting due to the limitations of the XPD.

The XPD is *not* the good basis for a spectrum analyzer.

The XBee modules have a 5MHz channel spacing with a 1.6MHz channel width.

Spektrum has a 1MHz channel spacing.

This means it's more than likely a Spektrum system will *not* be operating on a frequency the XPD is able to receive. What you're seeing is simply some vestigial sideband energy, not the primary signal transmitted by the Spektrum (that is lost *between* the XPD channels)

To understand a bit further -- Assume the Spektrum starts on channels 7 and 53 of its 1-79 channel arrangement.

As far as the XPD is concerned, these are not frequencies it is sensitive to -- since channel 7 is 7x1MHz from the start of the band right between channel 1 (+5MHz) and channel 2 (+10MHz) on the XPD's receiver.

Likewise channel 53 is +53MHz from the start of the band which is proabably right between channel 10 and channel 11 of the XPD receiver.

Of course all the XPS channels coincide directly with the frequencies the XPD *can* receive, so the XPS signal shows up as being *much* stronger.

To get an accurate indication of the relative powers involved you will need a *real* spectrum analyzer with a resolution of 1MHz or better (which is five-times that of the XPD).

[davebnz]

Kiwi,

great test, and interesting thank you.

I was concerned, being a user of the XPS modules on four heli's.

Does Jim's explanation solve this for you ?

http://www.rcgroups.com/forums/showthread.php?t=816984

Cheers

David

[KrisW]

	Quote: Originally Posted by davebnz
	Kiwi, great test, and interesting thank you. I was concerned, being a user of the XPS modules on four heli's. Does Jim's explanation solve this for you ? http://www.rcgroups.com/forums/showthread.php?t=816984 Cheers David

I read the first three pages of that thread. JD SOUNDS so good. . . . . . but the arrogant and condescending attitude that came through the words just blew me away. And refusal to show a video of frequency hopping just nails the coffin shut, IMHO.

It's very simple to recreate the hopping environment, and show it on a video. Takes an hour, and saves you all these hassles and questions. If people are complaining about lockouts and problems and lost aircraft, there IS a problem. As a responsible person, if I was in JD's position, I'd be doing a lot of explaining about how the hopping actually works, what triggers it, and be showing people what to expect and how to recreate the trigger for hopping. Ending the questions is as simple as providing the answer everyone wants. NOT doing it is destructive to the business, and kills your credibility and trustworthiness.

I read these posts and keep getting the "Used Car Salesman" Impression.

And THAT is the main problem. Keith Baker did the same deny deny deny at BME for the last year he owned it. . You can't "deny" things out of existence. . you need to prove that they are fixed and no longer a problem.

[Julez]

Richard Corby: Please consider this:
Xtreme Link Experiences

[JKos]

> For example, my understanding is that in situations where interference is short in duration, the
> transmitter system will retransmit data until the receiver acknowledges receipt of uncorrupted data.
> This may happen many times before the next frame

The XPS tx module will transmit up to four times. So that is up to three retransmits if the original transmission is not successfully acknowledged.

On collision avoidance, as pointed out, it certainly does not appear the XPS system actually makes use of it. Collision avoidance is typically only used in network situations in which you have multiple devices which need to communicate on the same "pipe", if you will.


- John