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Old Dec 20, 2012, 10:29 AM
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RyanNX211's Avatar
Upper Arlington, Ohio
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Originally Posted by JimDrew View Post
Any radio frequency above about 300MHz is immune to ignition noise.
Interference must be in the servos
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Old Dec 20, 2012, 12:02 PM
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United States, AZ, Queen Creek
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Originally Posted by JimDrew View Post
Absolutely! Both real world, and theoretical calculations... you can do those yourself by knowing the link budget and frequency. There are plenty of applications available that will calculate your range based on that info.
I was once the systems engineer for the Lunar Orbiter satellite at Goldstone.
My job was to test the sensitivity of the system prior to each pass of our station.
Normally the data link sensitivity measured better than -150dbm. We were using a cryogenically cooled maser as the preamp.
One day I couldn't get better than -110dbm. I looked out the window and found the antenna was pointed at the horizon instead of its normal zenith storage position. There was that much RF noise in the low atmosphere.
There is a difference in theoretical calculations and real world application.
I ask again, have you tested it?
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Old Dec 20, 2012, 12:23 PM
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Quote:
Originally Posted by dirtybird View Post
I was once the systems engineer for the Lunar Orbiter satellite at Goldstone.
My job was to test the sensitivity of the system prior to each pass of our station.
Normally the data link sensitivity measured better than -150dbm. We were using a cryogenically cooled maser as the preamp.
One day I couldn't get better than -110dbm. I looked out the window and found the antenna was pointed at the horizon instead of its normal zenith storage position. There was that much RF noise in the low atmosphere.
There is a difference in theoretical calculations and real world application.
I ask again, have you tested it?
did you see the "Both real world" part?... I think Jim gets more CV's posted at him than any other vendor I know...
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Old Dec 20, 2012, 06:46 PM
Xtreme Power Systems
Lake Havasu, AZ
Joined Jun 2005
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Quote:
Originally Posted by dirtybird View Post
I was once the systems engineer for the Lunar Orbiter satellite at Goldstone.
My job was to test the sensitivity of the system prior to each pass of our station.
Normally the data link sensitivity measured better than -150dbm. We were using a cryogenically cooled maser as the preamp.
One day I couldn't get better than -110dbm. I looked out the window and found the antenna was pointed at the horizon instead of its normal zenith storage position. There was that much RF noise in the low atmosphere.
There is a difference in theoretical calculations and real world application.
I ask again, have you tested it?
Well, you have proved my point. Real world testing that you did showed you got -150dBm of sensitivity. The theoretical sensitivity was probably very close to the real world you saw, until you changed the conditions to no longer match the expected test setup. Likewise, I can stick the antenna in the dirt and suddenly the real world range is diminished greatly. However, using the equipment as it was intended, the real world range matches the theoretical range.
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Old Dec 20, 2012, 11:03 PM
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You don't have to stick your antenna in the dirt. You have a very non directional antenna. It picks up all the crud that happens to be around. My antenna was very selective.( 85 ft parabolic)
Its been years since I have been in that business but the noise floor at ambient temperature is about -120dbm as I remember. That would mean your equipment would have a 0db noise figure. Our preamp was at +4 degrees K. It was ina liquid helium bath. Even so we could get only about -150 dbm at the 50cycle bandwidth. Our threshold for the video channel was -107dbm with a 100kc bandwidth. The maximum at +0 degrees K is -173dbm.
Perhaps you can see why I am a bit skeptical about your claims.
Here is a picture I took: http://www.spaceimages.com/luoreaph.html
I just had the help of about 500 other engineers.
We did it a year before Apollo.
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Old Dec 21, 2012, 09:51 AM
Plaurence
Joined Jul 2009
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Originally Posted by JimDrew View Post
We have always had frequency hopping. In October of 2008 we added 4 new frequency hopping modes (to conform with European EN300-328 rules changes). We also added a low voltage warning, and 80% of our customers immediately reported the warning coming on!

900MHz is definitely a better frequency for flying. We will have both 900Mhz and 2.4GHz versions of our transmitter, as well as the exact same receivers available in both 900MHz and 2.4GHz versions.

Why is 900mh better for flying than 2.4?

Peter
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Old Dec 21, 2012, 12:12 PM
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Originally Posted by plaurence View Post
Why is 900mh better for flying than 2.4?

Peter
...there's less people using 900mhz and there's less noise on the band in general, there's less chance of the receiver being shadowed by something in the plane (read, harder to put the receiver in the wrong spot), there's less chance of the ground attenuating the signal if you get near the ground or over a hill, carbon fuselages wont cut the signal, better range, etc etc...
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Old Dec 21, 2012, 02:26 PM
Xtreme Power Systems
Lake Havasu, AZ
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Originally Posted by dirtybird View Post
You don't have to stick your antenna in the dirt. You have a very non directional antenna. It picks up all the crud that happens to be around. My antenna was very selective.( 85 ft parabolic)
Its been years since I have been in that business but the noise floor at ambient temperature is about -120dbm as I remember. That would mean your equipment would have a 0db noise figure. Our preamp was at +4 degrees K. It was ina liquid helium bath. Even so we could get only about -150 dbm at the 50cycle bandwidth. Our threshold for the video channel was -107dbm with a 100kc bandwidth. The maximum at +0 degrees K is -173dbm.
Perhaps you can see why I am a bit skeptical about your claims.
Here is a picture I took: http://www.spaceimages.com/luoreaph.html
I just had the help of about 500 other engineers.
We did it a year before Apollo.
Well, there is no 'magic' involved. Like I said, you can easily calculate the theoretical range using basic formulas. 900MHz propagates better through everything, including the air.

Here are some simple calculators:

http://www.fpvhobby.com/fpv12.htm
http://www.fpv.lt/lang/en/range-calculator/

The chip used for the transceiver is Atmel's AT86RF212, Rx sensitivity of -101dB in the mode we are using it in:

http://www.atmel.com/Images/doc8168.pdf

The chip used for the front-end is RF Axis' RF1010, 12dB of Rx gain and +25dB of output gain (without saturation):

http://www.rfaxis.com/downloads/RFX1010.pdf

So, you if you figure frequency at 915Mhz (center frequency) with a 10dB margin, under rate the Rx sensitivity to -110dBm sensitivity (should be -113dBm) using a 1.5dBi strip antenna, set the power to 25dB through a 3dBi dipole, you can calculate the theoretical range yourself using one of the above (or any other) RF range calculators.

If you are interested in what happens with receiver sensitivity differences, change the sensitivity to -113dB and look at the range. Sensitivity is where its at!
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Old Dec 21, 2012, 03:44 PM
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I am not really interested in range. The equipment we have has more than adequate range for me.
I see the FPV receivers have only -85dbm sensitivity. That's a hell of a lot more believable to me. I would think they should do better than that.
The receiver chip you pointed out has a sensitivity of -110dbm. That is in the range I would expect to see. I am sure you will agree that's a lot different from -120dbm.
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Old Dec 21, 2012, 05:30 PM
Xtreme Power Systems
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Range is the ONLY thing I am interested in, because that is what gives you the headroom when installations are poor. FPV systems use analog signals, which are not the same as the digital spread spectrum signals (DSSS). Remember, you get big processing gains by using spread spectrum, hence the higher sensitivity.

The transceiver chip has a sensitivity of -110dBm if you use it in low speed mode. The mode we are using it in has a sensitivity of -101dBm. When you add 12dB of gain from the front-end chip, you could have -122dBm (low speed) or -113dBm (our case) of sensitivity. I under rated everything. The transmitter antenna is really 5dBi, not 3dBi. The receiver antenna is really 1.7dBi, not 1.5dBi.

With +27dB and -122dBm, that is a link budget of 149dB! That would give you a theoretical range of a few hundred miles. You could talk to some satellites in orbit with this kind of range.
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Old Dec 21, 2012, 06:39 PM
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Quote:
The transceiver chip has a sensitivity of -110dBm if you use it in low speed mode. The mode we are using it in has a sensitivity of -101dBm. When you add 12dB of gain from the front-end chip, you could have -122dBm (low speed) or -113dBm (our case) of sensitivity. I under rated everything. The transmitter antenna is really 5dBi, not 3dBi. The receiver antenna is really 1.7dBi, not 1.5dBi.
.
This is why I ask if you tested this. You just can't add the gain of that front end chip to your sensitivity. It will just amplify the noise along with the signal.
Your sensitivity will be what is quoted on the other chip. -110dbm
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Old Dec 21, 2012, 09:33 PM
Xtreme Power Systems
Lake Havasu, AZ
Joined Jun 2005
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Huh? If you add gain the to Rx circuit you add gain, this includes any noise. The ability to reject that noise is part of the digital signal processor that is built into the RF212 chip. There is also noise suppression built into the front-end chip as well. Of course, there is also separate bandpass circuitry that filters frequencies several orders both above and below the center frequency. Yes, the range has been verified as fact. Atmel has quite a few reference designs that you can build yourself for testing. One is a 900MHz walkie-talkie that samples your voice, compresses it in data packets and sends it. A pair of them are required of course, and the results are pretty impressive with no front-end chip at all!
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Old Dec 21, 2012, 11:31 PM
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Take the time to read this over:
http://www.phys.hawaii.edu/~anita/we...t/rcvr_sen.pdf

You will see receiver noise is dependent on your bandwidth and the temperature you are operating at. You cannot reject noise.It is all frequencies.Your chip will add processing gain but this is what will get you to -113dbm. Normally at that frequency, maximum sensitivity is -108 dbm.
I don't live to far from you. If you have a test set up that shows you have a better sensitivity than -113dbm let me know. I would like to see it. I will make the trip over to your factory just to see it.
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Old Dec 22, 2012, 11:59 AM
Xtreme Power Systems
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Please take some time and read information on digital noise rejection and also spread spectrum and how it works. Not only can you severely reduce noise, spread spectrum can pull valid data from below the noise floor.

There are receivers with much higher sensitivity, for example GPS receivers are typically in the -150dBm to -165dBm sensitivity range. Consider that GPS satellites are typically 12,650 miles in altitude and have to transmit through all of the cloud layers. They use 1.575GHz or 1.227GHz.

Like I said, there is plenty of evidence all over from many developers of this technology to prove its not even a big deal anymore to achieve this type of range.
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Old Dec 22, 2012, 03:34 PM
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Read it again. GPS receivers are quoted as -160w not dbm. There is a 30 db difference.
They also require very low bandwidth.
Spread spectrum systems require much wider bandwidth. The do not reduce noise. They add to it. They can,however, read below the noise threshold. In my experience you get about 3db improvement.
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