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Old Mar 08, 2012, 12:45 AM
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How does it work?

Folks,

Just out of interest, I an trying to understand how TX and RX remain in sync during the lightening-fast rate at which the frequency is changing.
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As I understand it, Frsky RX during bind process learns of the TX's unique ID and this is somehow used to determine the hopping order.
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How on earth does the receiver manage to keep on track through such a fast changing spectrum of frequencies?
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Any Ideas?..
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Rgds, Steve
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Old Mar 08, 2012, 02:29 AM
Oxford Panic
AndyOne's Avatar
United Kingdom, Oxford
Joined Feb 2003
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Steve,

Once the receiver is bound to the transmitter, the receiver knows where next frequency is going to be so it hops there in synchronism with the transmitter. If for any reason it looses track of the transmitter, there are mechanisms in place for it to find it again, like at switch-on it has to lock on and this is the cause of a slight delay before the servos become active.

A.
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Old Mar 08, 2012, 10:25 AM
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United States, TX, Comal
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Hey! How exactly is a rainbow made? How exactly does a sun set? How exactly does a posi-trac rear-end on a Plymouth work? It just does.
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Old Mar 08, 2012, 11:49 AM
T-Squared
United States, GA, Lawrenceville
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Magic!
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Old Mar 08, 2012, 11:52 AM
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Illinois
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Oh boy, this made me start humming "The End of the World" by The Carpenters.

Andy
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Old Mar 08, 2012, 03:26 PM
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Bruce Abbott's Avatar
Hastings, New Zealand
Joined Jan 2001
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Quote:
Originally Posted by T-Squared View Post
Magic!
Magic crystals made of quartz.

Seriously, the timing crystals used in 2.4GHz transmitters and receivers 'tick' at 12 million times per second with an accuracy of 0.003%, enabling very precise control of frequency and synchronization.
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Old Mar 08, 2012, 06:34 PM
"Unnecessary Necessity"
coriolan's Avatar
Canada, BC, Vancouver
Joined Sep 2006
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Quote:
Originally Posted by Bruce Abbott View Post
Magic crystals made of quartz.

Seriously, the timing crystals used in 2.4GHz transmitters and receivers 'tick' at 12 million times per second with an accuracy of 0.003%, enabling very precise control of frequency and synchronization.
T-Squared explanation was more poetic
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Old Mar 08, 2012, 06:52 PM
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Upstate NY
Joined May 2005
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Here is a good explanation....

http://www.rcmodelreviews.com/spreadspectrum03.shtml

And this video really helps
How spread spectrum RC systems can share the same part of the 2.4GHz band (8 min 25 sec)
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Old Mar 09, 2012, 03:08 AM
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Tks for your replies.
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I have seen the rcmr vid before.... very nice explanation, although i didnt realise that a momentary transmission on one single frequency doesn't even make up one bit in the data stream.
It is the initial sync issue i was most interested in, and how often some kind of "sync check" frame was sent/needed.
If , as Bruce says, the clocks are that accurate, then this would explain a lot.
Rgds, steve
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Old Mar 09, 2012, 05:15 AM
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No matter how accurate the clocks are in the TX and RX there will always be some drift between them so I would expect there to be a mechanism to resynchronise them. This used to be known as "tau dither" a fancy term which means the receiver is always hunting slightly up and down the synch range to see where the edges of the pulses lie so that it could tell how much drift is happening and can compensate for it.

A.
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Old Mar 09, 2012, 06:06 AM
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Joined Dec 2002
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That sounds really clever.... dynamically adjusting to keep in sync.
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I realise we all don't really need to know how these things work, I am just really impressed this all happens in a product so cheap (well, Frsky anyhow).., and find it really interesting.
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Rgds, Steve
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