R/C Tech Forums

Go Back   R/C Tech Forums > General Forums > Electric On-Road

Reply
 
Thread Tools Search this Thread
Old 11-22-2002, 03:30 AM   #1
Tech Apprentice
 
Join Date: Nov 2001
Posts: 67
Radio frequencys: AM vs FM

Just wondering how much better is FM over AM?
I currently own a futuba megatech juniour AM controller, and thinking of buying another radio for my other r/c car. AFAIK the FM has less interference... but from my experience with my AM controller i haven't had any interferences(yet). Is there more to it then just radio interference? Can someone enlighten me.
Thanks.
taybelZ is offline   Reply With Quote
Old 11-22-2002, 05:48 AM   #2
Company Representative
 
Join Date: Aug 2001
Location: Calgary, Alberta, Canada
Posts: 1,633
Default

The difference between the two is the size and length of the radio wave they broadcast. FM is a tighter, smaller wave, and is much less suceptable to interference than AM. Also most FM radios are a bit better in quality than AM radios. I can go into more detail if you like, it just may seem like a physics class tho TTYL.


Gary F.
Gary Fliegel is offline   Reply With Quote
Old 11-22-2002, 05:57 AM   #3
Tech Elite
 
Cole Trickle's Avatar
 
Join Date: Sep 2001
Location: Denmark / Europe
Posts: 2,571
Default

Usually, FM radios are computerized and got more features.

About interference, most guys at our club are using AM with good results. Most of us run stock only. I did also use AM in the beginning, but when I ran together with cars with hot modifieds, I got loads of inteference, when they passed me.

At my opinion, the great advantage of better radios, are better grip and adjustment of throttle, so the operation are smoother. I also find throttle / steering travel adjustment very usefull. A little detail like throttle travel as 80% forward and 20% brake, make a big difference.

Model memory are nice too. But if your buddy should borrow one of your cars so you can drive together, then one TX and several RX aint a solution.

All in all, I'm very happy with my computerized FM TX, but if you are a basher or a casual club driver like me, then it should be something of the last to spend money on.

At my opinion, KO Presto or Futaba 3PDF got very good value for money (both FM). If you prefer to spend less, the KO EX-5 (AM) might be a good choice. I've never heard anything bad about those.

Last edited by Cole Trickle; 11-22-2002 at 06:02 AM.
Cole Trickle is offline   Reply With Quote
Old 11-22-2002, 08:26 PM   #4
Tech Initiate
 
Join Date: Nov 2002
Location: SF Bay Area
Posts: 42
Default

Quote:
Originally posted by Gary Fliegel
The difference between the two is the size and length of the radio wave they broadcast. FM is a tighter, smaller wave, and is much less suceptable to interference than AM.
Please go into Physics class mode.

As I understood, frequency dictates wavelentgth and RC surface frequencies are fixed. If I have channel XX on frequency xx.xxx then how is the waveform longer or shorter with an AM or FM radio? Don't they both have to transmit on xx.xxx, which would determine how long the wavelength is? At least that is what I think I remember from Physics class... it's been a while.

I understood the difference was that AM transmitted on one frequecy only, while FM transmitted on two frequencies (equal amounts above and below the median frequency) or some such. Is this true?

I've actually heard one argument that AM was actually better in terms of noise rejection than FM when referring to RC radios because FM can be affected equally by interference on either of the two frequecies it uses to transmit, yet AM can only be affected on the one freq.

And that the real difference between the noise rejection differences was a marketing issue... that people would never buy "high end" AM radios with electronics equivalent in quality and noise rejection to the FM radios simply because a layman(person) typically considers AM inferior to FM. Even though in this case it may not be, the general perception and feeling of people is enough to dissuade manufacturers from making AM radios with the same quality inside as the FM radios.

Anyway, most of that is from hearsay, I, for one, would be interested in a more technical explanation.

Last edited by Concillian; 11-22-2002 at 08:29 PM.
Concillian is offline   Reply With Quote
Old 11-23-2002, 06:20 AM   #5
Company Representative
 
Join Date: Aug 2001
Location: Calgary, Alberta, Canada
Posts: 1,633
Default

Ok a short run down on the two. AM is amplitude modulation, where the radio wave is a set length and the Height of the wave is changed to broadcast. AM has a very long wave length.

FM is frequency modulation where the the amplitude(Height) of the wave is set but the frequency of the wave is changed. FM has a very short wave length.

You can check this and this out for diagrams and such.

Because FM uses a shorter and stronger wave than AM it is less suceptable to outside interference. TTYL.


Gary

Last edited by Gary Fliegel; 11-23-2002 at 06:25 AM.
Gary Fliegel is offline   Reply With Quote
Old 11-23-2002, 08:12 PM   #6
Tech Elite
 
GTO2's Avatar
R/C Tech Charter Subscriber
 
Join Date: Jul 2002
Location: Don't Blink !!
Posts: 2,264
Default

Hi guys,
a bit off side here: anyone knows what's the freq difference on FM 40MHz Tx and Rx Xtals? Is it the same as AM 27MHz? a 455kHz image bandwidth? thanks.
GTO2 is offline   Reply With Quote
Old 11-25-2002, 11:17 AM   #7
Tech Apprentice
 
Join Date: May 2002
Location: Las Vegas
Posts: 50
Trader Rating: 3 (100%+)
Default

This link may help. Very technical and long but good stuff.

http://www.torreypinesgulls.org/Radios.htm
Kdonks is offline   Reply With Quote
Old 11-25-2002, 11:32 AM   #8
Tech Elite
 
GTO2's Avatar
R/C Tech Charter Subscriber
 
Join Date: Jul 2002
Location: Don't Blink !!
Posts: 2,264
Default

thanks, that is very helpful!
GTO2 is offline   Reply With Quote
Old 11-25-2002, 06:41 PM   #9
Tech Initiate
 
Join Date: Nov 2002
Location: SF Bay Area
Posts: 42
Default

Gary, those were some useful links, thank you for the information.

It makes more sense now about FM, it has to have more precise control of frequncy (as that is how it controls servo movement) and can always operate at full amplitude, where AM will be operating at significantly lower amplitude for center, and thus be more susceptible to noise (same noise, less signal, lower SNR/more susceptibility.)

I think when you say shorter and stronger, you are more referring to the aspect ratio of the wave, in that the amplitude can be higher relative to the wavelength. I still don't see how the wavelength would change, however the aspect ratio difference between AM and FM is now obvious to me.

Last edited by Concillian; 11-25-2002 at 06:43 PM.
Concillian is offline   Reply With Quote
Reply

Similar Threads
Thread Thread Starter Forum Replies Last Post
2 DTX ICE - Car stands, 8000 7.4 lipos, B4 Spares, Tires, 3PK radio M8 radio spektrum racer34v R/C Items: For Sale/Trade 7 07-16-2008 06:10 AM
gt2 truck,nitro extras,spetrum radio,ae radio,charger,battery more....cheap xsxracing R/C Items: For Sale/Trade 0 08-07-2007 08:40 AM
Understanding transmitters and frequencys tc3team Rookie Zone 1 12-28-2006 12:03 AM
Good radio:fm, any brand, no offer refused! needed radio(3 channel t to the maxx2 R/C Items: Wanted to Buy 16 02-05-2005 11:20 AM
Futaba 3PJS FM/PCM radio w/7 sets of crystals and MATT FRANCIS radio bag Ron Meloni R/C Items: For Sale/Trade 1 01-08-2003 04:43 PM



Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are Off



All times are GMT -7. It is currently 02:40 PM.


We are a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for us to earn fees by linking to Amazon.com and affiliated sites.
Powered By: vBulletin v3.9.2.1
Privacy Policy | Terms of Use | Advertise Content © 2001-2011 RCTech.net