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differences on AM and FM

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pike

Member
hey guys,

ive been trying to build a remote controlled vehicle using MC's. For radio recption i'm using "off the shelf" walkie talkies but i have a choice between an AM based radio and an FM based radio. I know that the FM based one is much better for it's low distortion figures but why???

What makes AM reception different from FM reception. I already know that FM means frequency modulation and that AM means amplitude modulation, But how do they differ ???

And yes i'm intending to use the 433.420 Mhz CB radio band, but both the AM version and FM, work in this frequency band.
 

stevez

Active Member
Taken from a handbook of mine - "The communications effectiveness (of FM) depends almost entirely on the receiving methods. If the receiver will respond to frequency changes but is insensitive to amplitude changes it will discriminate against most forms of noise ..."

My understanding, which may be oversimplified, is that in some FM receivers a bunch of the incoming signal has the top chopped off - where the amplitude is varying - because it doesn't need that - it only needs to see the variation in frequency. In AM you need the variations in amplitude because that is where the information is contained.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
stevez said:
Taken from a handbook of mine - "The communications effectiveness (of FM) depends almost entirely on the receiving methods. If the receiver will respond to frequency changes but is insensitive to amplitude changes it will discriminate against most forms of noise ..."

My understanding, which may be oversimplified, is that in some FM receivers a bunch of the incoming signal has the top chopped off - where the amplitude is varying - because it doesn't need that - it only needs to see the variation in frequency. In AM you need the variations in amplitude because that is where the information is contained.
From a practical point of view (as an ex radio ham), FM provides better quality, less interference, and more reliability - as long as the signal is reasonably strong. AM, while being poorer quality, more prone to interference, and less reliable, will work at lower signal strengths.

This basically applies to voice communications, where your ears can make sense of amazingly poor signals - for a data communication system FM is likely to be prefered.

It seems like hard work though using walkie talkies, you will have to add a modulator at the transmitter end and a demodulator at the receiver end (modem) for a data link. Plus, do the licence exempt conditions allow data transmissions on that band?. It will probably be easier to use the small licence exempt data transmission modules - just serial data in and out, or you can get them that add manchester encoding as well.
 

pike

Member
thanks for your replies guys,

I was thinking about using the serin and serout commands directly to the CB radios.

Transmitter-The serout pin goes through a voltage dropping resistor then to the electret microphone.
Reciever-And then the signal is amplified to TTL levels for the serin pin]

As for the license, from what i know, these radios operate on the LIPD band (Low Interference Potential Device). But data communication is limited to only the lower 23 frequencies of the 40 available channels, whereas voice transmission is permitted all over this band frequency.

I know it would probably cheaper to buy one way data modules but i'm getting these radios for free- My uncle runs a store.
 

Nigel Goodwin

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Most Helpful Member
pike said:
Transmitter-The serout pin goes through a voltage dropping resistor then to the electret microphone.
Reciever-And then the signal is amplified to TTL levels for the serin pin]
If you could do that you wouldn't need a modem for your computer to access the Internet :lol:

A simple way to do it is use the serout pin to frequency shift an audio oscillator (555?), then at the receiver end use a PLL (a 567 or similar) to demodulate the FSK (Frequency Shift Keying) signal. With a simple scheme like this you can probably get 1200 baud.
 

pike

Member
Have you got an example on how to wire up a 567 ??? I have never heard or had any experience with such devices. I was able to dig up a data sheet from National Semiconductors, but that has raised more questions than answers.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
pike said:
Have you got an example on how to wire up a 567 ??? I have never heard or had any experience with such devices. I was able to dig up a data sheet from National Semiconductors, but that has raised more questions than answers.
The output is on pin 8, this is open collector so requires a pull-up resistor, under no signal this pin will be high (held high via the pull-up). The oscillator in the transmitter during resting should be one frequency, and when a start bit appears a second frequency should be produced - I don't think it matters which is high and which is low.

The oscillator in the 567 should be set to the second frequency, so while no data is being send pin 8 remains high - when the second frequency appears pin 8 will go low, and will follow the serial data from the transmitter as the two switched frequencies are sent.

Basically pin 8 goes low when the 567 receives the correct frequency input.

I'm sure there are circuits about somewhere for doing this, I've seen them in the past - but I'm talking a long time ago!.

You might have a look at http://www.qsl.net/vu2upx/Projects/rfmodem.htm which is doing a similar thing, Packet Radio is basically slow speed computer networking over radio links.
 
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