Is AM Radio transmission AC coupled?

[NJ Roadmap]

Because I'm sure FM is. Atleast Nigel Goodwin told me that!

If it's not AC coupled then I'm pretty sure I don't need to worry about NRZ codes while transmitting which is a bonus speed-wise!

 

[Sceadwian]

All radio frequency methods are AC coupled. Static electricity can't transmit information.

 

[Leftyretro]

All radio frequency methods are AC coupled. Static electricity can't transmit information.

"Static electricity can't transmit information"

On the other hand static can send quite a message

Lefty

 

[Sceadwian]

Nahh, static electrictiy is fine, as long as it stays right where it is =) As soon as becomes mobile electricity, that's when the message gets sent

 

[NJ Roadmap]

All radio frequency methods are AC coupled. Static electricity can't transmit information.

Are you sure about that? I'm not doubting you at all but just making sure that you're confident thats true.

 

[j.p.bill]

Your information comes from an amplitude modulation of an AC carrier. DC put on the transmitting modulator would result in a constant signal level that would produce a level out of the rectifier, but no information. All stages of the AM receiver are AC coupled, so that level produces no output.

 

[RadioRon]

Of course the AM carrier and final signal need not be DC coupled, but sometimes it is useful to DC couple the modulator. For example, if you have a slow bit rate and you are using an asymmetrical coding scheme, then you might find that your modulation spectrum goes down to a lower frequency than you think. For example, if you are sending a 100 bps bit stream and you are not doing any coding to distribute the ones and zeros, it is possible that you might need a flat frequency response down to 1 Hz or less. In cases like this it is sometimes easier to simply DC couple so that you dont' get any unwanted ripple in the modulation. I've faced this problem numerous times when designing modulators for older POCSAG codes, and for the old 4800 bps data streams we used to send through NBFM mobile radios. In addition, if you AC couple you may also face the problem of on/off transients on your modulator caused by initial charging and discharging the coupling capacitors. These transients corrupt your signal for the first moments of transmission. You can avoid such transients by DC coupling.

Of course, if you DC couple you have to pay attention to the quality of the DC. Any drift will cause variation in output power in an AM system and will cause frequency drift in an FM system.