Is AM Radio transmission AC coupled? - Electronic Circuits Projects Diagrams Free

NJ Roadmap · 7th April 2007, 09:06 PM

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 · 8th April 2007, 01:08 AM

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

Leftyretro · 8th April 2007, 01:45 AM

Quote:

Originally Posted by Sceadwian

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 · 8th April 2007, 01:57 AM

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 · 8th April 2007, 08:50 AM

Quote:

Originally Posted by Sceadwian

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 · 8th April 2007, 03:07 PM

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 · 8th April 2007, 05:32 PM

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.

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode

Similar Threads
Title	Starter	Forum	Replies	Latest
Need advice about this AC Line Current Detector	Avocado	Electronic Projects Design/Ideas/Reviews	6	21st March 2008 06:37 PM
Simple fixed frequency AM radio receiver	cjk2	Electronic Projects Design/Ideas/Reviews	1	4th March 2007 12:28 PM

7th April 2007, 09:06 PM	(permalink)
NJ Roadmap	Is AM Radio transmission AC coupled? 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! __________________ www.myspace.com/crimsonroadmap

8th April 2007, 01:08 AM	(permalink)
Sceadwian	All radio frequency methods are AC coupled. Static electricity can't transmit information. __________________ "Because I be what I be. I would tell you what you want to know if I could, mum, but I be a cat, and no cat anywhere ever gave anyone a straight answer, har har."

8th April 2007, 01:57 AM	(permalink)
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 __________________ "Because I be what I be. I would tell you what you want to know if I could, mum, but I be a cat, and no cat anywhere ever gave anyone a straight answer, har har."

8th April 2007, 03:07 PM	(permalink)
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.

8th April 2007, 05:32 PM	(permalink)
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. __________________ RadioRon