Re: sound card AM Transmitter

cs_posting@xxxxxxxxxxx wrote:
On Jul 30, 9:31 am, "ypg" <yogesh_ghar...@xxxxxxxxx> wrote:

1)At 20Khz carrier freq (say) , wavelength will be of the order of 15Km ,
antenna of that order is not feasible.

Unlicensed hobbyist VLF transmitters are legally limited to antennas
of short dimension that are a small fraction of a wavelength. They
aren't intended to be good antennas !

2) Music may have freq like 15Khz , u cannot amplitude modulate 15Khz
signal with a carrier of 20Khz. Carrier should be of a comparatively high
freq as compared to the signal being modulated.

A common misconception. You can quite successfully modulate a carrier
of zero frequency if you feel like it, though you will need a signal
path that is good down to DC. Needless to say it won't propagate very
well between antennas. If your bandwidth of interest crosses through
zero Hz and is not symmetric, then you will probably need separate
quadrature signals (I & Q) to be able to tell positive and negative
frequencies apart. But if your carrier is say 10 Khz and your total
bandwidth 15 Khz centered there, then your frequencies of interest are
from 2.5 Khz to 17.5 Khz and a single phase will suffice. This is
actually how a lot of PC-based software radio work started out - mix
the RF frequency down to a carrier within the range of the soundcard's
bandwidth. And note that telephone quality double sideband AM voice
has a modulated bandwidth of 6-7 KHz.

To answer the original poster's question: synthesize a sine wave of
the desired carrier frequency and multiply it by the signal you wish
to modulate it with. Send the resulting data to the sound card.

How far do you expect it to propagate with an antenna of reasonable (say 50 meter) size? What will serve as a receiver?

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
.

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