QAM or QPSK over audio RF link

data over audio

Hi, i´m planning to do a project based on QAM or QPSK for transmitting data over a RF audio link.
I have sucessful buitl a FSK FM data link, but I want to get more data transferer, so QAM or QPSK or superior digital modulation is needed.

Any ideias, schematics or any thing that could help me?

Tanks for any help

schematics for a qpsk

What is an "RF AUDIO" link? unless it has very high quality components in it, you will be unable to trasmit much higher modulation than qpsk. 64 QAM or higher requires great linearity and very low phase noise.

qpsk demodulator 8051

Based on this project **broken link removed**
it is possible to use audio bandwidth to transmitt 16 or 64QAM modulation and get data rates up to 100kbps or more over a FM RF link.
Fully software designed it was an excelent idea!
My ideia is collect some information that would help me design a simple 4QAM modulator by hardware or even more. I dont want to have lots of data like Mbps, just want to test a simple QAM modulation of a data stream input.

I´ve seen this schematics : http://electronicdesign.com/Files/29/2756/Figure_03.gif
from the article
http://electronicdesign.com/Articles/ArticleID/2756/2756.html (fig3) for a modulator
and http://electronicdesign.com/Files/29/3168/Figure_03.gif
from the article
http://electronicdesign.com/Articles/ArticleID/3168/3168.html (fig3) for the demodulator

Acording to this on the modulator the QPSK modultation out would be fm/8, so if I get 10khz modulated output, I would need to have a fm= 80khz

Any comments, examples or schematics are welcome

Tanks

audio qam

I'm the one who wrote the QAM software you mention above.

QPSK is very easy compared to 16QAM because you don't have to deal with keeping the amplitude exact all the time. so yup, QPSK is a good place to start.

I've thought about doing a hardware implementation (for QAM16 and 64) but have done nothing so far.

My idea was to use a dsPic (cheep) chip and a DAC. they run at 40Mips. Weather the 12bit ACD that come with them is enough for demodulation of QAM64, or problems of using a 16bit operating system means its infeasible, I don't know.

If you want to stay more hardware like, make something like http://electronicdesign.com/Files/29/2756/Figure_01.gif
if it be like they say or buy a few mixers on a chip or make them using op amps.

Have a look at some data sheets ( http://www.datasheetcatalog.com/ ) and see whats already available.

I don't have any schematics telling you how to make it tho.

why qpsk easy to implement

Jontio, that's an impressive project! I don't think I have ever seen sound cards used for high data rate QAM.

qpsk schematic

Hi jontio congratulations for that project !
I´m am planinng to make some simple qam modulator using 8051 microprocessor an DAC and ADC , the modulator simple get the data via serial than if 00 it generates like a cosine wave at 5Khz for example and DAC will generate analog voltage , if it receives 01 it generates a cosine with 90 degrees phase , if 10 it generates a cosine with 180 degrees phase for a 11 it generates a cosine 270 degrees phase.

For the receiver ADC to get digital convertion for the 5KHz wave , then the demodulation of the wave is a bit more complicated, I need to have some wave for reference or using a integrator with ampop for detecting phase diferences in the wave received.
More ideias will come about that, I don´t have much time now, but I will study a way to do digital signal processing for demodulation.

Some time ago I have used you RS232 sender for broadcasting digital audio over an FM transmitter, than get with an receiver connected to a data slicer , it worked nice

Cya

qam modulation project schematic

Thanks all.

8051, that's a very old 8bit cpu. something like a dspic30f4012 would probably be a better choice.

for demodulation read up on symbol timing recovery and tracking, carrier frequency recovery and tracking. also the idea of matched filters (root raised cosines are good). the constant modulus algorithm (CMA) is useful for channel equalization and is easy to implement.

QPSK is very robust, so you will probably be able to get away with using only symbol timing tracking (read up on the Gardner algorithm). However the more things you implement the more you can push your setup.

qam schematics

QPSK may be the better optio,
Project is ver intresting, after getting result please update eda board folks

__Sree