Demodulation possible using PC !

Hi Guys,
I am doing a project on Software Defined Radio receiver..
I have generated a AM signal which is coming from a microphone and it
goes to a signal mixer. I have a local oscillator and the two signals
are mixed (Superhet). The output from the mixer forms the
IF(Intermediate Frequency).Now what I have to do is to demodulate  the
signal and feed it to the speaker using a AF Amplifier. So I was
wondering if if there is a way to do the Demodulation (I think its like
a modulation) on a computer.
Please help me in this issue.
Regards
Nitin

Hello Nitin,


>                        ... The output from the mixer forms the
> IF(Intermediate Frequency).Now what I have to do is to demodulate  the
> signal and feed it to the speaker using a AF Amplifier. So I was
> wondering if if there is a way to do the Demodulation (I think its like
> a modulation) on a computer.
> Please help me in this issue.


If the IF is low enough, like 12-15kHz or so you can do that with the 
sound card. I don't remember where I saw it but it was on one of the ham 
radio web sites. Somebody did an SSB demodulator which was actually part 
of a spectrum analyzer software. Could also do AM. It let you pick I/Q 
building blocks and piece them together as you like. IIRC it was from a 
German (but documented in English).

It can probably be found here:
http://personal3.iddeo.es/ea3qp/soft1.html

A long time ago I tried that. It worked but was weird: I ran the regular 
IF/demod of the receiver in parallel and the audio from the PC came 
about 1/2sec later.

Regards, Joerg

http://www.analogconsultants.com

"Joerg" <notthisjoergsch@removethispacbell.net> wrote in message
news:NLKPf.34962$_S7.30701@newssvr14.news.prodigy.com...
> Hello Nitin,
>
>
> >                        ... The output from the mixer forms the
> > IF(Intermediate Frequency).Now what I have to do is to demodulate  the
> > signal and feed it to the speaker using a AF Amplifier. So I was
> > wondering if if there is a way to do the Demodulation (I think its like
> > a modulation) on a computer.
> > Please help me in this issue.
>
>
> If the IF is low enough, like 12-15kHz or so you can do that with the
> sound card. I don't remember where I saw it but it was on one of the ham
> radio web sites. Somebody did an SSB demodulator which was actually part
> of a spectrum analyzer software. Could also do AM. It let you pick I/Q
> building blocks and piece them together as you like. IIRC it was from a
> German (but documented in English).
>
> It can probably be found here:
> http://personal3.iddeo.es/ea3qp/soft1.html
>
> A long time ago I tried that. It worked but was weird: I ran the regular
> IF/demod of the receiver in parallel and the audio from the PC came
> about 1/2sec later.
>
> Regards, Joerg
>

You could write software for a PLL, recover the carrier with the software
PLL  and then multiply into the original AM and low-pass filter.
Another way is to use the dreaded I and Q. I and Q is found by multiplying
the original AM by a sin and cosine at the carrier frequency. Once you have
I and Q the demodulated output is

sqrt(I^2+Q^2)

Tam

You can look into the gnu radio project. It has everything you would need.

If you want something very cheap you need to find something to digitize your
IF (maybe a high end sound card that runs at 96 kHz) to get the IF data into
your computer. Once there you simply:

1. mix the signal to baseband, i.e. multiply by a complex sinusoid of the
same frequency as the IF
2. low pass filter the baseband I/Q data
3. calculate the magnitude (I^2+Q^2)

At the audio rates you are talking about virtually any PC can handle this
amount of math.

If you have Matlab (or download Octave) you can prototype the sgnal
processing very quickly. The hardest part is probably getting the data into
the computer and the sound samples out.

-Clark

"Nitin" <indianitin@gmail.com> wrote in message
news:1141859263.438452.20760@i39g2000cwa.googlegroups.com...
> Hi Guys,
> I am doing a project on Software Defined Radio receiver..
> I have generated a AM signal which is coming from a microphone and it
> goes to a signal mixer. I have a local oscillator and the two signals
> are mixed (Superhet). The output from the mixer forms the
> IF(Intermediate Frequency).Now what I have to do is to demodulate  the
> signal and feed it to the speaker using a AF Amplifier. So I was
> wondering if if there is a way to do the Demodulation (I think its like
> a modulation) on a computer.
> Please help me in this issue.
> Regards
> Nitin
>

Why multiply with a complex sinus? Why not with a sinus?

"Anonymous" <someone@microsoft.com> skrev i meddelandet
news:TeXPf.37461$915.13404@southeast.rr.com...
>
> You can look into the gnu radio project. It has everything you would need.
>
> If you want something very cheap you need to find something to digitize
your
> IF (maybe a high end sound card that runs at 96 kHz) to get the IF data
into
> your computer. Once there you simply:
>
> 1. mix the signal to baseband, i.e. multiply by a complex sinusoid of the
> same frequency as the IF
> 2. low pass filter the baseband I/Q data
> 3. calculate the magnitude (I^2+Q^2)
>
> At the audio rates you are talking about virtually any PC can handle this
> amount of math.
>
> If you have Matlab (or download Octave) you can prototype the sgnal
> processing very quickly. The hardest part is probably getting the data
into
> the computer and the sound samples out.
>
> -Clark
>
> "Nitin" <indianitin@gmail.com> wrote in message
> news:1141859263.438452.20760@i39g2000cwa.googlegroups.com...
> > Hi Guys,
> > I am doing a project on Software Defined Radio receiver..
> > I have generated a AM signal which is coming from a microphone and it
> > goes to a signal mixer. I have a local oscillator and the two signals
> > are mixed (Superhet). The output from the mixer forms the
> > IF(Intermediate Frequency).Now what I have to do is to demodulate  the
> > signal and feed it to the speaker using a AF Amplifier. So I was
> > wondering if if there is a way to do the Demodulation (I think its like
> > a modulation) on a computer.
> > Please help me in this issue.
> > Regards
> > Nitin
> >
>
>

"Nitin" <indianitin@gmail.com> wrote in message 
news:1141859263.438452.20760@i39g2000cwa.googlegroups.com...
> Hi Guys,
> I am doing a project on Software Defined Radio receiver..
> I have generated a AM signal which is coming from a microphone and it
> goes to a signal mixer. I have a local oscillator and the two signals
> are mixed (Superhet). The output from the mixer forms the
> IF(Intermediate Frequency).Now what I have to do is to demodulate  the
> signal and feed it to the speaker using a AF Amplifier. So I was
> wondering if if there is a way to do the Demodulation (I think its like
> a modulation) on a computer.
> Please help me in this issue.

http://www.flex-radio.com has a plethora of documentation and source code 
for a Windows based system. Originally it downconverted directly to I/Q 
baseband using a quadrature NCO and quadrature sampling detector, although I 
believe it now downconverts to about 11kHz because of noise close to DC.

Regards, Howard 

Blipp wrote:
> Why multiply with a complex sinus? Why not with a sinus?

To get I and Q.

Jerry
-- 
Engineering is the art of making what you want from things you can get.
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Hi Joerg,
Thanks for the reply.
I was just wondering if it is possible to do the Demodulation part
using MATLAB.
First of all, I have doubts that MATLAB can take inputs from the
soundcard in Real Time.
O.K. If it is not able to take in Real time, what I can do is take the
signal, record it in my computer using a Goldwave in (.mat) form. Then
I can link this file into a source in the MATLAB simulink and feed it
into the [DSB AM
Demodulator
Passband] block and take the output and again store it in a (.mat)
format.

But I am really not able to use the [DSB AM
Demodulator
Passband] signal block.Can anyone suggest if I am going in the right
direction.I will take the output in the .mat file and then play it
using a speaker.
Thanks
Nitin

Thanks for the reply.
I was just wondering if it is possible to do the Demodulation part
using MATLAB.
First of all, I have doubts that MATLAB can take inputs from the
soundcard in Real Time.
O.K. If it is not able to take in Real time, what I can do is take the
signal, record it in my computer using a Goldwave in (.mat) form. Then
I can link this file into a source in the MATLAB simulink and feed it
into the [DSB AM
Demodulator
Passband] block and take the output and again store it in a (.mat)
format.

But I am really not able to use the [DSB AM
Demodulator
Passband] signal block.Can anyone suggest if I am going in the right
direction.I will take the output in the .mat file and then play it
using a speaker.
Thanks
Nitin

Thanks for the reply.
I was just wondering if it is possible to do the Demodulation part
using MATLAB.
First of all, I have doubts that MATLAB can take inputs from the
soundcard in Real Time.
O.K. If it is not able to take in Real time, what I can do is take the
signal, record it in my computer using a Goldwave in (.mat) form. Then
I can link this file into a source in the MATLAB simulink and feed it
into the [DSB AM
Demodulator
Passband] block and take the output and again store it in a (.mat)
format.

But I am really not able to use the [DSB AM
Demodulator
Passband] signal block.Can anyone suggest if I am going in the right
direction.I will take the output in the .mat file and then play it
using a speaker.
Thanks
Nitin