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Topic Title: Baseband and RF Signal
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Created On: 16 February 2012 12:40 PM
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 16 February 2012 12:40 PM
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jls14ce

Posts: 11
Joined: 05 February 2012

Hi Guys
I am simulating radio receivers using heterodyne and homodyne structures and I dont know how to get the frequencies. I need to input a RF signal at the receiving end and then the Baseband signal will be produced at the output.
But how do I get the RF signal and its corresponding baseband signal, can someone help me with this please?
Thank you!!
Jenny
 16 February 2012 01:15 PM
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eswnl

Posts: 144
Joined: 29 November 2008

When you say simulating, are you simulating in schematic capture software or are you trying to write a program? Or are you physically building a receiver and want to inject an RF signal into it?
 16 February 2012 08:04 PM
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johnnmann

Posts: 53
Joined: 18 October 2006

You are talking about some basic radio communication concepts. If these are new to you can I suggest you take a look at the book "Foundations of wireless and electronics" by M.G. Scroggie. It's an old book (the first edition came out before the second world war I believe) but it has been updated and revised over the years.
 17 February 2012 01:28 PM
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jls14ce

Posts: 11
Joined: 05 February 2012

Hi Eswnl
Thank you for your reply.
I am not writing a program. I am using Matlab Simulink to simulate the receivers. For this, I will have to input the RF signal and I should get its corresponding Baseband signal at the output. But I dont know how to get these signals and if you could help me with this, I would really appreciate it.
Thank you
Jenny
 17 February 2012 01:30 PM
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jls14ce

Posts: 11
Joined: 05 February 2012

Hi Johnnmann
Thank you so much for your suggestion. This is a new concept to me indeed. I will look into this book. Would it provide me with the signals that I'm trying to find out?
Thanks once again
Jenny
 17 February 2012 02:30 PM
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jls14ce

Posts: 11
Joined: 05 February 2012

Hi g3xoi
Wow!! So thats how its done?! Thank you ever so much!! I was expecting it to be something really complicated but never thought it would be like this. So in the example that you gave, the RF signal was 2 Mhz and the Baseband Signal is the result. Now which one of the result would be the baseband signal exactly? Is it the sum or the difference?
Thank you once again!!
Really appreciate this.
Jenny
 17 February 2012 04:21 PM
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eswnl

Posts: 144
Joined: 29 November 2008

I think g3xoi is talking about direct conversion recievers (homodyne), where the RF from the antenna is mixed with the local oscillator and the output is baseband audio. Think about definition of baseband. It starts at 0Hz and then think about what frequencies audio signals use.

In heterodyning, you downconvert the RF to a fixed frequency(IF) and then demodulate to get the audio baseband. This gives better selectivity and performance.

I think the next question you need to ask is what demodulation you are using (SSB, FM, AM).

Edited: 17 February 2012 at 04:29 PM by eswnl
 23 February 2012 02:11 PM
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jls14ce

Posts: 11
Joined: 05 February 2012

Thank you eswnl
That was really helpful!!
Will look into this more.
 25 February 2012 12:29 PM
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jencam

Posts: 608
Joined: 06 May 2007

Originally posted by: jls14ce
I am not writing a program. I am using Matlab Simulink to simulate the receivers. For this, I will have to input the RF signal and I should get its corresponding Baseband signal at the output. But I dont know how to get these signals and if you could help me with this, I would really appreciate it.


Use an oscillator component with a sine wave output for the local oscillator and set the frequency and amplitude accordingly.

The input RF signal will be an already modulated RF signal, basically, what's picked up by a radio aerial. The simplest is a sine wave with a frequency that corresponds with the broadcast frequency suitable modulated (AM, FM, SSB, etc) by an audio frequency sine wave. Matlab has a component which can be used for this. The baseband signal at the output can be viewed as a waveform either in the time (oscilloscope) or frequency (spectrum analyser) domain.
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