SDR reception of legacy AM, FM waves

Started by DevBee 3 years ago6 replieslatest reply 3 years ago246 views


I am a newbie to the SDR and technology.

I understand that SDR is fundamentally based on I/Q modulated waves where any discrete value  that can be transmitted corresponds to a symbol which is represented as amplitude and phase through I and Q signals.

How can a latest SDR receive legacy AM waves ? as the SDR uses I,Q components whereas legacy AM doesn't have any such component applicable? I presume SDRs are able to receive and demodulate existing legacy FM transmissions - please correct/add.

Thank you in advance,

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Reply by DSP_BigManApril 19, 2021

I and Q are characteristics of the RF wave form.  You can reduce the sample rate by sampling I and Q each to separate channels.  AM is not a symbol in this context.  

There are 4 kinds of Amplitude Modulation.  

More complex schemes such as 16-QAM transmits four bits of data per symbol resulting in a bit rate of 4 times the sample rate.

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Reply by fharrisApril 19, 2021

Hell0 DevBee

The SDR radio can demodulate any analog modulation format as well as digital modulation options>

If you have access to IEEE explore, look up paper Software Defined Radio....Part 22 in a series of tutorials on instrumentation and Measurements.

I you don't have access... drop me an e-mail fjharris@ucsd.edu and I'll send you a copy... it is too big to load as an attachment here.

fred  harris

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Reply by DevBeeApril 19, 2021

Thank you very much Fred.

Please let me look at the paper. I could get it.

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Reply by Y(J)SApril 19, 2021

I'll try to give the simplest possible answer.

Modulation means changing the parameters of a signal in order to carry information. For example, in audio AM one starts with a sinusoid and changes its amplitude according to the audio coming from a microphone (the frequency of the unmodulated sinusoid being much higher than the frequencies in the audio).

Demodulation means recovering the original information (in our case the audio) from the modulated signal. This can be done using analog electronic circuits (for example, an "envelope detector" follows the largest values the modualted sinusoid takes) or by sampling the modulated signal (thus converting it into a vector of numbers) and using software to recover the information.

SDR is the latter approach.


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Reply by DevBeeApril 19, 2021

Hello Y(J)S, 

Thank you for the reply.

How does SDR know that it has to switch to envelope detection mode instead of I/Q demodulation? Unless the user gives a trigger to SDR, its software functionality for legacy AM is not triggered and sampled signal continues to be perceived as combination of I and Q symbols thus creating bit pattern that is junk - Isnt it? Or am missing some fundamental..

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Reply by Y(J)SApril 19, 2021

An SDR would not usually do envelope detection at all - that is a primitive method used because of limitations of analog electronics.

If the input is IQ then the demodulated AM audio is simply the square root of the sum of the squares of the I and Q A[t] + SQRT(I^2[t] + Q^2[t]).

And the PM demodulation is the arctangent of the ratio (but extended to 4 quadrants and possibly unwrapped).

All this was assuming you are IQ sampled with rate at least the highest frequency.

If the input is real sampled at twice the highest frequency (i.e., only the I) then you can either reproduce the Q using the Hilbert transform, or by complex downmixing to zero and low-pass filtering.