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Re: [Discuss-gnuradio] Rectifying and Low Pass filter


From: Robert James
Subject: Re: [Discuss-gnuradio] Rectifying and Low Pass filter
Date: Sun, 17 Nov 2013 14:53:14 -0500

I see.  For those of us who have some background in analog circuits,
is there a way to work with real valued passband signals? For example,
is there a block which takes a complex baseband signal and converts it
to a real valued passband signal?


On 11/17/13, Johnathan Corgan <address@hidden> wrote:
> On 11/17/2013 11:01 AM, Robert James wrote:
>
>> I'm trying to demodulate AM (I know there are existing blocks to do
>> that, but I'd like to learn how to do it on my own).
>>
>> In a real world analog circuit, I would find the envelope by first
>> rectifying the signal (using a diode), and then using a low pass
>> filter - that would give me the envelope.
>>
>> How can I rectify with GNU Radio?
>
> In GNU Radio, one is typically dealing with complex baseband I/Q
> signals, where a "real valued" passband signal has had its carrier
> shifted down to zero and what remains is the complex envelope
> represented by I and Q.  There are many resources indexed by Google to
> help you learn more about this.
>
> To demodulate a passband AM signal represented at baseband, simply take
> the magnitude of the I/Q representation, i.e., sqrt(I^2 + Q^2).  GNU
> Radio has a block to do this, gr::blocks::complex_to_mag, which you can
> also access in Python as blocks.complex_to_mag(), or from GRC under
> "Type Converters/Complex to Mag".  It is a synchronous block that
> accepts a complex signal stream at an arbitrary sample rate and outputs
> the magnitude of that signal as a float stream at the same sample rate.
>
> Depending on what the original signal was that went into the AM
> modulator on the transmitter (such as audio like in AM broadcast radio),
> you may need to further process the signal in order to use it, such as
> removing DC offset, filtering, amplification, or changing the sampling
> rate.
>
> --
> Johnathan Corgan, Corgan Labs
> SDR Training and Development Services
> http://corganlabs.com
>



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