Hey Fernando,
not quite sure I get what you need; I'd say the Amplitude Spectrum you'd
be looking for is
$$A_{|\cdot|}[f]=|X[f]| = \left\lvert\sum_{n=0}^{N-1} x[n]\cdot e^{j2\pi
\frac {nf}N}\right\rvert $$
or, rather, the decibel representation of that. There's no way to get a
negative number out of the absolute of something – it's by definition a
positive real number.
Now, we could also use our freedoms to define our amplitude spectrum to
take the shape
$$A_\text{signed} = s(X[f]) |X[f]|\text{ with }
s(X[f])=\begin{cases}1&\text{for } -\pi \le \angle X[f] < \pi \\ 0
&\text{else.} \end{cases}$$
But: that's really only useful if you have phase-coherent reception – as
an analytic tool for an unsynchronized observation of the spectrum, it
doesn't help you much, since you have a random $\angle$ due to having
random relative phase.
So, maybe it'd be a good idea to formulate what purpose you're doing
this for :) You can, indeed, tell 180° out-of-phase signals apart by
this, but I'd argue that being 180° out-of-phase, for the most things I
can think of, is only meaningful on one and the same frequency – and
hence, I'm not quite sure this is what you're looking for!
Best regards,
Marcus
On 25.04.2017 12:01, Fernando wrote:
> Hello.
>
> Yes, with Time sink I can see the difference, but if the signal is
> compound of some other signals (for instance signal=1K/amplitude +1
> +2K/amplitude -1 +3K/qamplitude +1 +4K/amplitude +1 ) i would like to
> see the 2k signal as -1 amplitude, but in the power spectrum it will
> appear as possitive and in the QT time sink it is very difficult to
> see the signal as it is a complex one.
>
> regards
>
>
> El 25/04/17 a las 10:57, Jinyang Lee escribió:
>> Hello Fernando,
>>
>> I think the QT GUI time sink displays the relationship between time
>> and amplitude. You can see the signal through it. But when I use the
>> channel model block,the QT2 can see the signal which is zero.
>> Enclose is running result with channel model and with channel model.
>>
>> Regards,
>> Lee
>>
>> 2017-04-25 15:45 GMT+08:00 Fernando <ferna...@samara.com.es
>> <mailto:ferna...@samara.com.es>>:
>>
>> Hi.
>>
>>
>> Is there a way of visualizing ampitude spectrum (with + and -
>> signals)
>> instead of power spectrum?
>>
>>
>> regards
>>
>>
>>
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>
>
>
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