12 votes
Accepted

Amplitude modulation vs sampling rate?

The OP's opening statement is incorrect: $f_s > f_{max}/2$ prevents frequency aliasing for a bandlimited signal, but not amplitude aliasing $f_s > 2 f_{max}$ prevents aliasing. It's as simple ...
Dan Boschen's user avatar
  • 51.4k
11 votes
Accepted

Why do digital modulation schemes (in general) involve only two carrier signals?

Ah! They don't generally involve only two degrees of freedom. The ones you've been looking at mostly have, but that's not because they're the only ones used or taught. Chances are you'll meet one or ...
Marcus Müller's user avatar
9 votes

Why Hilbert Transform is terrible choice for amplitude demodulation of broadband signals?

It's not only a matter of "broadband or not": The Hilbert estimate degrades for multi-component signals - that is, whatever we can't draw without lifting our pen, left-to-right, in time-...
OverLordGoldDragon's user avatar
5 votes

How harmonics happens when you mix two signals?

Amplitude modulation is a linear operation (excluding the carrier in the classical AM) and does not introduce harmonics due to the modulated signal. Sidebands are not harmonics. They are created as ...
Fat32's user avatar
  • 28.2k
5 votes
Accepted

Absolute value based AM envelope detection viewed in the frequency domain

$\DeclareMathOperator{\sgn}{sgn}$ The modulating signal in AM is $$s(t) = C + a(t)\text,$$ where $a(t)$ is the (audio) amplitude, and $C$ is a constant so that $s(t) \ge 0 \;\forall t$. (Otherwise, ...
Marcus Müller's user avatar
5 votes
Accepted

Why is the carrier suppressed by AM-DSB-SC modulation?

It's a simple trigonometric identity: $$\cos(\omega_mt)\cos(\omega_ct)=\frac12\left[\cos((\omega_m+\omega_c)t)+\cos((\omega_m-\omega_c))\right]\tag{1}$$ Multiplying two sinusoids of different ...
Matt L.'s user avatar
  • 90k
5 votes
Accepted

Why Does ASK Modulation Create Fourier Sidebands?

If you did a continuous on off keying of a 10101010... pattern, then you would see sidebands as described since this is simply an up-conversion of the Fourier Transform of a 50% duty cycle square wave ...
Dan Boschen's user avatar
  • 51.4k
5 votes

Are there modulation schemes that use more bandwith than they need to, but don't require a spread-spectrum code to decode? (besides AM)

Yes, analog wideband FM is another example where the occupied bandwidth is wider than the bandwidth of the modulation signal, and provides a similar processing gain to digital spread spectrum ...
Dan Boschen's user avatar
  • 51.4k
4 votes

Amplitude modulation vs sampling rate?

The claim is wrong. Sampling of a pure sinusodial whose frequency is below but arbitrarily close to the Nyquist frequency (half the sampling frequency) is a perfectly valid operation, as long as you ...
Fat32's user avatar
  • 28.2k
4 votes

Why Hilbert Transform is terrible choice for amplitude demodulation of broadband signals?

Amplitude extraction / AM demodulation criteria I shall prove, $y(t) = x(t) \cos(\omega_c t)$ demodulates perfectly to $|x(t)|$ if A) $x$'s highest frequency, $\omega^\text{max}_x$, is $<\omega_c$...
OverLordGoldDragon's user avatar
3 votes
Accepted

Amplitude modulation and FFT query

The math says: there has to be there ripples! Thing is easy: A sine with a period that fits exactly an integer time into the DFT window (the FFT is just how you compute the DFT) will become a single ...
Marcus Müller's user avatar
3 votes

FFT of a AM modulated signal

Time-domain multiplication of signals that are each a sum of only a few sinusoidal components is simple to understand as frequency-domain convolution: showing first your signal 1: $$\sin(2\pi f_m t) \...
Olli Niemitalo's user avatar
3 votes
Accepted

FFT of a AM modulated signal

There two definitions for the AM modulated signals The first one is called as the classical-AM (or conventional AM) and is given by $$x_{AM}(t) = (A_c + m(t)) \cdot \cos(2\pi f_c t) \tag{1}$$ and the ...
Fat32's user avatar
  • 28.2k
3 votes

Is there any relation between QAM and streaming?

QAM is a digital modulation scheme. As such it is one way of implementing a physical layer that allows to convey digital information over a given medium. QAM is frequently used in all kinds of systems,...
Florian's user avatar
  • 2,463
3 votes

Why complex signal doesn't remove "beatings" while sampling?

In this case FFT shows only one (positive) frequency component, but beatings of the sampled signal still exist. My question is why? Why would it? Real vs. complex has nothing to do with the ...
Hilmar's user avatar
  • 44.7k
2 votes

Diode demodulation for AM

Because of the diode at the input, you get a DC offset at the output. Notice that a single diode is already a primitive rectifier, because it blocks the negative half wave. But you usually will want ...
Max's user avatar
  • 2,343
2 votes
Accepted

What's the maximum allowable amplitude modulation frequency for a given carrier frequency?

In the frequency domain, an AM modulated sinusoid looks like a carrier plus two side-bands in complex conjugate symmetry above and below the carrier. If you permit the lower sideband to cross into ...
hotpaw2's user avatar
  • 35.3k
2 votes
Accepted

Fractional Bandwidth of a Gaussian Amplitude Modulated Signal

I'll give this a shot. The Fourier Transform of a Gaussian is also a Gaussian. The standard deviations in each domain are related as $\sigma_t \cdot \sigma_F = \frac{1}{2\pi}$ The time standard ...
Hilmar's user avatar
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2 votes
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Do we assume modulating and carrier signal uncorrelated in AM modulation?

The total power of an amplitude modulated signal is $$\begin{align}\overline{s^2_{AM}(t)}&=\overline{\big(A+m(t)\big)^2\cos^2(2\pi f_ct)}\\&=\frac12\overline{\big(A+m(t)\big)^2}+\frac12\...
Matt L.'s user avatar
  • 90k
2 votes

How do I determine the carrier frequency of a given AM signal?

AM Broadcast is large carrier AM, in that the carrier is always present and the amplitude is modulated consistent with the waveform of interest. You can simply hard limit the signal to remove all ...
Dan Boschen's user avatar
  • 51.4k
2 votes
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Why does the Superheterodyne AM Receiver use an envelope detector?

The AM receiver could be done with a second Local Oscillator to baseband which would work fine if there was no frequency offset between the transmitter and receiver; otherwise the baseband signal ...
Dan Boschen's user avatar
  • 51.4k
2 votes
Accepted

Eb/N0 on a VLC simulation using OOK modulation

$E_b$ refers to the energy spent per information bit. That's the whole point! You're technically right that you've got a visible light carrier, but you're using this as a baseband system, to be honest ...
Marcus Müller's user avatar
2 votes

Why Hilbert Transform is terrible choice for amplitude demodulation of broadband signals?

Assume that we have a signal $$x(t)=\cos(\omega_1 t) + \cos(\omega_2) + 3$$ with a carrier $$c(t) = \cos(\omega_c t)$$ that is not known apriori. Than, for the amplitude modulated signal$$y(t)=x(t)c(t)...
Gideon Genadi Kogan's user avatar
2 votes

Why don't I hear the carrier wave or higher-pitched audio when tuning slightly away from the center of an AM station?

well, it depends on what type of demodulator your SDR-based receiver uses. But possibly, it's an envelope detector, i.e. what matters is the instantaneous received power, and that doesn't change with ...
Marcus Müller's user avatar
2 votes
Accepted

Why don't I hear the carrier wave or higher-pitched audio when tuning slightly away from the center of an AM station?

The super short, super cynical* answer is that if someone invents an AM demodulator that lets the carrier wave through as you are positing, they say "goodness, this is a crappy demodulator" ...
TimWescott's user avatar
  • 12.7k
2 votes

Omission of 8-PSK and 32-QAM in LTE/5GNR

Because link adaptation consists of not only modulation order but also channel coding code rate. You can check 38.214 to see that NR has already covered a wide range of target spectral efficiency ...
AlexTP's user avatar
  • 6,595
2 votes
Accepted

Why don't we need frequency and phase correction in AM and FM?

With AM all information is encoded in the amplitude regardless of any phase modulation occurring. Therefore we can demodulate it with an amplitude discriminator (envelope detector) as long as the ...
Dan Boschen's user avatar
  • 51.4k
1 vote
Accepted

How to compute a complex modulated signal from an audio file?

If the signal is a real audio waveform that only has AM components, the baseband equivalent waveform will be real unless the carrier has a phase offset. If the intention is to model the effect of ...
Dan Boschen's user avatar
  • 51.4k
1 vote
Accepted

How to choose frequency of local oscillator for heterodyne principal

Your bandwidth is between 550 kHz - 1720 kHz, but that is centered around a particular frequency. Now it depends if you want to move the signal to baseband (centered at zero frequency) or some other ...
Envidia's user avatar
  • 2,471
1 vote

How to analyze multiple amplitude modulated signals?

Well, you can of course instead of using the Discrete Fourier Transform as filter bank just employ bandpass filters to extract your individual tones. (The DFT is really just a filter bank, if you ...
Marcus Müller's user avatar

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