7 votes

Why do SDR receivers emit I/Q samples?

The SDR (or any general digital signal processing system) receiver takes the received RF signal, and down-converts it from the carrier frequency to the baseband, converting a real bandpass signal to ...
Maximilian Matthé's user avatar
7 votes

FSK and IQ modulation

What is the advantage of performing the FSK using IQ modulation? You only need one RF oscillator operating at a single frequency, instead of having 2 (or more in the case of M-ary FSK) oscillators ...
GordonFreeman's user avatar
7 votes

How to "draw" the function/wave to send symbols using QAM?

The baseband QAM signal is complex, and the only way to draw it is by doing two drawings, one for the in-phase (real) component, and one for the quadrature (imaginary) component. The passband QAM ...
MBaz's user avatar
  • 15.3k
6 votes

Sending complex signal over a single wire

Note that signals sent over wires (and over the air, and over any medium) are always real. What quadrature means is that, on a passband channel (wired or otherwise), you can transmit two signals at ...
MBaz's user avatar
  • 15.3k
5 votes

In-Phase and Quadrature components of low-pass equivalent of a band-pass signal?

i will call the bandpass signal: $$\begin{align} y(t) &= \Re \Big\{ (x(t) + j\hat{x}(t)) \, e^{j \omega_0 t} \Big\} \\ \\ &= x(t) \cos(\omega_0 t) - \hat{x}(t) \sin(\omega_0 t) \\ \...
robert bristow-johnson's user avatar
5 votes
Accepted

Quadrature modulation for FM and AM

Both amplitude and frequency modulated radio signals nowadays use quadrature modulation and demodulation as a mean to transfer and receive radio signals. This question is ill-defined – a signal ...
Marcus Müller's user avatar
5 votes
Accepted

Fast phase calculation

Below summarizes efficient phase estimators for this application updated to include both a phase range of +/- 30 degrees and +/- 60 degrees. This is given in two parts, estimators for a real IF (...
Dan Boschen's user avatar
4 votes

FSK and IQ modulation

There are many ways of demodulating the FSK signal Indeed! What is the advantage of performing the FSK using IQ modulation? Depends. Generally, IQ is the only shape you have your signal in, so ...
Marcus Müller's user avatar
4 votes
Accepted

How to reconstruct RF signal using IQ data

The RF signal $r(t)$ is obtained from the complex baseband (IQ) signal $s(t)=x(t)+jy(t)$ in the following way: $$r(t)=\text{Re}\{s(t)e^{j\omega_0t}\}\tag{1}$$ where $\omega_0$ is the carrier ...
Matt L.'s user avatar
  • 90k
4 votes

Detecting I/Q imbalance

As long as the assumption of equiprobable data with independent I and Q reasonably holds (and over the long duration with whitened transmissions this is typically a good assumption), then IQ imbalance ...
Dan Boschen's user avatar
4 votes

Quadrature demodulation, homodyne detection, lock-in detection - what's the difference?

They are essentially the same but may have key differences varying across implementation. The process is identical in terms of multiplying a signal by a sine wave at the center frequency of the signal ...
Dan Boschen's user avatar
4 votes

How to test I/Q modulation with only one branch (I or Q)?

The FSW is superheterodyne. This means that it will not have a DC offset or I/Q imbalance, but the IF path will appear as an asymmetrical lowpass (because it is really a shifted bandpass), which is ...
Simon Richter's user avatar
4 votes
Accepted

Quadrature component - condition on equivalent lowpass signal

Your idea with the Hilbert transform doesn't work. The only signal (apart from $x(t)=0$) for which the Hilbert transform is zero, is a constant signal. A band pass signal $x(t)$ can be written in ...
Matt L.'s user avatar
  • 90k
4 votes
Accepted

In case of using a Doppler radar, do we have to use IQ demodulation to find the direction of a target?

Radar designer here: It sounds like you’re talking about pulse-Doppler (PD) radar systems. For PD radars, the process is essentially as you described: Generate a waveform (typically at IF) and then ...
vintagevogue's user avatar
4 votes
Accepted

I Q sampling and baseband version of analytic signal

The problem in your derivation is the way you discard the high frequency components. Note that $\hat{s}(t)$, the Hilbert transform of the bandpass signal $s(t)$, has frequency components around $\...
Matt L.'s user avatar
  • 90k
4 votes
Accepted

Obtain I/Q Components from a Real Signal On the Fly - Hilbert Transform or Digital Downconversion?

[Update: I mentioned a possible +3dB processing gain by including the Hilbert Transform prior to DDC for the case of a real IF signal in the first version of this post, which @MattL questioned so I ...
Dan Boschen's user avatar
4 votes
Accepted

I/Q sampling with just one ADC

Is this an entirely stupid idea? No, but you've just came to the conclusion that instead of sampling complex, with Nyquist rate being the bandwidth, you should do twice as many samples. That simply ...
Marcus Müller's user avatar
4 votes

What tricks can one play on a bandwidth-limited IQ modulator?

Your mixer's bandwidth is a physical property of both the nonlinear part that implements the mixing as well as the strictly necessary filters to eliminate the unwanted intermodulation products. You ...
Marcus Müller's user avatar
4 votes
Accepted

What tricks can one play on a bandwidth-limited IQ modulator?

The choice of an IF frequency of 100 MHz limits the achievable single-sided bandwidth for a real signal centered on it to be 100 MHz, so any pulse out of the modulator would be limited in rise/fall ...
Dan Boschen's user avatar
3 votes

What type of information can be contained in I/Q (quadrature) data?

IQ data samples can contain double the bandwidth of information within the same frequency bandwidth and/or sample rate of single channel (non-IQ or strictly-"real") data samples. When heterodyning a ...
hotpaw2's user avatar
  • 35.3k
3 votes

Play IQ data in the soundcard

I'm replying to Oleg's recent comments here in an "Another Answer" block because the "add a comment" capability prevents me from providing a complete reply to Oleg. Here's my reply: You can indeed ...
Richard Lyons's user avatar
3 votes

Play IQ data in the soundcard

If the spectrum of your I/Q samples is centered at zero then you'll have to perform either AM or FM demodulation before routing any real-valued audio samples to a sound card. For AM demodulation you'...
Richard Lyons's user avatar
3 votes

IQ imbalance amplitude

What it looks like you are experiencing is carrier offset and not IQ imbalance, at least from this view that you show. Looking at the scale of the two axis it looks balanced and it is just that each ...
Dan Boschen's user avatar
3 votes
Accepted

IQ signal FM descrimination using phase derivative

I simply use unwrapped atan2(IQ(i)) - atan2(IQ(i-1)) to estimate a discrete derivative, then low pass filter to below 15 kHz. Although with a shallow slope, the 1st order approximation to atan() ...
hotpaw2's user avatar
  • 35.3k
3 votes
Accepted

Why is In-phase Quadrature sampling not used to record and store digital audio?

It is because the audio signals are real and already at baseband. In contrast radio frequency signals are often represented as complex numbers once they are brought back to baseband. Real signals can ...
Dan Boschen's user avatar
3 votes
Accepted

How does analog television chroma signal work with QAM and vestigial sideband?

I am not an expert on the demodulation of NTSC, but noting here that the OP has made an interesting observation in that "I" as a real AM modulated waveform would have a baseband equivalent ...
Dan Boschen's user avatar
3 votes
Accepted

Quadrature Detector with a single ADC

The sampling offset is a time delay between the I and Q paths. The Fourier Transform of a time delay $T$ is $e^{-j\omega T}$ and thus we see that this will impart a linear phase that is proportional ...
Dan Boschen's user avatar
3 votes
Accepted

How are discontinuities in quadrature modulated signal QAM physically produced?

I am unclear how the "discontinuities" in the waveform at the junction of each data point can be physically produced? They can't, exactly. In the real world you may come close, but ...
TimWescott's user avatar
  • 12.7k
2 votes
Accepted

What is the advantage of measuring IQ data non-digitally?

IQ demodulation is normally done with the "I" component being sampled at a rate that is at or near (including below) the carrier frequency of the signal, or the center of the band of interest. Since ...
hotpaw2's user avatar
  • 35.3k
2 votes

Quadrature modulation for FM and AM

Both amplitude and frequency modulated radio signals nowadays use quadrature modulation and demodulation as a mean to transfer and receive radio signals. The use of a quadrature transmitter / ...
Andrew W.'s user avatar
  • 121

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