13
votes
Accepted
What does correcting IQ do?
Here are some graphics I have to add to the other good answers, as well as typical correction schemes for DC Offset and IQ Imbalance.
DC OFFSET
Below is an example DC Offset on a 16 QAM signal (with ...
- 48.8k
11
votes
Accepted
Are Software Defined Radios only Oscilloscopes?
All the three devices you mention have mixers, so that's what you wouldn't find in an oscilloscope usually. The amplifiers are optimized for weaker signals, and typically, the quality of ADCs in ...
- 29.6k
10
votes
Accepted
Large spike at the center frequency when using Ettus X310
The problem that I have is that I always have a big spike (10-15 dB)
directly on the center frequency (no matter what frequency I set). I
am relatively new to all this so I would appreciate any ...
- 29.6k
7
votes
What does correcting IQ do?
Real RF devices are not ideal as in a simulation or theoretic calculation. In particular, RF devices produce (among others) the following effects:
DC Offset
IQ Imbalance
The correct IQ option is ...
- 6,208
7
votes
Accepted
pyrtlsdr: Why is my output different than universal radio hacker?
Your signal recording clearly shows that you have long streaks of 1.0 – that probably means you're clipping.
Your signal is thus broken. Make a new recording with less gain.
- 29.6k
7
votes
Accepted
Why do we need to increase sampling frequency at the transmitter?
Three reasons to increasing the sampling rate further are
1) To relax the requirements of the post D/A conversion filtering for image rejection.
2) Increase signal SNR by spreading quantization ...
- 48.8k
6
votes
Accepted
Decimation and filtering in the frequency domain
I do a lot of decimation in the frequency domain. Little details are important.
I assume you already know the basic rules for fast convolution: the FFT length N is equal to the data blocksize L plus ...
- 76
6
votes
Why do SDR receivers emit I/Q samples?
The SDR (or any general digital signal processing system) takes the received RF signal, and downconverts it from the carrier frequency to the baseband.
Now, the real bandpass signal from the antenna ...
- 6,208
6
votes
Accepted
DAC and ADC architecture in SDRs
To add here are two diagrams showing common transceiver architectures: (1) a super-heterodyne where the down-conversion is done first to an IF frequency and then to baseband and (2) a zero-IF receiver ...
- 48.8k
5
votes
Accepted
Recommendation for good ressources on digital processing and D-QPSK
Wow, I'm honored by Matt L. doing what I'm often doing: Referring people to GNU Radio.
The project actually has a list of recommended literature, but I don't know how well that'd fit you. It's ...
- 29.6k
5
votes
Accepted
Sampling frequency lower than half of sampled signal frequency?
The channel frequency controls the local oscillator on your SDR which is the frequency about which it covers. So you are receiving signals from 16KHz below 107.5MHz to 16KHz above.
The concept you ...
5
votes
What does correcting IQ do?
One may "Correct IQ" to correct gain and phase imbalances in the I signal and the Q signal. These imbalances can lead to artifacts in the output (resulting in signal distortion). The IQ signaling is ...
- 1,422
5
votes
Why is phase range between $-\pi$ and $+\pi$ (instead of $0$ and $2\pi$)?
It is just a convention, but it is useful in some cases. For example, the phase of the DFT of a real discrete-time signal is odd only if the angles are expressed in the range $[-\pi, \pi)$. Sometimes ...
- 14.9k
5
votes
Why is phase range between $-\pi$ and $+\pi$ (instead of $0$ and $2\pi$)?
First, when you're talking angles, in DSP pretty much all angles are $\mod 2\pi$. So $2\pi \equiv 0$. Usually it's more convenient to keep angles on the interval $\left [-\pi, \pi \right )$, because ...
- 11.8k
4
votes
Accepted
Possible to Derive Receive Signal Power from RSSI?
I also read this from a response to a USRP user's question about RSSI measurements:
[The] Received Signal Strength [Indicator is] always relative to some signal model, incorporating ...
- 29.6k
4
votes
Accepted
Compare the quality of IQ data from different Software Defined Radios (SDRs)
First and foremost, I would recommend against over the air testing for this given the significant challenge in really being able to provide the same signal to each radio (since you have both temporal ...
- 48.8k
4
votes
Recommended device for testing my own SDR?
For both receive and transmit SDR experimentation/testing at RF/HF/VHF/etc. frequencies, I use a LimeSDR Mini (far less expensive than an Ettus). But I stream from/to it using C programs and files, ...
- 35.2k
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 $\...
- 88.9k
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 ...
- 48.8k
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 ...
- 29.6k
4
votes
Accepted
Digital demodulator (SDR) for receiving legacy analog modulated wave
Sure, an SDR can transmit and receive conventional, analog AM. I've done it many times as an introductory exercise in my courses.
A quadrature signal $s_{\text{QAM}}$ is made up of two signals, $s_I(t)...
- 14.9k
3
votes
IQ sampling vs 2X f0 sampling for SDR
I can envision advantages (or convenience) in doing this for lower IF signals certainly, as long as the sampling rate is significantly higher than the signal bandwidth to contain the phase dispersion ...
- 48.8k
3
votes
Accepted
GQRX Frequency tuning
It is common for low-cost direct-conversion (zero-IF) RF receivers to have a DC offset in their A/D hardware, which would correspond to a large spike at the frequency that you're tuned to. That's ...
- 24.5k
3
votes
Accepted
am I recording LTE uplink signals?
There's a lot of different domains of knowledge coming together here, so I'll split my answer into multiple sections, each answering an implicit question that you raise in your explicit question. Hope ...
- 29.6k
3
votes
Accepted
Getting the bitstream of an unknown RF signal
First I found the sine wave extrema (black markers at the bottom) and stretched and superimposed a sine wave graph (red) on top of the signal graph with the extrema in the same locations. The signal ...
- 13.5k
3
votes
Getting the bitstream of an unknown RF signal
The IQ data looks like a sinusoidal modulated pulse train with binary phase shift keying (BPSK) superimposed. The BPSK symbol rate appears to be at the same rate as the pulse repetition frequency (PRF)...
- 156
3
votes
Accepted
Use of I/Q representation for unmodulated Rx Signal
If you are "observing" the source, this implies there is some sort of information you are looking to get out of it, whether it be the total background noise, interference levels etc.
Do you ...
- 48.8k
3
votes
Accepted
Cosine of amplitude 1 but GNU Radio shows FFT has amplitude > 30,000
Consider the formula for the DFT (which the FFT efficiently computes as an algorithm):
$$X(k) = \sum_{n=0}^{N-1}x(n)e^{-j2\pi nk/N}$$
Notice that it is a summation over $N$ samples total. Also note ...
- 48.8k
3
votes
What can be done with I/Q data which would otherwise be impossible?
The main motivation for using quadrature (I/Q) signals is spectral efficiency.
Most signals are naturally baseband: they have no carrier, their energy is concentrated around DC, and they are limited ...
- 14.9k
3
votes
Are Software Defined Radios only Oscilloscopes?
That depends on how much your definition of "just an oscilloscope" limits functionality. There's probably enough hardware in a high-end O-scope to make at least a receive-side SDR.
The big ...
- 11.8k
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