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13 votes
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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 ...
Dan Boschen's user avatar
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11 votes
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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 ...
Marcus Müller's user avatar
11 votes
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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 ...
Marcus Müller's user avatar
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

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 ...
Maximilian Matthé's user avatar
7 votes
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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.
Marcus Müller's user avatar
7 votes
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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 ...
Dan Boschen's user avatar
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6 votes
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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 ...
Phil Karn's user avatar
6 votes
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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 ...
Dan Boschen's user avatar
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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 ...
hops's user avatar
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5 votes
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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 ...
Marcus Müller's user avatar
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 ...
MBaz's user avatar
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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 ...
TimWescott's user avatar
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4 votes
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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 ...
Marcus Müller's user avatar
4 votes
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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 ...
Dan Boschen's user avatar
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4 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)...
rjan87's user avatar
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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, ...
hotpaw2's user avatar
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4 votes
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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
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4 votes
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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
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4 votes
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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
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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)...
MBaz's user avatar
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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 ...
Dan Boschen's user avatar
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3 votes
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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 ...
Marcus Müller's user avatar
3 votes
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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 ...
Olli Niemitalo's user avatar
3 votes
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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 ...
Dan Boschen's user avatar
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3 votes
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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 ...
Dan Boschen's user avatar
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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 ...
MBaz's user avatar
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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 ...
TimWescott's user avatar
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3 votes
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G3RUH Multiplicative scrambler in C++

I believe this line bit = (m_nLSR & 1)^((m_nLSR >> 12) & 1)^((m_nLSR >> 17) & 1); in your Descramble ...
Peter K.'s user avatar
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3 votes

How to achieve a DC portable power source with low noise in 400 MHz frequencies

I can't give you a recommendation for a specific power bank, but I can give you a couple of general observations and recommendations: First, if you're building a sensitive receiver, there is almost no ...
TimWescott's user avatar
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