14
votes
How does IQ modulation work (intuitively)?
I think it's more intuitive if you unlearn some things first.
Oscillation is not:
$$ \cos(\omega t) $$
where $\omega$ is the angular frequency in radians per second, and $t$ is time.
Rather, ...
- 51.4k
14
votes
Why is 20m band waterfall showing signals every 50 kHz?
Those equally spaced lines in the waterfall are very likely from EMI/RFI noise generated by modern electronics, often harmonics and/or intermodulation coming from DC-to-DC power converters. Could be ...
- 13.1k
13
votes
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Confusion about SDR
Is the raw data coming from the USB dongle literally samples of the ~1090 MHz wave? Or is the carrier frequency first demodulated (in that case, what actually am I receiving?)
It's downconverted, not ...
- 24.2k
13
votes
Accepted
GNU Radio sinusoid multiplication does not produce perfect results
I expect to see the sum & difference frequencies.
You're multiplying two complex sinusoids, not a $\sin$ and a $\cos$, but
$$e^{j2\pi f_1t}\cdot e^{j\left(2\pi f_2t-\frac\pi2\right)}= e^{j2\pi(...
- 15.2k
13
votes
Accepted
What limits a PIN diode or varicap from being used in a "Software Defined [Radio] Filter"?
What limits the use of "solid state variable capacitors" like these for dynamically filtering (low power/received) RF signals?
The main issue is bandwidth.
Although there are some varicaps ...
- 246
12
votes
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Understanding WFM modulation
Your “B” is the stereo difference signal of broadcast FM stereo.
It is placed at twice the pilot frequency so that it can be recovered by having the receiver lock onto the pilot signal and frequency-...
- 24.2k
12
votes
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Can SDR receivers be detected remotely?
You might be able to detect a regenerative receiver. This is because this receiver topology uses positive feedback to increase the Q of the tuned circuit. If the feedback is too much it becomes an ...
- 51.4k
10
votes
What kind of modulation is this?
The Hilbert block in GNU Radio creates an analytic signal from a real-valued one by applying the Hilbert transform to create the Q (imaginary, quadrature) component.
The Complex to Mag block then ...
- 24.2k
10
votes
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Multiple simultaneous tunings with SDR
Software-defined radio is radio where the signals are sampled and converted to digital data early in the process. This means that like any digital data, they can be copied without loss of quality. ...
- 24.2k
9
votes
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Signal Source Block of GNU Radio
You're on to something very right!
In signal processing, we define a basic waveform by its frequency, number of samples within the period and its amplitude.
I'd go a step further: In digital ...
- 15.2k
8
votes
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How to choose the right sampling rate and bandwidth for RTL-SDR dongle?
What the FFT sink shows as frequency axis actually has no basis in "real world signal" – it just takes the sample rate you set (here, you set 1 MHz), and scales the full nyquist bandwidth to that. If ...
- 15.2k
8
votes
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Oscillators inside SDR hardware
Are the oscillators in SDR hardware (that are used to mix the input to sinusoidal waves) purely digital?
The answer is the same as to your other questions:
The term "SDR" doesn't describe a single ...
- 15.2k
8
votes
How can quadrature mixing adjust the phase of all frequencies by the same amount? Can it be done in the digital domain?
Let's first start by developing some intuition of what a complex signal looks like. We can use GNU Radio to generate a signal that's just an unmodulated carrier, and then put that into a UI that will ...
- 51.4k
7
votes
7
votes
Cause of harmonic spurs?
You've nailed this!
So, I'm sometimes involved in these kinds of investigations. It all gets easier when you have control over the transmitter – when some spurs disappear as soon as you turn it off, ...
- 15.2k
7
votes
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RX antenna and RTL SDR with upconverter ..How to hear something?
Fun setup! I have many of the same components and need to try them out myself, but here's a general outline. (Plenty of this may be review for you, but I'm including it for others who may not be as ...
- 7,229
7
votes
Accepted
I want to know the bandwidth of this signal; problems using "QT GUI Frequency sink" to show it
GNU Radio, like any DSP system, works primarily in terms of sample counts, not time. Therefore, you have to add on time information — a sample rate — to get correct frequency-domain information. GNU ...
- 24.2k
7
votes
Fast TCP/IP radio modem on UHF band
As you might suspect, the data rate impacts the required RF bandwidth. Besides the data rate, the modulation method, encoding, packet overhead, duplexing, and other factors ultimately determine the ...
- 18.5k
7
votes
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Where does this common spurious transmission come from? Is there a quality difference?
This is very likely to be be due to overload of the receiver. A quick, rough way to tell the difference:
Tune your SDR receiver so the waterfall center frequency is not the same as the transmitter's ...
- 24.2k
7
votes
What is IQ in the context of SDRs?
"IQ" refers to the practice of having two mixers with their oscillator phase 90 degrees apart, and then the digitization and processing of those two streams of data.
One practical problem is in a ...
- 51.4k
7
votes
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Can these in-band spurs from rpitx be eliminated somehow?
I can't dig up my source for this, but I'm pretty sure these spurs are just the result of rpitx using a fractional-N PLL (that isn't in any way optimized for phase noise) to generate a square wave. ...
- 11.5k
6
votes
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HackRF One — Expected 'hackrf_transfer -t' file format and its creation?
Outside of the -w receive options, hackrf_transfer always deals in 8-bit signed IQ samples, raw with no file headers.
You must ...
- 24.2k
6
votes
Any SDR radios that receive FM BCB
Almost every SDR receiver is capable of receiving the FM broadcast band. The ones which can't are typically SDR transceivers designed for specific HF bands.
The harder part of your requirements is ...
- 24.2k
6
votes
Accepted
Is there a name for the demodulation technique described here using the parallax propeller?
Very interesting (and clever)!
Yes, the first counter is being used as a delta-sigma modulator to turn the incoming analog signal into a high-speed string of bits. Then the counters generating the I ...
- 1,210
6
votes
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33cm 5W(effective output power) tx amplifier for ATV (HamTV)
DVB-T signals are OFDM (orthogonal frequency division multiplexing), which means that you take your symbols, put them in a vector, put that vector through a discrete fourier transform, and get the ...
- 15.2k
6
votes
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DIY 433MHz antennas from copper wire
Based on the information you have given in the post and comments, the simplest way to construct a "omni" for 433 MHz, is by using an SO239-chassis-connector, and solder/screw the antenna elements ...
- 2,127
6
votes
Accepted
How to transmit images and video through a hurricane?
In general, the HF bands (1.8 MHz through 30 MHz) do not have sufficient bandwidth to support live transmission of a video signal. In the US, the FCC does not authorize an emission mode for live video ...
- 18.5k
6
votes
How to find transmitting (voice and CW) Amateur stations in UK?
The UK band plan for the 2m (VHF) band can be found here on the RSGB website. In the UK, the 2m band is between 144.000MHz and 146.000MHZ.
Of note is the section 145.5935 - 145.7935MHz, which are the ...
- 4,519
6
votes
Playback entire FM Band
Yes. To actually do it, you need:
SDR receiver hardware with (slightly more than) 20 MHz bandwidth to capture the entire FM broadcast band.
A computer, fairly powerful (definitely not a Raspberry Pi ...
- 24.2k
6
votes
Accepted
What is "Billion Code" for garage door openers from around 1996?
The billon code refers to a generation of transmitters that could transmit up to ~3.5 billion different codes. Although there were a large number of codes possible, the transmitter always transmitted ...
- 18.5k
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