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 ...
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13 votes
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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 ...
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13 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 ...
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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-...
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12 votes
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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(...
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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 ...
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12 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, ...
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10 votes
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Why is dynamic range relevant for an SDR?

Signal strength and dynamic range Generally speaking, dynamic range is the ratio between the strongest and weakest signal that can be received. In a digital signal, the dynamic range is determined by ...
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9 votes
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How to receive a signal from multiple sources simultaneously (helical antenna)

I'm working on a system (theoretical at the moment) whereby I wish to receive data from multiple sources all transmitting at the same time. I've read that a helical antenna is capable of this. I'm ...
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9 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. ...
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8 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 ...
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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 ...
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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 ...
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8 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 ...
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7 votes

SDR CB Transmission

An RTL Dongle is Receive-Only. You can't transmit with it.
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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, ...
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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 ...
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7 votes
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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 ...
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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 ...
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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 ...
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7 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 ...
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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. ...
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6 votes

Why doesn't GNU Radio Companion's wx Constellation Plot doesn't give me the clean QAM constellation I get from its Qt sibling?

There are a lot of things wrong here. Neither plot looks correct. There is no way that Qt plot is realistic for anything but a signal generator. Where's the noise? Where are the three missing ...
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6 votes

Getting Started with Digital Modes on VHF on Linux

2m isn't a good band if you want to do anything digital. Digital modes just aren't very popular on 2m. I think the only digital activity you are likely to find is: D-STAR APRS D-STAR uses GMSK, but ...
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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 ...
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6 votes
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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 ...
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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 ...
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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 ...
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6 votes
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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 ...
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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 ...
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