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14 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(...
Marcus Müller's user avatar
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, ...
Phil Frost - W8II's user avatar
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 ...
hotpaw2's user avatar
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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 ...
Neil_UK's user avatar
  • 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-...
Kevin Reid AG6YO's user avatar
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12 votes
Accepted

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 ...
Phil Frost - W8II's user avatar
10 votes
Accepted

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. ...
Kevin Reid AG6YO's user avatar
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9 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 ...
Marcus Müller's user avatar
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 ...
Marcus Müller's user avatar
8 votes
Accepted

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 ...
Marcus Müller's user avatar
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 ...
Phil Frost - W8II's user avatar
7 votes

SDR CB Transmission

An RTL Dongle is Receive-Only. You can't transmit with it.
Marcus Müller's user avatar
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, ...
Marcus Müller's user avatar
7 votes
Accepted

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 ...
natevw - AF7TB's user avatar
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 ...
Kevin Reid AG6YO's user avatar
  • 24.6k
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 ...
Glenn W9IQ's user avatar
  • 18.6k
7 votes
Accepted

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 ...
Kevin Reid AG6YO's user avatar
  • 24.6k
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 ...
Phil Frost - W8II's user avatar
7 votes
Accepted

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. ...
hobbs - KC2G's user avatar
  • 12.5k
7 votes

How is the Hackrf able to achieve 20MHz of bandwidth with an ADC that only allows 22Msps?

The Nyquist limit is half the sampling rate because otherwise you can't distinguish a signal at a frequency $x$ from a signal at $f_s - x$ which starts 180° out of phase from the first one — they give ...
hobbs - KC2G's user avatar
  • 12.5k
6 votes
Accepted

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 ...
Marcus Müller's user avatar
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 ...
Dave Tweed N3AOA's user avatar
6 votes
Accepted

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 ...
Edwin van Mierlo's user avatar
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 ...
Glenn W9IQ's user avatar
  • 18.6k
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 ...
Scott Earle's user avatar
  • 4,599
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 ...
Kevin Reid AG6YO's user avatar
  • 24.6k
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 ...
Glenn W9IQ's user avatar
  • 18.6k
6 votes

IQ imbalance after quadrature mixing

A quadrature mixer contains two signal paths, I and Q. For a sinusoidal input into the mixer, the outputs will be in quadrature. It's this property that allows a quadrature mixer to distinguish ...
Phil Frost - W8II's user avatar
6 votes
Accepted

How does one know how a signal has been modulated

Yes, you'll have to look at it. I can't go into detail about every possible modulation, because there's just too many. But typically, a look at the spectral representation gives you an idea of ...
Marcus Müller's user avatar
6 votes

How is a crossed-ellipse display generated from FSK?

The display is simple: Pass the incoming signal through two band-pass filters, one at the mark frequency and one at the space frequency. Use the two filters' output signals as the X and Y coordinates ...
Kevin Reid AG6YO's user avatar
  • 24.6k

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