# Tag Info

15

QAM modulates both the phase (= frequency) and amplitude of the signal.

13

It turns out that we DO simultaneously use frequency and amplitude modulation: our everyday term for this is "single sideband." As described on Wikipedia, single-sideband is a special case of quadrature modulation in which the quadrature component (Q) is derived directly from the in-phase component (I) of the modulating signal. This effect becomes evident ...

12

I believe you have encountered a Part 15 radio station. United States Code of Federal Regulations, Title 47, Part 15 (47 CFR 15) covers such things as garage door openers and the like. As such, and due to the fact that they are unlicensed by definition, there is no station-identification requirement. Note that this section covers both intentional and ...

12

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-double it to obtain the subcarrier signal marking the position of the difference signal. The receiver uses this subcarrier to shift it in frequency down to the ...

8

First of all, you're not 100% right that SSB will reduce transmit power. I often keep talking on SE about power spectral density. So let me start with that again and first assume that we have a radio that has fixed output power of 100 W. So let's first take a (simplified) look at AM: We have a carrier and two sidebands. One sideband, the part in red on ...

8

There are actually three different distinctions one could mean by referring to “narrowband” FM. Wideband FM in this context generally refers to the type of FM used for broadcast stations — those picked up by consumer FM receivers — as opposed to that used by two-way communications, including amateur transceivers. You are right that there is no fundamental ...

8

Assuming that the two transmitters operate at the same carrier frequency, and that the receivers receive similar power from each transmitter, then the FM signals will suffer destructive interference and the demodulated signal will be heavily distorted. PL tone, which is part of the transmitted signal, will also be lost. If one of the RF signals gets to the ...

8

You'll need an experimental radio license (Versuchsfunk) from Bundesnetzagentur link for research operation. For demonstration/teaching purposes, a demonstration radio license (Demonstrationsfunk für Bildungseinrichtungen) would be in order. I'm not a lawyer nor have I ever applied for either of these; I'd recommend contacting BNetzA on these aspects. ...

8

The story is about Eric LeMarque who got lost after snowboarding out of bounds in the Sierra Nevada mountains. Radio receivers usually contain some kind of oscillator. Detecting the unintentional radiation from the oscillator or the mixing products in the receiver is technically possible, but in modern receivers the power is so weak this power is so weak it ...

7

Whatever modulation we use, there's a baseband signal we wish to transmit (music, voice recording, whatever), which somehow modulates a carrier to produce the output signal. Your question suggests you are primarily concerned about how the frequency domain representation of the baseband translates to the frequency domain of the output. This is a valid thing ...

7

As for other legal alternatives, one might be transmitting inside a suitably shielded Faraday cage. I’ve seen such facilities at labs where various prototype and pre-production (and competitors?) electronics systems were being tested to measure how far out-of legal compliance they were.

7

"Tuning" an FM receiver sets the center frequency of the receiver, but its internal circuitry is designed to pass enough of the Bessel sidebands produced by frequency modulation of that waveform to be demodulated with low noise and distortion.

6

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 the waterfall of “the band … or a significant portion of that range”. The FM broadcast band is 20 MHz wide, and to display all of it straightforwardly requires ...

6

Multipath distortion can be problematic on fast digital modulations since the receiver sees multiple copies of the transmitted signal, each delayed by some appreciable fraction of a symbol. For example, digital TV uses a symbol rate of something like 4 million symbols per second. That means each symbol is about 250 nanoseconds long, which corresponds to a ...

6

This is a link budget question. Transmit power is just one factor: others are terrain, antennas, frequency, quality, noise, and so on. If we restrict the variables to typical values for FM commercial broadcast stations, we can use the FCC's rules to make some estimations. By that reckoning, you need a transmitter of around 50kW with an antenna 150 meters ...

6

Firstly, FM is a constant-envelope modulation, meaning the amplitude is constant. SSB, being a kind of amplitude modulation, obviously does not have a constant amplitude. Consequently, FM can employ a non-linear amplifier which is more power efficient, and thus has a longer battery life in mobile operation. Secondly, SSB signals add with each other and also ...

6

FM modulated radio is much more resistant to noise and can deliver better sound quality than AM radio. Under certain conditions, that is! Namely, additive noise, and a fading channel. or is this noise resistance also an inherent property of FM? Huh, that surprisingly can be a pretty philosophical question about what bandwidth actually is, but let's ...

6

Bandwidth: Broadcast FM stations are spaced 200 KHz apart, with the intention of allowing this kind of signal structure without interference: The FM broadcast bandwidth of just more than 50 KHz is quite sufficient for near-CD quality stereo. Normal broadcast FM uses multiplexing so the main channel includes L+R - both channels - what we call Mono. Then ...

6

SSB phone requires good frequency stability to be intelligible. Anything over 10Hz of frequency difference between transmitter and receiver and the signal starts to sound funny; beyond 50-70 Hz of error, intelligibility starts to drop. Hitting that target is more difficult at higher frequencies than at low frequencies. In the HF bands, getting within 10Hz ...

5

The simplest squelch, applicable to almost all modes, and the one provided by most amateur radios' “squelch knob” as far as I've experienced, is power squelch: measure the signal power (in SDR, $I^2 + Q^2$), average over some short time interval, and compare against a threshold. This is done before the demodulator as the demodulator removes amplitude/power ...

5

So, the bandwidth argument is the dominant one. You need to incorporate the full Carson Bandwidth in your channel spacing, not only the frequency deviation! As explained in my answer to your previous question, that must be significantly larger than the bandwidth of the same audio signal in AM. Aside from that, as always in Ham usage: historical reasons. If ...

5

General principle: In GNU Radio you cannot ever have a flow graph with a loop in it. If you wish to have feedback of some kind, it must be implemented in a single block. (There are many existing blocks that do this, such as AGC blocks and IIR filter blocks, and classes to help create them, though they still require writing C++ code.) However, you do not ...

5

I'm not familiar with the behavior of the particular programs you've named, but the obvious (guessed by ear and confirmed with a spectrogram/waterfall tool) difference between these two clips is the audio bandwidth is different. The clip from SDR Console has audio occupying about 5 kHz of bandwidth, in a file with a 12 kHz sample rate. The clip from SDR# ...

5

There are two ways to look at this: center frequency and bandwidth. In FM, the frequency varies as the amplitude of the audio varies. If the audio was a sinusoidal tone, the frequency would vary symmetrically around a center frequency, which is constant. More complex audio is generally still symmetrical. The FM receiver actually locks on to that center ...

4

As Michael noted, a repeater's CTCSS system is looking for a very specific frequency with a very low deviation (typically 10% or less) as low as about 40 dB down. What you are hearing is of course the whole audio range. The decoder in a CTCSS system is based on a very narrow bandpass filter which passes the desired CTCSS tone. There is a balance between a ...

4

FM works by varying the frequency of the signal around the nominal carrier frequency. Because the frequency is varying, the signal is not a pure sine wave. Therefore, it necessarily has some energy in sidebands as well as the instantaneous frequency. The higher the audio frequency, the more the signal deviates from a sine wave, so the more energy ends up in ...

4

The waves (to be correct: the signals) are exactly as suspectable to noise for both modulations! What's different is the receiver: As the name suggests, Amplitude Modulation (AM) works by taking the input (audio) signal, and using its strength, to modify the amplitude of your carrier wave. The AM receiver hence only has the amplitude of the received signal ...

4

I generally understand that AM side bands occupy sufficiently wide band width to recreate the spectrum of the modulating audio signal (while varying amplitude at each frequency in the spectrum of the sound wave)--so the spectrum of a side band should look basically like the spectrum of the underlying modulating audio wave. Is this understanding wrong? This ...

4

The way that stereo sound is broadcast using a single 'channel' is to broadcast the L+R (the sum of the left channel and the right channel) signal on the centre frequency, and include the L-R (difference between the left and right channels) signal modulated on a frequency at a known difference from the main carrier. What you are seeing is the main (L+R) ...

4

This grouping is what frequency modulation looks like whenever the input to the modulator is a constant single tone of a fairly high frequency. (If the tone were of a much lower frequency, then you would instead see a single peak moving in a sinusoidal fashion as a straightforward understanding of frequency modulation would suggest. This can be observed on ...

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