# Tag Info

27

It's a feature not a bug. When turned on, the weather alert function checks the local NOAA station for its alert tone every 5 seconds for half a second... doing so takes over the receiver. I'd turned it on, and forgotten I'd done so; and how that feature is implemented never registered in my mind. Turning it off solves my "problem." Thanks to ...

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 ...

9

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

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 ...

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

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 ...

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

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

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

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 ...

6

So, coming at this from an engineer's perspective: If a device has a datasheet that doesn't tell you how to use it, it's not the device you need – end of story. So, by the way, ebay searches aren't a clever way to design electronics! There's several large, reliable component distributors that have searches, too. Try https://digikey.com. Sure, there might ...

6

...I don't know what I am supposed to connect to the input... Am I expected to frequency shift the signal from the radio station down to whatever the chip may support... Several of the demodulator chips you linked would occupy the dashed box in the architecture of a superheterodyne FM stereo receiver: (diagram courtesy ARRL Handbook for Radio ...

6

That AN7411 is not an FM demodulator. It's a stereo demodulator, taking audio (0 to 57 kHz) from an FM demodulator, recovering the stereo multiplex signal (DSBSC at 38 kHz carrier), adding that (L - R) to the baseband mono (L+R) signal to give L and R outputs. The 19kHz "clock" is a pilot tone, frequency doubled to generate the 38 kHz carrier. The ...

5

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 ...

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

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 ...

5

That looks like you're strongly overdriving your receiver. Drastically reduce gain. This signal might not come from 137.5 MHz at all but be mixed there by the nonlinear effect of overdriving your amplifiers. The fact you can receive FM (a different signal at a different frequency) doesn't mean the gain is appropriate for this signal. Also, FM, being FM, is ...

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