Questions tagged [modulation]

Generic term for coding information onto another carrier signal

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37
votes
9answers
58k views

What is the basic difference between AM and FM radio?

I want to know the basic/fundamental difference between AM and FM radio. Why nowadays FM radio has replaced AM and has become more popular?
21
votes
10answers
7k views

How exactly does AM/FM carry both pitch and loudness of voice?

Almost every tutorial on AM/FM modulation shows the modulating signal as something like a simple tone or continuous sine wave. Now that's easy, and for AM you just superimpose the modulating signal ...
21
votes
8answers
13k views

fastest way of doing on/off-modulation of a LED?

I need to modulate a LED quickly (multi-megahertz range) on/off. It's a high-power LED. I have had some trouble locating any well-known methods to do this. Simply switching the voltage with a FET ...
16
votes
4answers
6k views

If an FM signal has different frequencies, how does the radio receive the signal in a fixed frequency?

If an FM signal has different frequencies according to the modulation, how does the radio receive the signal in a fixed frequency like 103.2 MHz?
15
votes
3answers
37k views

What is Overmodulation?

The Wikipedia article is very short and doesn't explain the concept very well, and there aren't any other sites that I can find that give a simple explanation. What does it mean by 100% modulation? I ...
14
votes
1answer
888 views

Why FM stereo uses DSB-SC instead of VSB?

Using vestigial sideband instead of double sideband supressed carrier could make the baseband stereo signal more compact. Would there be any other benefits or downsides of employing VSB?
12
votes
5answers
4k views

Why is amplitude-modulated radio broadcast (AM) still active?

In my country (Germany) we have frequency-modulated (FM) and amplitude-modulated (AM) radio broadcasts. However, the reality is that no one ever listens to the AM since you just don't hear anything (I ...
12
votes
2answers
1k views

How does the plastic-tube inductor in this wireless transmitter work?

This single layer PCB (solder side shown here) is from an old fan remote: The inductor in the plastic tube is not connected physically to any components; however, the two loops of wire surrounding it ...
10
votes
1answer
2k views

What could this signal at 119.9 MHz be?

I had some fun with an RTL-SDR dongle, and I noticed this weird signal with no noticeable pattern at 119.9 MHz. What's even weirder is that the signal is symmetrical. What might this be? Or is ...
8
votes
5answers
4k views

What does it mean for a signal to have an amplitude below 0 dB?

I am a software developer (using high level languages like .NET,C,C++ etc) trying to understand how computers work at a lower level. I understand that amplitude is always positive because it is ...
7
votes
3answers
1k views

Why do spectrum analyzers use envelope detectors?

Spectrum analyzers can usually be represented by the following block diagram: Before entering the video filter, the signal passes through an envelope detector. What I don't understand is why this is ...
7
votes
3answers
6k views

Transfering data using ultrasound

Yamaha InfoSound and ShopKick application use technologies that allow to transfer data using ultrasound. That is playing an inaudible signal (>18kHz) that can be picked up by modern mobile phones (iOS,...
7
votes
3answers
785 views

Information content of amplitude modulation versus single-sideband transmission

An amplitude-modulated radio signal with carrier frequency C, which includes frequencies from 0 to F, will use output frequencies in the range C-F to C+F, or a total bandwidth of 2F. A modulation ...
6
votes
4answers
2k views

What does “shift keying” mean in the context of digital modulation?

English is not my native language and I do not understand why the words "shift keying" were used to describe some types of digital modulation. What does "keying" mean in the context of signals and ...
6
votes
3answers
3k views

modulation in phase and frequency

what is the difference between frequency modulation and phase modulation?
6
votes
3answers
5k views

Any ideas to make IR LEDs identifiable during position-tracking?

Background Johnny Lee demonstrated various interesting ideas (detailed, e.g., in this video as well as this page) that take advantage of the Infrared camera from a Wii-Remote. The IR camera has ...
6
votes
2answers
157 views

MOSFET channel length modulation - Glossing over physics

I am having trouble understanding an aspect of the channel length modulation/pinch off of MOSFETs. My current understanding (For NMOS) is that when a positive voltage is applied to the gate, the P-...
5
votes
5answers
452 views

modem free high base AM data transfer possible?

not a very experienced EE, but I was thinking about a system for high speed data transfer, and it is fairly simple conceptually, so I imagine that there must be serious flaws with the method, because ...
5
votes
2answers
1k views

Why are the resistors in an AM modulator necessary?

In preparation for the amateur radio exam, I have been following the preparation course material by DJ4UF. In the section about modulation it presents this diagram of a simplified AM modulator. The ...
5
votes
3answers
29k views

RC time constant and diode detector

For a better detection of the modulated signal with the diode detector below, one requirement is that the time constant of the RC filter conforms to: $$ \frac{1}{\omega_{c}} \leq RC \leq \frac{ \sqrt{...
5
votes
3answers
825 views

Is antenna length for a baseband signal infinity?

I am studying RF communications and learned that antenna length is proportional to a wavelength of a signal. And many literature say that a high frequency is preferred in wireless communication ...
5
votes
1answer
659 views

What limits the modulation of digital signals?

I am trying to understand why there are different speed on different frequency bands. I see that e.g. ZigBee can do 250kb/s for both 2.4GHz and 868/915MHz, but <...
5
votes
1answer
4k views

FM Transmitter Modulation

I am designing an FM transmitter as a learning experience and for fun, but I am having some difficulty with FM modulation. I am using a VCO with a Colpitts topology and common emitter configuration. ...
5
votes
1answer
2k views

Questions about heterodyne receivers

All radio receivers seem to use the heterodyne principle. I can see that it will be easier to work with a fixed frequency than demodulating from a wide range. Questions: what's the difference ...
5
votes
2answers
252 views

Can AM and FM signals at the same frequency be separated?

Suppose one radio station emits an amplitude modulated (AM) signal at 100 MHz, and a nearby station emits a frequency modulated (FM) signal at 100 MHz. Is it possible to separate these two signals, ...
4
votes
3answers
2k views

FM transmitter design?

I stumbled across a relatively simple FM radio transmitter online. Credit: makezine.com I wanted to understand how it works (it is called "super simple" on the website), but several hours I have ...
4
votes
1answer
438 views

Why it has used BPSK modulation after OFDM?

I am quite new to OFDM modulation. I am reading this from a tutorial. In one of its parts, for showing amplitude variation of the OFDM signals, there is a graph that has used 8 sub-carriers and BPSK ...
4
votes
2answers
1k views

Is DC modulated with a sine wave actually AC?

So we have a DC level that, for example, varies between 0VDC and 10VDC in sinusoidal fashion. Is that classified as alternating current even though it does not go negative?
4
votes
3answers
2k views

Modulation scheme for simple signal generator

As part of a vintage radio restoration project, I decided to build a simple signal generator for alignment. The idea was straightforward, a short distance AM transmitter with a variable carrier ...
4
votes
2answers
1k views

How to Shift Carrier Frequency in QAM Signal?

If I wanted to apply a 100MHz frequency shift to a QAM signal with a 400MHz carrier frequency, I would Demodulate it at the carrier frequency Apply a frequency shift to the original signal Modulate ...
4
votes
2answers
614 views

How does frequency modulation takes place?

It is said that in frequency modulation the normal wave represents a '0' (False) and a more frequent wave represents a '1' (True). Is there any particular rule that the normal wave's frequency should ...
4
votes
1answer
193 views

Modulation And Other Communication Terms

Could somebody explain the concept of modulating a signal? It's such a broad term I never really understood what it meant. Are phase shift, frequency modulation, and amplitude modulation all ...
4
votes
2answers
748 views

Adding LFO Modulation to VCA in synthesizer

I just started sketching ideas to add LFO to VCA modulation in a device that already has a VCA circuit employed, but I’ve run into a snag… I can design a circuit to give proper modulation at full open ...
4
votes
3answers
1k views

Question about FM and PM and Angle Modulation

I am having a bit of trouble understanding how FM works mathematically. I fully understand the conceptual basics regarding how information is transfered by modulating the frequency, but what I don't ...
3
votes
5answers
2k views

Transmission of a digital signal over a medium

A digital signal is said to have a infinite bandwidth. If a transmission medium cannot have infinite bandwidth then how can a digital signal be transmitted over such mediums? Please correct me if I ...
3
votes
2answers
27k views

RF Bandwidth vs Data Rate (modulation)

I was reading some articles about digital and analog modulating processes. One of them was this: Modulation techniques At a certain point is says: The bandwidth produced is a function of the ...
3
votes
2answers
14k views

What is the required bandwidth for QAM modulation?

Why is the allocated bandwith smaller with QAM-16 or 64 compared to e.g. QAM-4? I am not a professional so I'm looking for an intuitive explanation.
3
votes
3answers
231 views

Network protocol over audio

I'm looking for open source, simple (simple enough for arduino, for example) protocol for transfer of some messages over audio (eg. in the apartment environment, not by analog line, but between ...
3
votes
2answers
341 views

Orthogonal Frequency Division Multiplexing (OFDM)

I am not a professional but I'm trying to figure out how OFDM works. It is clear to me that OFDM works with closely spaced subcarriers with orthogonal frequency. Each subcarrier gets modulated with a ...
3
votes
2answers
2k views

Advantages of a Gray code in 16-QAM

I'm looking at a system using a gray code on 16-QAM. When a symbol changes, you move from one of the 16 symbols in the constellation to any of the 16 symbols. I'm wondering if there is in fact an ...
3
votes
4answers
1k views

How Does FM Get Around the Gabor Limit?

The Gabor Limit states that it's impossible to simultaneously localize a signal in both frequency and time. FM communication modulates the instantaneous frequency of the carrier in step with changes ...
3
votes
2answers
14k views

I want to build a Frequency Modulation circuit

Does anyone have a simple circuit design that can implement Frequency Modulation or even Amplitude Modulation?
3
votes
2answers
5k views

QAM modulation - how data in different frequencies are laid out?

I know that QAM modulation uses a change in phase and amplitude to send data. But We use a series of frequency spaces as bandwidth to send data. However, we know that different frequencies oscillate ...
3
votes
2answers
164 views

Is it possible to capture a DVB-T mux's signal on VHS?

What will happen if i tune a VHS VCR's analog tuner to a DVB-T mux's frequency, record the signal on a VHS tape, then use the RF out of the VCR to replay the captured signal into a DVB-T receiver box'...
3
votes
1answer
81 views

Help identifying modulation scheme

I'm trying to identify the following modulation scheme being used to transmit a infrared code. It looks like manchester, but there's some gaps where there's no transitions. The code being ...
3
votes
2answers
698 views

Determining type of modulation

The circuit below is a radio transmitter circuit of a typical RC car. The circuit makes use of IC TX2 encoder. The output of IC ("SO" pin) is a series of pulses at low speed (84 Hz). This signal will ...
3
votes
2answers
672 views

UHF Phase Modulation

I would like to design a system that produces -10 degrees and +10 degrees phase shifts at 409.6MHz (Basically BPSK). The way I'm thinking about approaching this is by using a DDS to generate the BPSK ...
3
votes
2answers
429 views

Why is Pulse Code Modulation called 'modulation' and not 'conversion'?

Since all that a pcm accomplishes is analog to digital conversion, why is it called a modulation technique? There is a message signal but no carrier. According to my knowledge, modulation requires a ...
3
votes
1answer
620 views

maximizing IR range and peak current

I ordered a bunch of infrared detector diodes (aka IR diode) and infrared transmitters. luckily the detectors are blue (and don't respond to normal room light). I am planning to order a bunch of ...
3
votes
2answers
656 views

Amplitude Modulation Concepts

I am currently learning about telecommunications but I have some concept issues. Firstly, does it matter if I am using cosine or sine in the equation? I understand that they are basically the same ...