# Questions tagged [frequency-modulation]

For questions relating to frequency modulation (FM). This is a scheme where a carrier is modulated by varying its frequency to carry information.

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11k views

### Are audio cassettes FM or AM?

I know that cassettes store an analog signal, but is the signal Amplitude-Modulated (AM) or Frequency Modulated (FM)?
9k 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 ...
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### 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?
3k views

What is a cheap way to receive and record on PC several (tens) of fm-radios (public, e.g. news, music, etc)? Such radios uses frequencies like 90.4 MHz or 102.7 MHz, sends a stereo (sometimes with ...
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### How can this FM transmitter modulate?

simulate this circuit – Schematic created using CircuitLab This is a minimalist FM transmitter I've found from http://www.talkingelectronics.com/projects/Spy%20Circuits/SpyCircuits-1.html I'm ...
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### FM Carrier Wave Generation

I'm trying to understand how the following FM radio schematic works. Specifically, I want to know how the carrier wave is generated. I understand the concept of an LC tank and I think I see it ...
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### Identifying an old RF circuit

I got an old RF circuit (I guess), probably used for pedagogical purposes. Here's the schematic: I chose the positive and ground rails as it seems right to me, looking at transistors and resistor ...
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### Why do FM radio signals occupy greater bandwidth than AM?

If a 5kHz baseband signal is modulating an AM carrier wave, the sidebands will be centered around the carrier wave and the the signal will occupy 10kHz bandwidth. AM radio limits the upper modulating ...
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5k views

As per this building simple radio it is the simplest AM radio receiver. obviously this is very poor AM receiver.(but it was working for someone in comment) I'm wondering what will happen when this ...
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### Why do EM waves with longer wavelengths propagate longer distances?

AM radio signals use carrier waves which are quite longer than in FM case. We know that in free space both high and low freq. RF signals should take the same distance in same period of time. There ...
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332 views

### What antenna is this?

I just disassembled an external FM antenna designed for an old Asus laptop. I expected to find some circuitry and a monopole or some other simple antenna. However, what I found was the antenna shown ...
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### Capacitor between emitter and collector

Considering the circuit below.: What is the purpose of capacitor C5? I know it provides positive feedback to sustain the LC oscillations, but how does this work ?
1k views

### Frequency spacing and the doppler effect

Suppose that I'm working on a system which needs to use a transceiver with an FM signal. If that system moves very fast (i.e. it is a satellite or whatever) in which the maximum doppler effect can be ...
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### What decides the range of FM band?

I read that the frequencies over which FM signals can be transmitted is limited between 88 MHz and 108 MHz. Why is it confined to a particular range and why specifically to this (88 MHz to 108 MHz) ...
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### Video Home System (VHS) bandwidth

Standard play VHS video cassette recorders offered only about 3 MHz luminance bandwidth by recording the signal on an FM carrier. What is that carrier frequency? Is it possible to confirm the carrier ...
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### Variable Capacitor?

I am working on a simple FM radio transmitter and I was looking for a variable capacitor to help tune the circuit. I rummaged around and took apart an old Sony radio that had this component in it. ...
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3k views

I have built an FM radio receiver from a kit. I'm very happy with the results! It is the Sinclair Micro FM. My goal is to learn about discrete electronics (NO IC's.) The radio works great, but I ...
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### 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, ...
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9k views

### How does Frequency Modulation work with radio frequencies?

Inspired by this question: What is the basic difference between AM and FM radio? I got to wondering: If I put my FM radio on some radio channel (Let's just say 100 MHz to pick some number), shouldn'...
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### FM Quadrature detector - is it the best choice

My input is an FM carrier of 80.00MHz. It is FM modulated with 625kbpsec data. The deviation from carrier is about +/-700kHz. The data never spends more than about 38us in a low state or high state i....
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990 views

### FM Receiver's Oscillator resonating to a frequency, but its modulated?

In an FM, the frequency is modulated. So whatever reaches the receiver is not a constant frequency. This being the case, at Receiving an AM/FM Signal: "In an FM radio, the detector is different, but ...
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6k 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. ...
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2k 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 ...
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### Good ICs for receiving and decoding FM RDS

I'm interested in doing a project to read FM RDS data. Can anyone recommend good chips for both the FM receiver and RDS decoder? For an FM receiver, I'd like to be able to control FM frequency (...
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 ...
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### Does this simple RF transmitter circuit actually work?

I've come across this circuit a few times while searching for simple transmitter circuits, and decided to try it. According to this site, it should be transmitting at around 90Mhz. http://www....
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344 views

### FM Receiver, how it works?

Im working on a Visible light communication project. Its a simplex transmitter receiver setup using an LED to send data to a photodiode receiver. Im thinking of ways to encode the data for ...
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### Strange op-amp circuit FM radio

I am working on a simple FM radio receiver. Built the circuit and it works very well. Learned few new things, and got some questions, especially the output stage. The Op-amp 2822 is a dual op-amp, but ...
2k views

### What is this type of radio fm antenna connector called?

I'm looking for an antenna replacement for my dad's busted radio (iHome IH8) and I'm having some trouble finding the name of this type of plug. The only things I find when searching online are coaxial ...
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### How do I determine the proper FM transmitter strength?

I want to build an FM transmitter that will transmit (on an unused frequency) from my laptop (near my video projector) to my receiver (about 20' away on the other side of the room). Then, I thought ...
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32k views

### How do I calculate the frequency of an FM transmitter circuit?

Below is an FM transmitter circuit I found on the web that I would like to build from scratch: How I would calculate the carrier signal frequency so that I know where to tune my FM radio? If I want ...
15k views

### Why did my FM transmitter mysteriously die?

I made a little FM transmitter project a few months ago. I used an Arduino, a litle 4-digit serial LED display, a couple of buttons and an FM transmitter component with breakout board I had this put ...
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### Would you explain the output waveform of this FM transmitter circuit?

I build this circuit and it works. I wanted to look at the output waveform by my soundcard oscilloscope software. I would expect something like the following picture: But the output waveform was this:...
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### Controlling DC (Pager) Motor with Audio Signal

I'm trying to control a DC (pager) motor with a sound signal so that I can use an audio synthesizer to control the speed of the motor. I'm hoping to modulate the speed of the motor at audio rate (1-...
3k views

### Can someone explain how this Ratio detector circuit works?

I understand how a Foster Seeley discriminator works, but in this Ratio detector, which is a modified version of the former, I don't understand the purpose of having three transformer windings, or ...
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### Why can't we divide the electromagnetic spectrum into arbitrarily many radio frequency bands?

I understand there is concern over the limited amount of wireless spectrum available for allocation but am somewhat confused as to why. Couldn't we just divide the electromagnetic spectrum into as ...
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### How to build an FM transmitter and how does it work?

I have become interested in radio transmission and would like to know how to build an FM radio transmitter. When I search for information I either get a block diagram or a video of someone putting it ...
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### Simple FM transmitter [duplicate]

How does this circuit perform Frequency modulation. I know the LC generates the carrier frequency. How does the transistor act as a varicap to perform FM. Which part of the circuit acts as a varicap ...
1k views

### Generating 19kHz and 38kHz phase-locked sine waves

As a personal project, I'm designing a stereo FM transmitter from scratch. Part of this entails design of a stereo multiplexer. Using this description, I've worked out how I'm going to implement most ...
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### Understanding this simple FM receiver

I know the theory of FM, but I just can't understand how this simple circuit works. So let's say a radio station is transmitted on 100 MHz, as I understand in this case the oscillator should also ...
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### What is the energy efficiency between FM radio tower broadcasting over an area vs point to point streaming via Internet?

Doing some research, it seems that an FM radio tower in a densely popoulated area can maximally transmit about 50kW of power. If you assume a certain amount of people tuning into the station, is that ...
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792 views

### In this Foster Seeley discriminator, what is the purpose of capacitor C3?

I understand the whole circuit, except for the use of capacitor 3. What's its purpose in here?
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I'd like to build an FM radio that is pre-tuned to a specific station. I could use something like the Si4703 FM tuner chip, but they're expensive and difficult to solder by hand (and require a ...
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### RDA5807M FM antenna question

I am trying to use RDA5807M to build a radio. My circuit is basically the same as the reference design in data sheet. I know the design uses the GND of earphone as antenna. But I don't understand the ...
873 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 ...
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158 views

### Using audio instrument cable for 1MHz transmission

I'd like to transmit digital data together with an audio signal along an instrument cable. In more detail, I'd like to send MIDI data and the audio signal of a guitar on a single cable. The rationale ...
241 views

### Building radios using RLC Meter

I'm interested in building AM and FM radios from discrete components. Has anyone ever used an RLC meter to precisely create the inductors and other elements used in these radios? I know cheaper RLC ...
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### SIMULINK: The spectrum of the modulated signal remains unchanged despite modification of the modulation index

I try to implement a frequency modulator. From what I know, the more I vary the modulation index (which is set by the gain block here,) the more the spectral lines should increase in number. When I ...