Hot answers tagged

73

This IR signal is indeed ignored by the AM radio. However, an AM radio is very sensitive the radio waves (yeah, DUH! ;-) ) When the IR remote operates (you push a button) the chip in the remote will switch on a clock resonator circuit which it needs to generate the IR signals. I have seen most IR remotes using a 455 kHz resonator. This is simply used ...


63

Short answer: FM is far less susceptible to disturbance of the signal. This is an AM modulated signal. The contours are the baseband signal which we recover by demodulation. Notice that there's a spike in the signal, which may be caused by a thunderstorm for instance. This is the demodulated signal. The demodulator doesn't "know" that the spike isn't ...


46

Actually, yes, a receiver can affect the transmitter. Passive RFID is based on this principle. However, RFID only works at very close distances, where the receiver is absorbing something on the order of 10-4 to 10-5 of the transmitter's signal. In other words, the transmitter is sending out hundreds of milliwatts, while the receiver is absorbing a few ...


34

Forget the quantum stuff for a moment. If you want to learn about quantum electrodynamics, read QED by Richard Feynman. (You should read it anyway; it may be the only really good pop physics book.) Classically, an electromagnetic field is a force field that acts on electric charge. It doesn't "look like" something any more than a mechanical push or pull ...


34

You could potentially break some radio transmitters by operating them without the antenna connected. Several things combine to make this possible. First, it's difficult to make power at RF frequencies, so the power devices are often fairly fragile, and run near their limiting conditions. Secondly, radio signals passing down a transmission line behave in a ...


30

AM radio is amplitude modulated, meaning that the amplitude of the carrier frequency is varying in the same manner as the audio signal you are transmitting. FM radio is frequency modulated, meaning that the frequency of the carrier frequency is varying in the same manner as the audio signal you are transmitting. Illustrative image:


30

That would be an inductor with a resistor style color code. Here's a picture from that Wikipedia page showing some similar 100 µH axial lead inductors: Vahid alpha at English Wikipedia CC BY 3.0, via Wikimedia Commons


26

It uses something called a filter. You can build filters out of all sorts of different things. RC filters made out of resistors and capacitors are probably the simplest to understand. Basically, the capacitor acts as a resistor, but with a different resistance at different frequencies. When you add a resistor, you can build a voltage divider that is ...


23

Your mention of the frequency (97.5 MHz) tells us this is an FM receiver. (AM will behave differently, as will other modulation schemes). Because FM is encoded by modulating the signal frequency, anything to do with AM is undesirable. To deal with this, most receivers over-amplify the signal until it becomes larger than the later stages can pass. The ...


23

A 1/4 wavelength trace or shorter can also have a substantial effect. The usual rule of thumb I've heard and used is that you can probably neglect transmission line effects when the length is less than 1/10 or 1/20 wavelength. For a simple example, say you terminate a 1/4 wavelength line with an open circuit and drive it with a single-frequency source. ...


22

It's technically possible to detect radio receivers if they are Superheterodyne receivers that use RF mixing to downmix the received signal to a well known intermediate frequency. You can scan for this frequency using a directional antenna and count the receivers around you. Though this doesn't sound like what you're inferring since the transmitter can't ...


22

This signal is an extremely inefficient use of bandwidth, as you can see because there is an essentially unused area between the center and ±125 kHz. Therefore, I expect that it is almost certainly unintentional radiation (a.k.a. RFI/EMI) rather than a meaningful transmission. The origin of the signal could be as follows — there are other ways it could ...


21

The frequency you tune to is the centre frequency of the spectrum created by the modulation. The input from the aerial is generally modulated down to an intermediate frequency (to make it easier to work with) and then fed into a Phase-Locked Loop (PLL) circuit which creates a signal proportional to the frequency shift from the centre frequency. This signal ...


20

That "fixed frequency like 103.2Mhz" is a bit of a lie: the radio is tuned to receive a signal in a small frequency band, which width is matched to the modulation depth (variation in the transmitted frequency).


18

Adding to Tom's answer: The wording is not very clear, but what this means is that digital signals do not actually exist in reality. All signals are analog. When we decide that a voltage above a certain threshold is a "1", a voltage below a certain threshold is a "0", and the space in between is "undefined", then we interpret an analog signal as a digital ...


17

The downside would be that the receiver would be more complicated. At the time that FM stereo was introduced, receivers were built entirely from discrete electronic components, and costs were directly related to the number of components required. Today, of course, you can add arbitrary numbers of transistors to an IC essentially "for free", so there are ...


16

Back when I was in grade school I had a crystal radio set. A crystal radio contains no amplifier. The output signal is completely powered by what is picked up from the antenna. I had around a 50 foot length of wire running out my bedroom window to a shed in the back yard as a antenna. With that I could pick up a 50 kW AM station over 20 miles away quite ...


16

Ferrite rod antennas were historically used for broadcast AM receivers. These signals are around 1 MHz, which corresponds to a wavelength on the order of 300 meters. A half-wave dipole at this frequency would be on the order of 150 meters long. The very high permeability of ferrite allows this antenna to be made smaller, at the cost of some performance. It ...


16

You may find this of interest. This is a spectrum allocation map, specifically the one for the United States, as determined by the FCC. It spans the entire radio spectrum. Frequency Allocation Chart Crowded isn't it? We are, as of October 2011, effectively allocated the entire RF spectrum. You probably noticed that it spans 9 KHz to 300 GHz. It of ...


15

I work in an industry affected by the LightSquared system and may be able to provide some insight. The issue at hand does fall into the area of being with the GPS receiver. The bands that LightSquared wants to use are near the GPS L1 wavelength. These bands are currently employed by systems that send command and control packets to satellite systems. However ...


15

There's the ISM bands that you can use for whatever you want, as long as you stay withing certain restrictions, mainly power level. The 2.4Ghz band is one such ISM band, which is why there is so much traffic on it (WiFi, Bluetooth, Analog video cameras, ZigBee). For that band, I believe that the power limit is 1 milliwatt for continuous transmission, and ...


15

Just to complement the excellent answer of Neil_UK and stress the fact that at RF frequencies voltages and currents don't really behave as those nice entities you know from KCL and KVL. You must drop Kirchhoff's laws and get your hands dirty with transmission lines theory, where the same concepts of voltage and current become a lot weirder! In other words,...


14

Some unscrupulous self promotion: Online Transmission Line Simulation Adjusting the transmission line length vs. the signal frequency is equivalent to adjusting time delay (tDelay) vs. rise time (tRise). Some interesting parameters: set tDelay=tRise/10. This is the case where the wavelength is much longer than the transmission line. Notice that the red ...


14

This is like asking "how bright is a red light?". It's as bright as it is. You can make a bright red light or a dim red light. As another answer points out, the energy per photon of an electromagnetic signal depends on the frequency of the signal. But you can make a brighter or dimmer (higher or lower power) source at any frequency by emitting more or fewer ...


13

One method is by actively steering the antenna (mechanically or electronically) to place a "null" in the direction of the jammer, reducing its signal strength significantly, while affecting the desired signal minimally, if at all. Also, assuming the jammer signal strength isn't so strong that it saturates the receiver front end, advanced DSP techniques can ...


13

This is done using a Heterodyne tuning system. For example, let's say you want to tune in a station at 1200kHZ. You set your tuning dial to "1200", which sets a local oscillator to generate a frequency of 745kHz. When you mix them, one of the resulting frequencies is the difference (the freq. you want to tune - 745kHz). The next stage is a narrow band ...


12

You asked a bunch of questions that are really too broad taken together, so I'll just answer what seems to be the underlying question about how to make a tuned ferrite rod antenna. Basically a ferrite rod antenna is a resonant L-C circuit. The ferrite rod and the coil wrapped around it form the inductor, and you connect a deliberate capacitor accross it. ...


12

When directional antennas are not practical, spread-spectrum techniques can be used. This causes the bandwidth of the signal to be very large, with very little energy at any particular frequency, making it much more difficult to jam. A similar approach is frequency hopping, where the carrier frequency is changed frequently according to a predetermined ...


12

Is it possible to build a very minimalist radio transmitter that can be built only with passive components, what would be the schematics for that? Sure. You can switch transients into a resonant circuit, as others have mentioned. The trouble here is that you end also transmitting a lot of those transients, which means a lot of broad-band noise. There's also ...


12

For FM, Carson's bandwidth rule informs you the approximate bandwidth of a transmission. The bandwidth value it calculates contains 98% of the energy of the whole transmission. It is expressed by the relation CBR = 2 (\$\Delta\$ f + f\$_M\$) where CBR is the bandwidth requirement, \$\Delta\$ f is the peak frequency deviation, and f\$_M\$ is the highest ...


Only top voted, non community-wiki answers of a minimum length are eligible