# Tag Info

41

Voice has both pitch, i.e. frequency, and loudness, which are two separate analog data streams. No. Voice is transmitted initially as one analog 'stream' of sound pressure waves in which the air pressure variation amplitude corresponds to the volume (at that instant) and the rate of change gives the pitch. No tutorial ... explain[s] how both aspects are ...

38

In the image below a amplitude modulated sine wave: 0% unmodulated, the sine envelope is not visible at all; < 100% modulation depth is normal AM use; 100% modulation depth, the sine envelope touch at y=0. Maximum modulation that can be retrieved with an envelope detector without distortion; > 100% modulation depth, "overmodulation", the original sine ...

36

It's not a bad question and shows quite a common misunderstanding of how Radio Frequency (RF) Systems work. The 900MHz and 2600MHz signals are called Carrier Frequencies. The actual information is contained in the 10MHz bandwidth. The original signal is a baseband signal that extends up to 10MHz. This is used to modulate the carrier signal. The reason we do ...

26

Forget about radio — how do you think voice is transmitted over a wire, which only has "voltage" — again, a single variable? The point is, "pitch" and "amplitude" are abstract parameters of a single-valued function of time. In fact, you can superimpose many different signals at different frequencies on a single wire. Each component of such a ...

24

In a nutshell: One antenna will give you a usable radius of 100...1000 km, depending on the power used. In Germany, for the example of my favored news station Deutschlandfunk, we used to have two long-wave AM stations (153 and 207 kHz, IIRC), and I do miss them every once in a while. The one at 207 kHz covered pretty much all of Southern Germany, and while ...

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

Because the bandwidth of the channel determines how rapidly the symbols on that channel can change, not the carrier frequency. It is the bandwidth of the channel which determines how fast the channel "rings down" from a sudden change in phase or amplitude. So your two 10MHz bandwidth channels can each sustain a symbol rate of no more than $10 \cdot10^6$ ...

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 ...

13

It's not (phase) (shift keying), for example. It's (phase shift) (keying). Another answer has explained why the word keying is used --- it dates to the days when modulation was controlled by a human operator using a telegraph key. The phase shift (or amplitude shift or whatever) is because something is changing (shifting) when keyed. If the system were ...

12

The inductor in the plastic tube is not an inductor. It's the inductor core. The loops of wire are the coil around that core. It's an adjustable inductor. Fixed to its given value with hot glue.

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 ...

12

After mixing and passing through the IF filter what remains is largely one small part of the spectrum that your SA is scanning through. If there's a signal present in this part of the spectrum, it'll be a sinewave and if you envelope detect it you are, in effect, measuring the peak amplitude of that sine wave. That peak signal is $\sqrt2$ higher than the ...

12

Sound is just a single-dimensional time-varying signal. Microphones essentially continuously track variations in air pressure. At any point in time, this is a single value. This value is what gets 'modulated' onto the carrier. This single-dimensional time-varying signal carries both the loudness and pitch information. It can actually contain the ...

10

Encoding: Information can't be "transferred to a set of photons (with no mass) and hold varying "states" within the analog light". The photons themselves do not store/carry the information inside them, but the variations in the number of photons (flux, intensity), or the variation in the frequency of photons can carry information. "Constituent state of ...

10

The concept here is that you are adding the signal currents before feeding the sum to the diode; the resistors are there to convert the voltage sources "NF" and "HF" to current sources. You'd get exactly the same effect by adding the signal voltages directly — simply connect the "NF" and "HF" boxes in series, without any resistors. The only downside ...

10

By quick analogy (the other answers are sufficient in their detail): If you own a hotel where all the rooms are the same size (bandwidth), it doesn't matter how high a floor (frequency) they're on--they all hold the same amount of stuff. If you have some larger rooms (more bandwidth), they'll hold more stuff, whether they're on the 2nd floor or the 26th.

9

The plastic sleeve is not active, it just holds the core and allows it to be adjusted by screwing in or out, to tune the transmitter. The two loops of wire are the antenna windings. Most likely the one surrounding the core is the tuned circuit, while the other provides positive feedback to sustain the RF oscillation. And the whole assembly can indeed act ...

9

Besides that also in Germany you can very well listen to AM stations, although the transmissions may not be as crisp clear as wide FM stations. There are several reasons that come to my mind why they might still operate: Old AM equipment may just still be there and function, and investing in replacing with FM is prohibitively expensive An FM frequency slot ...

8

AM over modulation causes the carrier wave to invert it's phase when the modulating signal has an amplitude that is above a certain level. Broadcast AM typically never does this because the complexity of an accurate demodulator is too great for the thousands and millions of receivers. AM is just the mathematical multiplication of two signals and regular ...

8

OFDM isn't a modulation technique. It stands for "Orthogonal frequency-division multiplexing" and is a way of splitting the spectrum into discrete channels for sending data. Each of those channels, which is just a single frequency carrier in its own right, requires the data to be modulated onto it. In this example BPSK is used.

8

Theoretically a square wave has infinite bandwidth but it still looks reasonably square even if the bandwidth is severely compromised. A square wave is "made from" a series of ever-increasing harmonics. See the picture below to get an understanding: - On the right is a sinewave then as you look down you'll see that it grows into a square wave. If all we had ...

8

Square-wave carrier systems are actually not uncommon in cable, fiber optic, or even line-of-sight optical communication systems, but it is important to note that these are closed-channel systems; effectively, all energy the transmitter produces for the most part can be assumed as being received by the receiver. The benefit here is that the transmitter and ...

8

Sort of. Certainly antenna length is proportional to wavelength, so if you take the limit going to DC then you'll need an infinitely long antenna. However, even if you make an antenna infinitely long, it won't work very well. Antennas have limited bandwidth. Say you design a 7.5 cm long antenna for a 1 GHz signal. This antenna will work very well at 1 GHz, ...

8

They have a much faster ADCs. Also, ADSL2+ has its downlink up to maybe 2.2 MHz, so you'd need at least 4.4 MS/s to do that. In reality, single-digit MHz ADCs are "old tech" and relatively cheap. Together with it being desirable to oversample the signal to apply digital filtering to increase your SNR, I'd presume that modern (as in: of the last 20 ...

7

RF bandwidth and data rate are related by the modulation format. Different modulation formats will require different bandwidths for the same data rate. For FM modulation, the bandwidth is approximately 2*(df + fm) where df is the maximum frequency deviation and fm is the frequency of the message. FSK is basically FM where the message signal is a square ...

7

A digital signal is composed of a fundamental frequency and an infinite number of odd harmonics. This is how the nice clean and sharp edges of a digital signal is made. As in the image below, you have the fundamental frequency, and as you add more and more odd harmonics, the signal begins to take the shape of an ideal digital waveform. Now you mention that ...

7

This block diagram is for an old-style CRT-based spectrum analyzer. The horizontal sweep represents the frequency being measured at a given time (like a sawtooth wrt time), and the vertical amplitude is the frequency content at that frequency (more accurately, over a narrow range of frequencies). If the input signal is a modulated carrier the display will ...

7

Put simply, for the same average energy per bit, the modulation scheme with more bits per symbol must have less distance between the points on its constellation diagram. Therefore a smaller noise excursion can produce a bit error.

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