80 votes
Accepted

Why does 60 Hz mean 60 refreshes and not 120?

Hertz means 'cycles per second', where a cycle is an event that's repeated. With the light bulb, that's cycles of the mains supply frequency. Being a balanced a.c. waveform, that crosses 0 V twice per ...
  • 20.4k
56 votes
Accepted

Why do push button telephones use dual-tone for signalling?

The two reasons are simple: Eight frequencies are easier to discriminate with simple analog electronics - banks of bandpass filters or even vibrating reeds - than sixteen frequencies. The equally ...
  • 66.3k
55 votes

How did 455 kHz end up being a commonly used IF (intermediate frequency)?

Piecing together information from a number of sources on the web, I believe this is the correct combination of circumstances: Early amplifiers were not stable much above 500 kHz, so the IF had to be ...
  • 166k
54 votes
Accepted

What's special about 14.31818 MHz?

It is exactly 4× the NTSC color-burst frequency of 3.579545 MHz. Since it is (well, used to be) used in huge quantities in commercial color TV sets, it is both commonly available, and ...
  • 166k
49 votes

Serbia, Kosovo power grid row delays European clocks. Why?

From the Reuter's article referenced: SARAJEVO, March 7 (Reuters) - European power grid lobby ENTSO-E urged Serbia and Kosovo to urgently resolve a dispute over their power grid, which has affected ...
  • 165k
36 votes

Where do overtones in a 555 generated square wave come from?

I know that a square can be constructed from sine waves. However, the 555 does not generate sine waves or multiple frequencies, it generates a single square pulse. So where have these harmonic ...
  • 144k
36 votes

Why doesn't higher frequency mean higher data rate?

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 ...
  • 833
35 votes

Why does 60 Hz mean 60 refreshes and not 120?

Light bulbs blink at twice the mains frequency because the AC waveform goes to negative side, and has two pulses per cycle: Compare that to the brightness you would see with an old time CRT monitor, ...
  • 5,460
34 votes
Accepted

Is current with a frequency less than 1 Hz still considered DC?

AC and DC are relative terms. If you're looking at a 10kHz waveform for 100ns, you will think it is DC. It works the other way around too: if you forget about what's providing you with "DC", who knows ...
34 votes
Accepted

Serbia, Kosovo power grid row delays European clocks. Why?

How a decrease in electricity production can lead to a decrease of the frequency on the grid on the long term? Isn't the frequency a parameter controlled by the power plant at the end of the day? ...
34 votes
Accepted

Are antennas specified for a certain frequency?

It's not so much that antennas are specified at certain frequencies (they usually are), but that they tend to work best over certain frequency ranges, determined by their physical shape and the ...
  • 39.9k
33 votes

How can a Cat 5 cable (frequency 100 MHz) transmit 2.5 gigabits/second?

No, you don't need 2.5 GHz bandwidth to transmit 2.5 Gbit/s. You can do it of course, but that would be very inefficient. The data is encoded into symbols. 100Base-T Ethernet uses one pair in one ...
  • 105k
32 votes
Accepted

How can over 24 GHz communication be possible?

RF comms do not transmit one bit of information per cycle of the carrier wave - that would be digital baseband communications and it requires incredible amounts of bandwidth. Incidentally, you can ...
32 votes
Accepted

Why do power grids tend to operate at low frequencies like 60 Hz and 50 Hz?

Higher frequencies are much more affected by the inductance of the power lines. 400 Hz is fine on an aircraft, but over long distances the power factor would be extremely poor. 60 Hz was an educated ...
  • 5,834
29 votes

Where do overtones in a 555 generated square wave come from?

A square wave can be viewed as a sum of the odd harmonics of a single frequency. A square wave can be generated by summing a bunch of sine waves. A square wave can also be generated by simply ...
  • 63.7k
29 votes

Why do power grids tend to operate at low frequencies like 60 Hz and 50 Hz?

What @Frog says about losses is true, however, that's not the real reason for utility frequency to be around 50-60Hz. HVDC systems have essentially no reactive losses, yet they did not really become ...
26 votes
Accepted

How does an AM radio filter out only the desired frequency?

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 ...
24 votes

Can I turn Radio waves into light?

Can I do this circuit practically o_O? Is there any transistor or IC that can oscillate 300 THz? Can I find an inductance (coil) of 0.0025 pH and capacitor of 1 pF? Not quite, no, and no. But ...
  • 45.6k
24 votes
Accepted

Why do most flyback converters / LED drivers operate under 100 kHz?

There are many reasons: - Cost: This is probably the biggest reason. The lighting market is quite competitive. There are lots of Chinese players offering aggressively cheap prices. This forces the ...
24 votes

How many watts does a 40 MHz radio frequency signal create?

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 ...
  • 123k
23 votes
Accepted

How can a phone wire have multiple frequencies at once?

The underlying assumption in your question - that the frequency being measured is the rate at which electrons reverse polarity - is incorrect. The frequency of a signal at the transmitter, receiver, ...
23 votes
Accepted

Can sampling rate be a floating point number?

Forget sampling rate for a few seconds... Think about sampling period for a second, which is the time interval between two consecutive samples. This time can be an integer or any real number (as long ...
  • 2,167
21 votes

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

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 ...
  • 4,102
21 votes
Accepted

How can Arduino send an RF signal at 433 MHz?

The short answer: It doesn't. Arduino doesn't send anything close to that high. It can only send a lower frequency (lower bitrate) signal over a 433MHz CARRIER frequency (or any other frequency for ...
  • 2,294
20 votes
Accepted

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

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 ...
19 votes

Is current with a frequency less than 1 Hz still considered DC?

Yes, you can have AC with a frequency less than 1Hz, in the same way you can have numbers between 0 and 1. Frequency isn't an integer number, but a "real" number. You can quite happily have a ...
  • 55.7k
19 votes

Why do push button telephones use dual-tone for signalling?

If you used single tones instead of dual tones, you'd need 16 of them instead of only 8 as in the DTMF system. Given that they have to be spaced far enough apart for reliable detection, and that you ...
  • 63.7k
19 votes

Serbia, Kosovo power grid row delays European clocks. Why?

How can a decrease in electricity production lead to a decrease of the frequency on the grid on the long term? Isn't the frequency a parameter controlled by the power plant at the end of the day? You ...
  • 20.1k
19 votes

How is it possible to drive VGA displays at such high pixel clock frequencies?

There are two main reasons you are finding this challenging. First, you are using older and more discrete (lower scale integration) parts than would have been used to do this in the era of VGA. But ...

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