The Mystery Behind Tuning a Radio

[Majid]

hi

does somebody knows why we hear a noise when tuning a radio? I mean when you turn the radio tuner to find a station. not for a typical station. during turning the knib.
I know it's becouse of a electromagnetic wave. but I don't know that wave and its source.

tnx.

 

[Claude Bile]

When you turn the knob, you are changing the frequency response of the circuit. Continuous frequency changes are often called chirp because they make a chirp noise in audio frequency.

Claude.

 

[tariq5]

Background cosmic radiations?

 

[Meir Achuz]

Most radios use "feedback" circuits to reduce distortion during amplification. Sometimes, if there is a weak signal, the current in the feedback circuit oscillates. If the frequency of oscillation is in the audible range, you here it as a whistle.

 

[Elrod]

You're certainly referring to AM (Amplitude Modulation) radio.
'Noises' in AM radio have different sources and causes.
'Crackling sounds' (similar to a bonfire) are generally from natural causes like atmospheric electricity (static and lightning). They can also be caused by 'plasma' or ionization in the high atmosphere caused by solar winds (aurora). Generally, they are not caused by cosmic radiation, though! Cosmic radiation interacts very poorly with the atmosphere. In fact, most of it goes through our bodies, concrete walls and cave walls!
But they can also be "man-made", like the noise from electric motors (sparks made by brushes) and iginition from cars (sparks made by sparkplugs).
A spark (arc) will cause broad band electromagnetic interference. It will be almost constant throughout the band (i.e. it will be there no matter where your tuning knob is).

Now, the 'whistles' and 'chirps' you sometimes get at your speaker, in an AM radio, are *ALL* man-made, that is, they all come from 'intentionally' emitted radio signals.
Some of them are actually emitted 'on purpose'. When you tune in SW (short-wave) you get lots of radio 'beacons'. Some of them are actually data transmissions. Some of them are even Morse-Code (single tone, intermitent 'beeps') others are RTTY (radio-teletype, generally two-tone 'beeps').

Other tones you hear come from spurious interference between two signals.
What you're hearing is the 'beat' between two different frequencies.
Let's say there are two AM radio stations emitting 10KHz apart (it's more common than you might think, taking into account the congestion in SW spectrum and the range SW signals can have - i.e. they can be heard around the globe).
So, we have these two carrier frequencies which differ only by 10KHz. The tuning circuits in your radio (filters) might pick them up both.
The two signals will add together, producing a 10KHz audible beep!
How does this happen? Well, try to imagine two sine waves of slightly different periods (distance between consecutive cycles).
If you add these two sine waves, you'll see that sometimes they add up producing a bigger amplitude (which means they are 'in-phase') and sometimes they will cancel each other ('out-of-phase').
This re-inforcement/canceling happens periodically at exactly the difference between the two frequencies (in the example, 10KHz, which is an audible tone).

Interference can also come from "out of band" signals (i.e. a band different from what you've selected in your radio).
Radio filters are not perfect. Plus, most radios are of the superheterodyne type, which means they have 'local oscillators' and tune by changing the frequency of a local oscillator, rather than modifying the filter's passband...
They 'mix' the incoming signal with a locally generated one, in order to get a third frequency, which is then filtered. This is a rather simplistic explanation.
But what I mean is that some out-of-band signals may 'mix' with a local oscillator frequency and produce a product that falls in the band you're tuning.

Hope this helps.
If you have further doubts, I'll be happy to clear them.

c ya

Elrod

 

[Majid]

thank you all.
I mean whistles those are due to an electromagnetic reason between wave source and radio (between transmitter and reciver). I know it's related to a plasma effect.

 

[Majid]

" They can also be caused by 'plasma' or ionization in the high atmosphere caused by solar winds (aurora). "
may you give me some refrences or direction?

 

[Elrod]

" They can also be caused by 'plasma' or ionization in the high atmosphere caused by solar winds (aurora). "
may you give me some refrences or direction?

Majid,
Actually, my setence may not be strictly correct.
Aurora certainly are associated with very noticeable disturbances in radio operation.
When they happen, radio "blackouts" and natural noise increases may occur in all bands of the radio spectrum.
But the major cause of radio noise during aurora is due to the very cause of aurora itself, i.e.: solar activity. More specifically, I'm talking about solar flares and/or Coronal Mass Ejections (CMEs).
Solar flares cause a huge increase in solar wind. That is, the flow of highly
energetic subatomic particles originating from the Sun.
They also cause strong electromagnetic disturbances. This is all tied together, of course. When we have electrically charged particles moving (i.e. electrons and protons) we also have an associated magnetic field.
Now, as you know, aurora is caused by these subatomic particles hitting the upper Earth atmosphere, ionizing the gases (and even turning some of them into plasma).
Plasma, is a fourth state of the matter where *all* the electrons break free from their orbitals around the atomic nucleous. Almost like a "soup" of electrons and protons. You can consider it as an extreme case of ionization.
The Sun, itself, is mostly "made of" plasma. And you can consider the solar wind also as plasma flow.
When electrons, occasionally, fall back into their orbitals, they emit energy in form of photons, and that's what causes plasma and ionized gas to "glow".
Plasma and ionized gases are also, inherently, highly conductive. So, when subject to varying magnetic fields, electrical currents will develop. When we have varying electrical currents and magnetic fields, we have, inevitably, electromagnetic radiation.
As I said, it's all tied together in a rather complex cause-effect scenario.
You can find lots of resources on the net about these subjects, including NOAA and NASA sites.
Here are some links to get you started:
http://www.solcomhouse.com/solar.html
http://www.lwca.org/sitepage/natrad/
http://www.sec.noaa.gov/primer/primer.html
http://science.nasa.gov/ssl/pad/solar/cmes.htm
http://science.howstuffworks.com/question471.htm

 

[Majid]

:!)
thank you. It's a great help.

 

[Majid]

unfortunally I can't find my answer. does anyone knows the source of natural radio (whistlers) those are in AM band? I know it's not becouse of solar activity.

 

[DaveC426913]

I've heard the term "whistler waves" which I believed had something to do with the Sun and the upper atmosphere but i don't really know.

Here's an article describing some radio phenomena: http://www.loscrittoio.it/Pages/MM-0700.html including this on whistlers:

WHISTLERS - Whistlers are descending tones generated through the propagation of sferics over very long paths formed by field aligned plasmas (ducts) m the magnetosphere. Whistler's magnetospheric propagation is between magnetic conjugate regions in northern and southern hemispheres. Terrestrial reception of whistlers results from subionospheric propagation
of these signals.

Whistler duration ranges from a fraction of a second to several seconds. The frequency range of whistlers can extend from above 30 kHz to below 1 kHz but those readily heard with simple equipment will mostly lie between 1 and 9 kHz, with their maximum energy usually concentrated between 3 and 5 kHz. Whistlers are categorized according to hops. One hop equals a
single traverse between conjugate regions. A one hop whistler is generated by lightning in the opposite hemisphere from the listener. It has traversed the magnetosphere just once and as a consequence, it tends to be a high pitched whistler of short duration. Since the causative sferic is very far away, it is rarely heard in association with single hop whistlers. Two hop whistlers are produced by lightning in the same magnetic hemisphere as the listener. The signal has traveled to the opposite
hemisphere and echoed back to the region of its origin. Subject to roughly twice the dispersion of a single-hop whistler, its duration is much longer than its one-hop cousin. Causative sferics can often be heard in very distinct association with 2-hop whistlers. Delays of 1.5 to 3 seconds between sferic and whistler are typical.

Odd order hops (1, 3, 5, etc.) indicate opposite hemisphere lightning while even order progressions (2, 4, 6, etc.) follow from same hemisphere lightning. On occasion, whistlers generate multiple echoes or progressions known as echo trains. While trains exceeding about a dozen echoes are uncommon, progressions of more than 100 have been observed on
rare occasions.

Whistler notes range from extreme] pure tones to breathy, diffuse swishes. The breathy quality is described as diffuseness. It results from whistler mode excitation of multiple ducts, with slightly different travel time for each duct serving to spread or diffuse the signal.

Whistlers were the first studied and most easily understood class of magnetospheric radio events but they are far from being the only ones that can be observed by a patient listener using basic tools.

 

[Majid]

thank you. I'm interested on any natural noise or frequency source that can be recived in AM band.

 

[richjoh]

Dave, it been a long while now but your link is no longer up. Is there another link for your post? I'd like to know more about AM whistle, i.e. heterodyne issue and how to deal with it.

Btw, I've installed an alpine 9886 car stereo and had to put a LPF (low pass filter) in series with the power ignition wire at the radio head. As your know the bigger the inductance the lower the cutooff freq. Well a 2mH inductor (partsexpress.com) cleaned up my alternator whine after replacing the window antenna with a 31 inch pole antenna. I now pick up fringe station, day and night :). My only issue is this persistent whistle up and down the AM dial. It's spaced every 30K (3 channels) between the whistles. I know it enter from the antenna, since whenever I go into a tunnel the whistle is silent.

Some car stereo enthusiast complain about AM reception. Since I have pretty good reception now, I wonder if OEM radios are really better on Rx than an OEM. What makes OEM radio better on AM reception than the after market?