AM RECIEVER

[prateeksikka]

HI EVERY BODY!AFTER MY SUCESSFUL WORK ON THE ASSEMBLY OF AN F.M SUPERHETERODYNE RECIEVER AND ELECTRONIC STETHOSCOPE(WITH HELP FROM ELECTRONICS LAB) I WISH TO WORK ON ASSEMBLY OF AN A.M RECIEVER I.E THE CONVENTIONAL A.M RADIO.A SIMPLEST CIRCUIT ISWHAT I REQUIRE.ANY SUGGESTION OR HELP EVEN WITH F.M OR A.M RECIEVER?AWAITING EARLY REPLY;
PRATEEK

 

[Kevin Weddle]

There are a lot of different types of modulators. The simplest only suffer from lack of performance. It is often desireable to stuff as much information as you can in a certain time frame.

 

[audioguru2]

Hi Kevin,
An AM receiver doesn't have a modulator, it just has a simple AM detector rectifier. Prateek wants to make an AM radio.

Hi Prateek,
Use a Google search to find an AM radio circuit. You may have a problem finding its coils and transformers.

 

[audioguru2]

Hi Kevin,
Nope. Changing the capacitance load of a signal won't change its frequency. Besides, at low AM radio RF and IF frequencies the few pF of diode capacitance is neglegible.

An AM radio's demodulator is simply a rectifier of the 455KHz IF frequency.

 

[Kevin Weddle]

It seems interesting that they call a diode and capacitor a demodulator. When in fact, the demodulator is just a capacitor filter. I think that since they remove part of the cycle with the diode they can say they have isolated the message. For the message will appear in the cycle that is passed. Incidentally, the magnitude of the voltage does determine the frequency of the message.

 

[audioguru2]

Hi Kevin,
If you look at a modulated AM carrier on a 'scope, you will see that the positive-going parts of the modulation equal the negative ones.
Therefore if you filter it, the modulation cancels itself and you are left with nothing.

If you rectify the modulated carrier with a diode, the modulation is recovered because then it modulates a single-polarity DC voltage.

The magnitude of the voltage does not determine frequency of the modulation. The magnitude is the loudness and the rate of modulation is the frequency.

 

[Kevin Weddle]

The voltage of the carrier does indicate frequency because a radical change in the amplitude of the carrier indicates a high frequency of the message.

 

[audioguru2]

No Kevin,
I don't know why you don't believe me, but this is how AM modulation works:
) A radical change in the amplitude of an AM modulated carrier indicates a change in loudness (or volume) of its modulation and occurs in its demodulated output.
2) If you AM modulate a carrier with a high frequency, the amplitude variation of the carrier occurs quickly, at the same frequency as the modulation. Likewise, if you AM modulate a carrier with a low frequency, the amplitude variation of the carrier occurs slowly.
3) The demodulated output's frequency is dependent on the cycles-per-second (Hertz) amplitude changes (AM modulation) of the carrier.

A Google search for "AM modulation" (with the quotation marks) reveals many university lectures full of math and sidebands. Some tutorials have animated pics of waveforms that are seen on a 'scope. You can see that the amplitude fluctuations of the AM modulated carrier occur at the same frequency as the modulation.

 

[Kevin Weddle]

Audioguru I just don't agree. I think the loudness is the voltage difference between any two consecutive cycles of the carrier wave.
If you modulate a high frequency on a carrier, you will get the carrier to change at that rate. This is what the diagram shows and I agree with that.

 

[audioguru2]

The IF frequency of an AM radio is nearly 100 times higher than the highest modulation frequency, so amplitude variations between any two consecutive cycles of the IF would be neglegible.
The loudness is the amount of variation in the entire envelope of the IF carrier, at a very slow audio rate. Maximum volume is full modulaton where the carrier's amplitude varies fom zero to double and extremely low volume is a steady carrier amplitude.

 

[trigger]

AM demodulation is envelope detecter only.
And the loudness is depends on the modulation depth of the source.

And IF is the resultant frequency of the difference between the local oscillator and the carrier frequncy, which is to lower the frequency for demodulation.

 

[surajbarkale]

Hay you folks battling on theory - http://www.fas.org/man/dod-101/navy/docs/es310/AM.htm

prateeksikka,
Take a look at IC ZN414, it is the most simple way to build an AM receiver. It can catch a local AM station but don't expect to get BBC ;D (well unless you are in UK)

 

[audioguru2]

Hi Suraj,
Thanks for the Navy link with its very good illustrations.
But even those Navy folks got the theory wrong when they say, "The process of demodulation usually involves detecting the envelope of the AM signal. This may easily be accomplished by using a low-frequency filter (like a capacitor) which will remove the quickly oscillating carrier signal, leaving only the slowly-varying amplitude".

You can't recover the symmetrical amplitude variations with just a filter, without using a rectifier so that the waveform isn't symmetrical anymore and therefore the amplitude variations can be recovered.

I've had enough of old lousy-sounding, interference-prone AM radio.
I like FM radio but wish the broadcasters would reduce the compression and limiting of the audio. If the listeners at the far away fringes of reception get some noise, they can just use a bigger, higher antenna or move closer! ;D
Even the FM sound on TV broadcasts is pretty good nowadays.

I wish the TV announcers would stop hiding the microphone down on their clothes and put it in front of their mouth where it belongs.

Have you ever heard the poor quality of an underwater throat mic?
Lavalier mics sound similar. A lavalier mic picks-up the fundamental chest vibrations very loudly and the compressor cuts the volume down to normal, severely reducing the volume of the harmonics, "s" and "t" and other important sounds of speech. Have you ever seen (heard) a singer use a lavalier mic? No way, man! ;D

 

[ante1]

Does anyone know if the ZN414 is still in production?

 

[surajbarkale]

It is available here in India. So somebody must be producing it ;D