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I was recently reading about and building low-power AM radio transmitters, and read/noticed that when the carrier wave is not yet modulated, the radio receiver goes silent.

For example, let's say my transmitter transmits at 1000 KHz. Before the transmitter is turned on, the receiver picks up the local radio station that also transmits at 1000 kHz. However, once my transmitter is powered on (but not modulated), the radio receiver becomes silent.

Why does this happen? I'm curious about the mechanics of the speaker. Does the speaker vibrate at the 1000 kHz frequency, and we just cant hear it because it's out of our range of hearing? Or does the speaker cone expand outwards and stay there? Or, does it not move at all? I had a lot of difficulty finding information about this online, so any help is appreciated!

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  • \$\begingroup\$ If you over-power the AM radio station, with an unmodulated carrier, there is nothing to demodulate. As you reduce your carrier power, then some of the local station modulation will appear, perhaps distorted as carrier phasing/beatnotes confuse the sidebands. \$\endgroup\$ – analogsystemsrf May 22 at 0:51
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There are several things going on.

First, the output of the actual radio receiver (the "tuner" if you're building a cool hi-fi stack) is audio frequency only -- no RF. This output is proportional to those variations in the strength of the radio signal coming into the receiver between about 300Hz and 3000Hz, more or less (and feel free to look up what the "right" numbers are). The carrier wave is completely filtered out.

This gets amplified and applied to the speaker -- if there's no variation in the radio wave, then you won't hear anything.

Second, AM radio receivers have something called "automatic gain control" that senses the strength of the carrier wave and adjusts the gain of the receiver to keep the overall gain constant. If you didn't have this, then going from a close station to a distant one would mean turning the volume way up, and then going from that distant station to a close one would involve getting your ears blasted out of your head.

So when you turn on your home-brew transmitter its carrier overwhelms the local station; your carrier is constant, and so you hear silence. It's just about as simple as that.

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  • \$\begingroup\$ Got it, so the speaker simply doesn't move. However, now I'm confused as to why this worked: To make a morse code device, I connected a 555 IC (at 500 Hz astable mode) as the voltage source for an AM transmitter, which can only transmit a carrier wave. When I turned the receiver on, I heard a high-pitch tone (like a 500 Hz audio tone). Given that carrier waves don't move the speakers, shouldn't I not hear any sound, since what is really happening is that the "silent" carrier wave is simply being turned on and off 500 times each second? \$\endgroup\$ – F16Falcon May 22 at 2:01
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    \$\begingroup\$ @F16Falcon Your timer is turning the carrier wave on and off 500 times a second, which moves the speakers 500 times a second. (well, 1000 times - 500 times outward and 500 times inward) \$\endgroup\$ – user253751 May 22 at 2:57
  • \$\begingroup\$ @immibis So the carrier wave also moves the speaker? Like it moves it outward (or inward) once and keeps it there until either the carrier wave is turned off or another signal is sent? That would explain the sound; the carrier wave moves the speaker outward (or inward), and then when the carrier wave is turned off, the speaker moves back to its original state. When this happens 500x, the 500 Hz sound is heard. I had assumed from Tim's post that the carrier wave doesn't change the speaker position at all... Or am I wrong again? \$\endgroup\$ – F16Falcon May 22 at 3:05
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    \$\begingroup\$ @F16Falcon The overall strength of the carrier wave moves the speaker. The speaker moves out when the carrier wave is turned on and moves back when it is turned off. Sound is transmitted by changing the strength of the carrier wave. The speaker does not move for every individual wave of the carrier wave. \$\endgroup\$ – user253751 May 22 at 3:08
  • \$\begingroup\$ @immibis Ah, that makes sense. The frequency of the carrier wave is so high that the current is "seen" by the speaker as continuous rather than as a wave, so the speaker moves out and stays there. I appreciate the help man! \$\endgroup\$ – F16Falcon May 22 at 3:09
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Your question describes a situation which, if accurate, goes a bit beyond a basic receiver.

First, the background:

I was recently reading about and building low-power AM radio transmitters, and read/noticed that when the carrier wave is not yet modulated, the radio receiver goes silent.

As you're probably away, an AM envelope detector demodulates only modulation, and does not produce sound from the mere presence of a carrier. This is not, however, the only type of detector in use - a product detector especially in the form of a tunable beat frequency oscillator can be tuned to produce an audible tone in the presence of a carrier alone, and is how both morse-code "CW" signals are demodulated (via a frequency difference) and how single sideband signals are demodulated (by matching the original carrier, or nearly enough for voice). It can also be used to demodulate ordinary AM, but unless the received is carefully tuned, the carrier will be audible as a fixed tone of nonzero frequency.

For example, let's say my transmitter transmits at 1000 KHz. Before the transmitter is turned on, the receiver picks up the local radio station that also transmits at 1000 KHz. However, once my transmitter is powered on (but not modulated), the radio receiver becomes silent.

On the contrary, this is not something that actually occurs with a basic AM receiver. Such a "capture effect" is something that occurs with an FM one, but the lack of this behavior in a basic AM system is famously the reason that aviation communications are done with AM - the ability to "talk over" other transmitters and and have both transmission be intelligible as if this happened acoustically in a room is considered a key safety factor.

Instead, it is likely that what you have is an automatic gain control, a system where the strength of the signal causes the receiver to turn down its own gain. In that situation, then a strong signal would indeed quiet weaker ones, as if a robotic hand had reached out of the chassis and adjusted the volume knob. And AGC is typically driven by the strength of the carrier, not the volume of the audible modulation.

But this would be an intentional feature. Non-linear distortion such as compression when stages are overloaded does not really have this effect, because independent signals will not line up only at the waveform peaks, but rather the weak one will also be present in the troughs of the strong. There will be distortion generating additional intermodulation frequencies, but the original signals will still be there as well.

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First, in most radio designs, only the detected audio frequency (AF) signal reaches the audio amplifiers and speaker or headphone. An exception is a reflex receiver, in which one stage amplifies both radio frequency (RF) and AF. In a reflex receiver, a low-pass filter may be added to block RF from the loudspeaker, not because the paper cone could vibrate at 1 MHz, but to prevent radiation of RF to other receivers.

Second, consider the average voltage of a sine wave: zero. Since the cone can't move at 1 MHz, why should it be pushed forward or backward?

Third, you can hear an unmodulated carrier... with a beat frequency oscillator (BFO). If your receiver doesn't have a BFO, you can use another superheterodyne receiver tuned a few hundred kHz away (~455 kHz, if it uses that as the IF). You will hear the audible difference between the carrier and the BFO. For example, if your carrier is at 1 MhZ, the other radio uses a 455 kHz IF, and you tune the "helper" radio to 544 kHz, it's local oscillator would be at 999 kHz, and the beat tone would be just 1 kHz. Try it, and let me know if you can get that to work!

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