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My iPod picks up FM stations and it's not supposed to!

N

nd2023

Banned
I was standing less than 50 feet from the 500+ foot tower of a 50,000 watt FM station, and I was listening to my iPod. When I paused the music, I could hear the station's audio and a 60 hz hum on my iPod headphones. The loudness of the audio and the hum changed as I moved the headphone cord. The station and the hum disappeared when I was a few hundred feet away, and it got louder when I set the volume on the iPod to zero. It was also not present when I turned my iPod off completely.
I wonder how it's possible for the radio station to be demodulated by my iPod when it doesn't even have a tuner or transmitter. And I thought only AM stations show up in electronic devices because the changing amplitude of the signal induces a changing voltage and the changing voltage is the original audio. With FM, only the frequency changes with the modulation so I don't know how an electronic device would respond to the changing frequency to (unintentionally) demodulate the audio.
The station does not have IBOC and that tower only has one FM station.

Now I wonder how my iPod will react when I stand near a 50,000 watt AM tower.
 
Your ear-buds will burst into flame.

Well, probably not that. Might have less of a problem with an AM than FM and TV transmitters. The length of the headphone cord is a better antenna at TV & FM frequencies.
 
Nick said:
I was standing less than 50 feet from the 500+ foot tower of a 50,000 watt FM station, and I was listening to my iPod. When I paused the music, I could hear the station's audio and a 60 hz hum on my iPod headphones. The loudness of the audio and the hum changed as I moved the headphone cord. The station and the hum disappeared when I was a few hundred feet away, and it got louder when I set the volume on the iPod to zero. It was also not present when I turned my iPod off completely.
I wonder how it's possible for the radio station to be demodulated by my iPod when it doesn't even have a tuner or transmitter. And I thought only AM stations show up in electronic devices because the changing amplitude of the signal induces a changing voltage and the changing voltage is the original audio. With FM, only the frequency changes with the modulation so I don't know how an electronic device would respond to the changing frequency to (unintentionally) demodulate the audio.
The station does not have IBOC and that tower only has one FM station.

Now I wonder how my iPod will react when I stand near a 50,000 watt AM tower.
Click to expand...

It's entirely possible that you ARE hearing AM. Any FM transmission system can -- and usually will -- generate a simultaneous very low level AM signal.

When the system (transmitter, filters, transmission line, antenna) is properly adjusted, AM noise is usually well below (< -50dB) the FM signal, but it's there, nonetheless. Some of that AM noise is modulation. Some is power supply ripple (hum).

As you observed, this low level signal is only audible when your receiver is relatively near a transmitting antenna.

But audio equipment only needs the barest of non-linearities to decode this AM. As an engineer who has had to suffer with (and fix) studio audio equipment picking up noise at a co-located transmitter site more times than I care to think, I can relate.
:)

Kind Regards,
David
 
So the transmitter or exciter is also modulating the amplitude of the carrier as well as the frequency?

I guess that the SWR changes with even a tiny change in frequency. When the SWR changes, the radiated power also changes. With 50,000 watts, it doesn't take much of an SWR change to affect the transmitted power. It might vary only a few watts, but near the transmitter, a few watts are enough to change the induced voltage. But what explains the hum being amplitude modulated on the FM signal?

I did notice that the audio was loudest when the headphone cord was vertically extended perpendicular to the top of the tower.
I'm surprised my iPod doesn't act up when my cell phone is near it, because my phone can cause noise if I place it near speakers. I assume the field strength of my phone 0.2 inches away is higher than 50,000 watts about 180 feet away.
I wonder if there would be enough potential between a metal rod and the ground near a transmitter site to show up on a voltmeter.
 
You have the idea for what's called synchronous AM. If there are reactive components in the transmission system (antenna, coupling systems, anything which is frequency determined) then they will be 50 Ohms j 0 at only a spot frequency, at any other frequency there will be a reactive component, and yes, the output level will change. This is basically why systems tuned for minimum synchronous AM - which usually isn't exactly mmaximum efficiency - will sound better, and have less multipath. This is why broadband components like antennas sound 'better' thanb narrower designs.

Your iPod is probably filtered for cell frequencies, either intentionally of from enough stray capacity in the wiring to effectively ground those frequencies.

Using a Simpson 260 multimeter, it's opften impossible to get reliable readings around a tower site due to RF pickup. I can set mine on AC and show a varying voltage by merely opening the strobe controller box at the base of the pole.
 
Nick said:
But what explains the hum being amplitude modulated on the FM signal?
Click to expand...

It's ripple in the transmitter's power supply. This is 'non-synchronous' AM, i.e., unrelated to the FM modulating signal.


Kind Regards,
David
 
So is there something wrong at the transmitter because it's amplitude modulating the FM carrier with the modulation and a hum?
 
Nick said:
So is there something wrong at the transmitter because it's amplitude modulating the FM carrier with the modulation and a hum?
Click to expand...

Probably not. Every FM transmitter modulates a bit of AM!

It only becomes a problem when it is high enough in level that it interferes with accurate detection of FM. But if it is minimized below a certain level, likelihood of reception problems are greatly reduced.

That's why there are well-known techniques and test gear dedicated to minimizing AM noise. Minimize, not eliminate.

Kind Regards,
David
 
Nick, I find it amazing that you've made over 2700 posts on Radio-Info.com and yet this is new information to you.

I'd understand that you may not know about syncronous and asyncronous amplitude modulation, but darn near everyone I know in the radio business knows (or so I thought) about transmitter neighbors complaining about picking up the FM station on all sorts of devices without a tuner. (Like the toaster, as the old joke goes.)

I say that to remind myself and others that what we think everyone knows may not be such common knowledge. To me, that's the most interesting thing about this thread.
 
Not knowing anything about the internal layout of the unintended FM receiver........I wonder if it was in fact acting as a slope detector?

(Courtesy of WikiPedia®)

However an AM radio may detect the sound of an FM broadcast by the phenomenon of slope detection which occurs when the radio is tuned slightly above or below the nominal broadcast frequency. Frequency variation on one sloping side of the radio tuning curve gives the amplified signal a corresponding local amplitude variation, to which the AM detector is sensitive. Slope detection gives inferior distortion and noise rejection compared to the following dedicated FM detectors that are normally used.

RJ
 
RJ Kanary said:
(Courtesy of WikiPedia®) However an AM radio may detect the sound of an FM broadcast by the phenomenon of slope detection which occurs when the radio is tuned slightly above or below the nominal broadcast frequency
Click to expand...

This can be true for some conditions and hardware. But does this Wikipedia quote truly relate to the original post in this thread?

The OP did not apply to detection of FM signals by an AM radio, as I recall it. The OP stated that this iPod device had no r-f tuner, I think.

Regardless, very high r-f fields (AM and/or FM) may be detected by non-linear processes in or near any device or apparatus.

//
 
I've heard about AM stations being heard in fences, washing machines, toasters, phone lines, and all sorts of electronic devices that aren't radios, even metal tooth filings. But I have not heard of FM stations being heard in non-radio devices till I heard the FM station on my iPod when I was in front of its tower.

I just don't think that frequency modulated signals can cause a voltage change proportional to the modulation inside electronic devices.
 
Nick said:
I've heard about AM stations being heard in fences, washing machines, toasters, phone lines, and all sorts of electronic devices that aren't radios, even metal tooth filings. But I have not heard of FM stations being heard in non-radio devices till I heard the FM station on my iPod when I was in front of its tower.

I just don't think that frequency modulated signals can cause a voltage change proportional to the modulation inside electronic devices.
Click to expand...

You're right, they can't. Which is why it is important to note that there is residual AM in every real-life FM signal, and that if that signal is strong enough, even this small amount of AM can and will be detected by a non-linear device.

Clear yet?
:)


Kind Regards,
David
 
RJ Kanary said:
Not knowing anything about the internal layout of the unintended FM receiver........I wonder if it was in fact acting as a slope detector?

(Courtesy of WikiPedia®)

However an AM radio may detect the sound of an FM broadcast by the phenomenon of slope detection which occurs when the radio is tuned slightly above or below the nominal broadcast frequency. Frequency variation on one sloping side of the radio tuning curve gives the amplified signal a corresponding local amplitude variation, to which the AM detector is sensitive. Slope detection gives inferior distortion and noise rejection compared to the following dedicated FM detectors that are normally used.

RJ

Way back when I was a teenager and transistors were discrete devices with three wires coming out the bottom I read an article in Popular Science (or Electronics) about slope detection and there was a circuit using a single tube in the article that I built. It worked quite well on local strong FM signals but of course there was no stereo yet. As described in the quote it wasn't clean but as a kid I was just excited to build something that worked. I also made my own crystal sets and even a spark gap transmitter though they were illegal then, just put it on long enough to see that it worked.
Click to expand...
 
The theory that I was putting forth R.Fry was based on the observation that many audio circuits can inadvertently detect RF into a discernible signal. If something ahead of the audio amps in the iPod happened to be acting as a slope detector, {Whose output I am ASSUMING <G> is an AM signal}, the subsequent stages could output it as a readable audio signal.

When I lived in West Deer Township in Allegheny County, I never really needed a radio receiver. KDKA 1020 was readable on just about ANY audio amp with an open input.:)


RJ
 
I used to live about 2 miles due north of a tower with three FMs and one TV station on it and I would hear the station on 105.9 through my computer speakers. It was tinny but it was definitely there but I just chalked it up to the little FCC warning sticker on the sub-woofer finally holding water :)
 
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