JBL system AM radio - constant audible tone

Well, considering that AM transmission is just a step above CW, it could be anything from skip RF to electrical noise generated by your car to HV power line noise. It's AM, get used to it!!!

 

Sounds like crosstalk. It might mean a faulty radio, it might not. Basic moves: Take two radiated signal. For this example, suppose that they're both sine waves. Next, run them though some non-linear device. In ye olden days, this would be the detection diode in a (really simple) AM radio. So, you get something that looks like A*sin(2*pi*freq1)*B*sin(2*pi*freq2): that is, the nonlinearity looks like a multiplication.

What you get out looks like C*sin(2*pi*freq1) + D*sin(2*pi*freq2) + E*sin(2*pi*(freq1+freq2)) + F*(2*pi*(freq1-freq2)). That is, you get the original two signals out, a signal that has the frequencies of the two signals added to each other, and a signal that has the two frequencies subtracted from each other.

Confusing, but true. So, it's not just that you have two different AM stations going into your radio. You've got the entire AM radio band, all the signals that are near the band, and all the TV, radio, microwave, and $DIETY knows what else heading down that antenna cable on the way to the mixer in your radio.

OK. So, durn near every radio known to Man these days has the following:
1) An antenna. Hopefully the antenna doesn't absorb that megawatt radar signal at the airport you're driving by, thus preventing smoke from coming out of your dash.
2) A selective "bandpass" circuit. Different bandpasses are switched in for different bands, so you get one for AM, another for FM, and who-knows-what for the Satellite stuff. Anything in the AM band gets through, everything else gets rejected, or at least attenuated.
3) This is the good part. Nearly every manufacturer then sticks in the most low-noise, linear amplifier that they can come up with. If the amplifier is linear, then crosstalk and heterodyning is Not Us. More on this in a bit.
4. We then hit the Mixer. The Mixer takes in as inputs the AM band and an oscillator that is adjusted by you tuning your radio up and down. Typically, with an AM radio, the oscillator would be running 455 kHz higher than the frequency you're trying to receive. Out of the mixer comes the original signal, the oscillator signal, a signal that looks like the original signal, but displaced high by 455 kHz, and a signal that looks just like the original signal, but running right at 455 kHz.
5) Narrow Bandpass filter. That takes in the signal at 455 kHz and knocks out everything that isn't right at 455 kHz, plus or minus 20 kHz or so. That kills the rest of the AM band, so you only have the signal you want to listen to.
6. Amplifier/demodulator. The amplifier cranks up the amplitude of the desired signal, usually to a couple of volts peak-to-peak; the demodulator then extracts the audio. That is further amplified and what all and sent to the speakers.

Now, when you start hearing intermod (tones, etc.), those tones and all are most likely not coming from the ether. Remember what I said about non-linearity up above? If there are non-linear elements in the front end (that is, up to the mixer), then intermodulation between the various radio signals and such can occur, and that's when you start hearing those tones.

Now, the front end amp has its limits. Yeah, it can be plenty linear when the incoming signal is a hundred microvolts or so off the antenna. But some of those big AM stations can blast away with a volt or three, at the radio input! Hence, that front end amp is specifically designed to be as linear as possible in the presence of any old signal. There are limits: You're not going to push 30 V on the thing and expect it to work. The intermod products tend to go up as the cube of the overall input power.

Engineers Do Things to try and help the radio along. There are sometimes linear, dynamic attenuators placed ahead of that first amplifier. Then, a feedback loop looks at the amplitude of the signal after the first amp and, if it's on the large side, puts more attenuation in to keep the amp in its linear range.

OK. The stretch of Route 95 through Waltham, Massachusetts is known as Intermod Alley for good reason, what with all the gazillions of commercial, AM, TV, Cell, Microwave, and what all in that area. At one time it was famous for bringing BMWs to their knees, and it's a place where 90% of all car radios likely have some pretty strange sounds. If you're in that kind of environment, welcome to life.

If, on the other hand, that tone is following you everywhere, it could be a sign of a malfunctioning AM radio. Stuff happens. Best bet: Get a decent table radio out of your house, or borrow one, get a 12V->120VAC inverter, and do an A/B comparison, preferably with a passenger to flip switches. If the car radio is worse than the table radio, find ye a repair guy at the dealer and tell them so.

Good luck!

Kbeck

 

Jesus, I was with you right up to the end, then this... see above quote.
Why introduce cheap inverter whine into the mix there are good quality BATTERY AM portable radios, yes I know, it's an O X Y M O R O N, out there that would do the trick.
But really, AM, not quite the class action material here, it's AM..Fuggiddaboudit...
Are there any othe stations on that frequency in that part of the county, say within a 750-1000 miles range???
Two signals, one weak, one stong, mixing, it's AM...

 

I ve noticed an audible tone on JBL as well when selecting a Prius.
And it didn't sound as much improvement over II...so I bought a II...

GL

 

73 from WA1MEE!

Hmm.. Well, I guess I'll argue this one a bit. A couple of things.

First, AM transmitters in the US are supposed to all be on standard frequencies. Hence, if you got near and far, yep, they'll interfere with each other, and you'll get squawking and stuff as the sidebands bang up against each other, but, unless one of them is actually transmitting a tone, a more-or-less constant tone is not what you're going to get.

I don't know if the international AM stations are on any fixed plan, but I'll grant that one could be, say, 500 Hz off from a local radio station. That would give you a tone, but I'd expect it to come and go as the ionosphere weather changes on its usual daily cycle. The OP was implying that this thing stuck around all the time.

The thing is, despite my quick and dirty on my first post, is that the typical nonlinearity doesn't just multiply a couple of signals together. It squares signals, cubes them, and then takes the whole bunch of them and multiplies them times each other. That also goes for the internal oscillators and such inside the radio. I've watched serious design engineers bang their heads against walls for weeks trying to come up with an internal frequency and filter plan that doesn't put internal intermod spikes on top of the narrowband receive filter, where it might show up as a receive spur.

OK, a battery powered radio might be a better try than one using an inverter. However, I was just thinking that the better quality of table radio might be AC only. I stand corrected. However, if there's no tone with the extra radio vs. tone with the car radio, then it's probably the car radio at fault.

Of course, if the OP can demo that, in his car, he's got a tone with the car, none with his semi-portable radio, and another Prius doesn't have a tone, and all on the dealer's lot, then he's got a slam-dunk on getting his radio replaced. Or at least trying to figure out where in the car some tone is coming from. (Bad filtering, anyone?)

Kbeck

 

Ken, nice to meet you OM, well, I just got off the phone with an Engineer from Bloomberg Radio in NYC!!! I wanted to see if they were adding to the signal, programming data, etc, that could possibly be causing the "tone" whine, etc.
The engineer did suggest, since its a Prius with XM, listen to it on XM and blow off the AM reception.
Valid point there.
But he did confirm to me that there are NO other signals other than "Normal Program Content" on their transmission. Also, 1130 is a Clear Channel for them, so there should not be anyone else on 1130 kc within thousands of miles, but there is that skip thing...but thats usually at night only, as you probably know!
That to me pretty much rules out the source being the problem.
My money is on a harmonic generated in the Prius being detected in the Prius AM system and thus the tone.....
Funny enough, he also suggested taking a second radio and standing next to the car, listen for the tone, and then start walking away from the car to see if the tone diminishes....
Well at least I got to speak with a engineer who has some ideas....

Looking forward to more posts from you, you never can have too many Electro Savvy folk around.