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FM radio frequencies that interfere with each other?

D

DXER1

Guest
I know that some one might have asked about this already.
I have two radios near each other one is tuned to 98.7
and the other is tuned to 88.1 and both signals cancel
each other why is that and what frequencies do this ?
Every trip I take with my parents I will try to find
the frequency that interfres with the frequency that
they are on.

I love doing that :) <P ID="signature">______________
"I'm a gonna go to hell when I die!" Connan O'Brien

"yay boo, yay boo, it's lots of fun to do, if ya like it holler yay, and if ya don't ya holler boo!"

Connan O'Brien
</P>
 
> I know that some one might have asked about this already.
> I have two radios near each other one is tuned to 98.7
> and the other is tuned to 88.1 and both signals cancel
> each other why is that and what frequencies do this ?
> Every trip I take with my parents I will try to find
> the frequency that interfres with the frequency that
> they are on.
>
> I love doing that :)
>
Its the difference of the 10.7Mhz IF (Intermediate Freq) in the local oscillator in the receiver circuit.<P ID="signature">______________

</P>
 
> I know that some one might have asked about this already.
> I have two radios near each other one is tuned to 98.7
> and the other is tuned to 88.1 and both signals cancel
> each other why is that and what frequencies do this ?
> Every trip I take with my parents I will try to find
> the frequency that interfres with the frequency that
> they are on.
>
> I love doing that :)
>
Matt correctly answered it but to expand on it, ALL FM radios use 10.7 as the IF.....shame since a higher IF of 21.4 or better would improve rejection (but I digress!).....The Local Oscillator which is the tunable signal that mixes with the on air signal to produce the IF that then gets decoded (typical super heterodyne receiver design). The LO is usually high side injection (10.7 MHz HIGHER than the signal you wish to pick up)...this is ONE reason why it is ILLEGAL to use a FM radio rcvr in an airplane. The LO falls inside the aircraft range on FM channels above 98 Mhz and can intefer with VOR directional or voice communications. On FMs that fall below 98MHz, the LOs falls within the FM band from 108 down to 98Mhz (with the rcvr listening to 98.1 to 88.1 MHz thus the LO falling on 108.8 down to 98.8 Mhz)....the LO is usually 100 kHz offset from an actual FM channel (all FM channels being ODD spaced such as xx.1, xx.3, etc and the LO will be falling on EVEN spaced channels such as xx.2, xx.4, etc). In the analog days, the LO could easily be DEAD on freq.....since analog radios would not stop exactly on the odd channels but could be as much as 50 or 100kHz off and still pull in a station ok...but the digital tuners still can blank a nearby rcvr....

ALL radios emit a signal from the receiver btw...two way, scanners, FM radios (AM radios too)...even cordless phones and even cell phones! They differ in the IF freq used and thus the offset of the LO from the received freq.
The internal oscillator does radiate a little bit (which is why rcvrs MUST pass Part 15 FCC requirements before being sold) Some emit it stronger than others but they ALL must be below a certain limit..sometimes having an external antenna on a scanner causes such a signal to be emitted farther than normal (I once tracked down a scanner that was bothering a radio base station 5 blocks away!..luckily changing the xtal from low side injection to high side injection cleared the problem! ;) Again, a design of the super hetrodyne receiver..
 
> > I know that some one might have asked about this already.
> > I have two radios near each other one is tuned to 98.7
> > and the other is tuned to 88.1 and both signals cancel
> > each other why is that and what frequencies do this ?
> > Every trip I take with my parents I will try to find
> > the frequency that interfres with the frequency that
> > they are on.
> >
> > I love doing that :)
> >
> Matt correctly answered it but to expand on it, ALL FM
> radios use 10.7 as the IF.....shame since a higher IF of
> 21.4 or better would improve rejection (but I
> digress!).....The Local Oscillator which is the tunable
> signal that mixes with the on air signal to produce the IF
> that then gets decoded (typical super heterodyne receiver
> design). The LO is usually high side injection (10.7 MHz
> HIGHER than the signal you wish to pick up)...this is ONE
> reason why it is ILLEGAL to use a FM radio rcvr in an
> airplane. The LO falls inside the aircraft range on FM
> channels above 98 Mhz and can intefer with VOR directional
> or voice communications. On FMs that fall below 98MHz, the
> LOs falls within the FM band from 108 down to 98Mhz (with
> the rcvr listening to 98.1 to 88.1 MHz thus the LO falling
> on 108.8 down to 98.8 Mhz)....the LO is usually 100 kHz
> offset from an actual FM channel (all FM channels being ODD
> spaced such as xx.1, xx.3, etc and the LO will be falling on
> EVEN spaced channels such as xx.2, xx.4, etc). In the analog
> days, the LO could easily be DEAD on freq.....since analog
> radios would not stop exactly on the odd channels but could
> be as much as 50 or 100kHz off and still pull in a station
> ok...but the digital tuners still can blank a nearby
> rcvr....
>
> ALL radios emit a signal from the receiver btw...two way,
> scanners, FM radios (AM radios too)...even cordless phones
> and even cell phones! They differ in the IF freq used and
> thus the offset of the LO from the received freq.
> The internal oscillator does radiate a little bit (which is
> why rcvrs MUST pass Part 15 FCC requirements before being
> sold) Some emit it stronger than others but they ALL must be
> below a certain limit..sometimes having an external antenna
> on a scanner causes such a signal to be emitted farther than
> normal (I once tracked down a scanner that was bothering a
> radio base station 5 blocks away!..luckily changing the xtal
> from low side injection to high side injection cleared the
> problem! ;) Again, a design of the super hetrodyne
> receiver..
>
Umm..yeah, what you said =)<P ID="signature">______________

</P>
 
Re: FM radio frequencies that interfere with each other? (kinda long)

> I know that some one might have asked about this already.
> I have two radios near each other one is tuned to 98.7
> and the other is tuned to 88.1 and both signals cancel
> each other why is that and what frequencies do this ?
> Every trip I take with my parents I will try to find
> the frequency that interfres with the frequency that
> they are on.

The frequency that causes interference is 10.6 or 10.8MHz lower than the one they're listening to. You of course noticed that in this case, where the interfering frequency is 88.1, the "listening" frequency 98.7, and the difference 10.6. If they were listening to 106.9, the "interfering" frequency would be 96.1 or 96.3.

Now, the long part<grin>...

Virtually all radios (and TVs) use Edwin Armstrong's "superhetrodyne" circuit. In this circuit, incoming signals are mixed with a "local oscillator" signal in a "mixer". What comes out of the mixer are the original incoming signal, the local oscillator signal, the sum of the two, and their difference.

So for example, if you're tuned to 88.1, the four outputs:

- The original signal (88.1)
- The local oscillator (98.8, I'll explain why it's here later)
- Their sum (186.9)
- Their difference (10.7)

The radio then splits out the difference signal at 10.7MHz, amplifies it, filters it, converts it to audio and sends it to the speaker.

Now, let's say you decide to listen to 88.9 instead. The local oscillator is retuned from 98.8 to 99.6. And now the four outputs:

- The original signal (88.1)
- The local oscillator (99.6)
- Their sum (187.7)
- Their difference (10.7)

Yep, the difference between the two is *still* 10.7. In fact, whenever you change stations, the local oscillator is tuned to a new frequency such that the difference between the oscillator and the desired station is always 10.7. The advantage of this circuit is that, when you change stations, all you have to do is retune the local oscillator. You don't have to make any changes to the rest of the amplifier or demodulator circuits.

Now, in theory, the only place the local oscillator signal would go is the output of the mixer. It would be easily filtered out by the 10.7MHz filters.

Problem is, theory and practice are two different things... Some of the local oscillator signal "escapes" the radio. I suppose for the most part it "backfeeds" through the mixer & gets radiated out the antenna. I'm sure some is simply radiated from the radio's wiring. Either way, it gets out. And as you see in the math above, if you're tuned to a frequency lower than 97.3, this local oscillator signal also falls within the FM band.

Note that AM radios work the same way. The "magic frequency" is usually 455KHz. (there is more variation on AM - while virtually all FM radios use 10.7, some AM sets use 450, 460, or even 262.5.) Radiation of the local oscillator isn't as much of a problem, but "image responses" can be. For example, when I visit my mother near Milwaukee, I get Radio Disney on 730:

- The original signal (730)
- The local oscillator (1185)
- Their sum (1915)
- Their difference (455)

But there is no Radio Disney station on 730, in Milwaukee or anywhere else.

Now, let's say there's also a station on 1640 in Milwaukee. With the local oscillator still on 1185, we get:

- The original signal (1640)
- The local oscillator (1185)
- Their sum (2825)
- Their difference (455)

Yep, the 1640 station will come in with the radio tuned to 730. At least it will if the 1640 signal is strong enough. (and it is. And yes, Radio Disney is on 1640 in Milwaukee.)

Hope that all made sense!
 
AM IF's in digital tuners are usually 450KHZ because 450 can be divided by nine for AM channels in most of the world, and by ten for the Americas.

How bad are most car radios on FM today? Do you feel that we still need the 10.6/10.8 mileage requirements on the books. There was a big stink a few years ago when a 99.1 in Miami moved their site and overlapped an 88.5 in Fort Lauderdale. Also, a 102.7 in South Florida was stuck in a poor location for many years until a 92.1 moved to 92.3.<P ID="signature">______________
Proud 2 B a pioneering satellite radio subs¢riber
Ai4i is always on the trailing edge of technology
______________</P>
 
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