[sdiy] 100 MHz EMI, what can it be?

[greg montalbano]

Railroad communications, cordless phones, baby monitors, car phones, FM & 
AM stations, police & emergency broadcasts -- these days, the soup is so 
thick I'm surprised we don't ALL live in Faraday cages.

I live in the San Francisco bay area -- VERY heavy radio saturation, with 
many hills & tall buildings to create standing-wave pockets.  I, too, have 
had the experience of trouble-shooting a noisy circuit, only to find that 
if I moved it to another part of my studio, the noise vanished.

(Back when I used to play bass, I found that the audio "sweet spot" in the 
room corresponded to the phantom radio sweet spot -- moving two feet to the 
right made the interference go away.  When several of us used to get 
together to play, we called the ensuing twisting & 
contorting  "interference tai-chi").

~GMM


At 07:17 PM 4/14/03 +0200, Rene' wrote:
>Hi Martin et al,
>
>Well, I have the Bonn AM Station (WDR2, 720kHz) in close
>proximity, and there are also a couple of FM stations in
>line of sight. If the probe is open, and I touch the tip
>I can see the 720k carrier riding on the 50Hz pickup.
>If I short the probe the 720k is gone, and I can see
>something in the neighborhood of 100MHz too. (Some 200uV)
>Probably pretty normal in FM Station dense countrys like
>Germany.
>
>Which reminds me that I have once built a FM-detector, which
>consisted out of a loop of wire with 10-15 cm diameter, and a
>20pF trim cap. And also a AA118 Ge-Diode for demodulaton of
>the signal. (Use a 2000ohms Headphone, or something suitably
>matched with a transformer.)  In a way this is similar to your
>arrangement, the loop and capacitance of the scope lead probably
>form a tuned circuit as well. Very interesting, I had 3 stations
>trong enough that you actually would hear something, although you
>can't really separate the individual stations (the tuned circuit
>is not haveing enough Q). But still I found that pretty amazing.
>
>Cheers,
>  René
>
>
>Czech Martin wrote:
>>Bingo,
>>107,0MHz or 107,7MHz FM , 500W
>>directly in my neighbourhood.
>>Do you see something like this with your local FM stations?
>>m.c.
>>
>>-----Original Message----- From: René Schmitz
>>[mailto:uzs159 at uni-bonn.de] Sent: Montag, 14. April 2003 12:01 To:
>>Czech Martin Cc: synth-diy at dropmix.xs4all.nl Subject: Re: [sdiy] 100
>>MHz EMI, what can it be?
>>
>>Could that be be a FM radio station? I would check with a receiver
>>what that signal is.
>>Cheers, René
>>Czech Martin wrote:
>>
>>>If I connect the (long) ground clip of the oscilloscope probe to
>>>the tip, I get a nice loop. I can see 10 mVpp, around 100MHz. No
>>>matter how I orientate the loop, it will not disappear.
>>>What can this be? Computer is of, cell phone is much higher, wireless 
>>>phone should also be higher.
>>>Of course, connecting the probe tip to shield via 5mm wire will
>>>make the noise disappear, so it is clearly something from outside
>>>the scope.
>>>I live near a railway track, is this railway communication radio?
>>>m.c.
>
>--
>uzs159 at uni-bonn.de
>http://www.uni-bonn.de/~uzs159
>
>
>
>