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

[Czech Martin]

When I was 14 or so, I got an "elevtronic kit", containing two transistors,
diodes, capacitors, resistors, and a tuneable capacitor (RF style).
Withe some wire and the tuneable capacitor and the diode I could
build a detector for AM radio (the strongest station).
A high impedance headphone capsule was also included.
This way you get radio without any batteries, just from the field.

I still have the headphone capsule. Sometimes, if some circuit
has problems it is usefull to listen to it, even if it is non audio.
E.g. a counter problem in a digital circuit can be detected
by unclear , noisy counter outputs. The scope would look
fine if the problem is less then 1% of the time, but you can hear it.


m.c.

-----Original Message-----
From: René Schmitz [mailto:uzs159 at uni-bonn.de]
Sent: Montag, 14. April 2003 19:18
To: Czech Martin
Cc: synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] 100 MHz EMI, what can it be?


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