J
Johnny Chang
- Jan 1, 1970
- 0
the noise is coming from a cell phone which we use to transmit. No
noise
when its not transmitting. I located the actual chip on the board, an
op amp,
that injects the noise into the signal. Since it is from the cell
phone GSM
TDMA accessing (4.6 ms packets, looked it up on scope, 217 hz and its
harmonics), I guess it is electromagnetic radiation. When I change to
WLAN the
noise completely goes away.
I am working with audible frequencies, the noise does not affect the
other
signals as we have a lot capacitance between power and ground at all
parts of
the board. I've already completely shielded the power source ( a
battery
several feet away ) using shielded power cables, and even wrapping the
whole
board and power cabling in aluminum foil which is grounded. all no
luck.
There is no 217 hz and its harmonics on the power and ground from the
opamp that
creates the noise, so i am completely dumbfounded. I even added 220 uF
capacitance (more than enough for 217 hz and above) and it doesnt
affect it. I've even
added capacitance to all the capacitors in the circuit surrounding
the op amp, and none help (some just cut the sound completely off).
The trace
is a couple inches in length which COULD be a problem and I could try
unsoldering the chip that it goes to but it would be a ton of work, or
I could
cut the trace with an exacto knife. However, even shielding it with
aluminum
foil and grounding it completely does not fix it!
Since everything else I've measured doesn't pick up the 217 hz it is
not the
oscilliscope cabling.
I tried disconnecting the microphone such that basically ground was
going into
the op amp. The noise very well could be coming from the line
attached to the
opamp output but I don't know how I can deal with it. There was still
noise
with none on ground/power.
I will try out some better than aluminum foil insulation and shielding
methods between them. We place
these devices within several inches from the cell phone in order to
have it small in the woods.
This happens across over 50 boards with the same layout so it can't be
the opamp. it only happens when the gsm phone is transmitting.I am
going to try increasing the resistance in the feedback loop but I
really see no 217 hz in the input of the opamp at all. so is it
possible that the output trace which is several inches in length and
whatnot could be picking up signal? What are some better ways to
shield it?
http://i38.tinypic.com/e7fo02.jpg
Does anyone have any ideas?
noise
when its not transmitting. I located the actual chip on the board, an
op amp,
that injects the noise into the signal. Since it is from the cell
phone GSM
TDMA accessing (4.6 ms packets, looked it up on scope, 217 hz and its
harmonics), I guess it is electromagnetic radiation. When I change to
WLAN the
noise completely goes away.
I am working with audible frequencies, the noise does not affect the
other
signals as we have a lot capacitance between power and ground at all
parts of
the board. I've already completely shielded the power source ( a
battery
several feet away ) using shielded power cables, and even wrapping the
whole
board and power cabling in aluminum foil which is grounded. all no
luck.
There is no 217 hz and its harmonics on the power and ground from the
opamp that
creates the noise, so i am completely dumbfounded. I even added 220 uF
capacitance (more than enough for 217 hz and above) and it doesnt
affect it. I've even
added capacitance to all the capacitors in the circuit surrounding
the op amp, and none help (some just cut the sound completely off).
The trace
is a couple inches in length which COULD be a problem and I could try
unsoldering the chip that it goes to but it would be a ton of work, or
I could
cut the trace with an exacto knife. However, even shielding it with
aluminum
foil and grounding it completely does not fix it!
Since everything else I've measured doesn't pick up the 217 hz it is
not the
oscilliscope cabling.
I tried disconnecting the microphone such that basically ground was
going into
the op amp. The noise very well could be coming from the line
attached to the
opamp output but I don't know how I can deal with it. There was still
noise
with none on ground/power.
I will try out some better than aluminum foil insulation and shielding
methods between them. We place
these devices within several inches from the cell phone in order to
have it small in the woods.
This happens across over 50 boards with the same layout so it can't be
the opamp. it only happens when the gsm phone is transmitting.I am
going to try increasing the resistance in the feedback loop but I
really see no 217 hz in the input of the opamp at all. so is it
possible that the output trace which is several inches in length and
whatnot could be picking up signal? What are some better ways to
shield it?
http://i38.tinypic.com/e7fo02.jpg
Does anyone have any ideas?
