J
Joerg
- Jan 1, 1970
- 0
Hello Folks,
In a really sensitive sub-audio application we are experiencing cell
phone pickup. It got a lot worse when switching from the LT1112 to the
LM833 in order to lower the sub-audio noise (and yeah, reduce cost...).
Anyhow, this surprised me because the LM833 doesn't even have the input
protectors like the LT1112 so the chances of something clipping and
demodulating should be lower. How could this be? Older process with
larger patterns and thus more pickup loop area?
The challenge here is that bypassing is all nice and dandy but
capacitors always have a residual inductance and at cell phone
frequencies that begins to matter. Cell phones are particularly nasty
because anything that gets AM demodulated produces this rat-tat-tat
pulse pattern between phone and cell tower. Shielding is going to be a
major pain in the neck in this application.
In case anyone wants to look at the datasheets:
http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1010,C1764,P1364,D1714
http://www.national.com/ds/LM/LM833.pdf
In a really sensitive sub-audio application we are experiencing cell
phone pickup. It got a lot worse when switching from the LT1112 to the
LM833 in order to lower the sub-audio noise (and yeah, reduce cost...).
Anyhow, this surprised me because the LM833 doesn't even have the input
protectors like the LT1112 so the chances of something clipping and
demodulating should be lower. How could this be? Older process with
larger patterns and thus more pickup loop area?
The challenge here is that bypassing is all nice and dandy but
capacitors always have a residual inductance and at cell phone
frequencies that begins to matter. Cell phones are particularly nasty
because anything that gets AM demodulated produces this rat-tat-tat
pulse pattern between phone and cell tower. Shielding is going to be a
major pain in the neck in this application.
In case anyone wants to look at the datasheets:
http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1010,C1764,P1364,D1714
http://www.national.com/ds/LM/LM833.pdf