Make sure the laptop is not the source of the problem. Connect your 1 meter
cable to the laptop, leave the other end open, and then measure your
emissions.
Note that a 1 meter cable is just about a quarter-wave monopole antenna at
72 MHz (good radiation efficiency).
Don Borowski
Schweitzer Engineering Labs
Pullman, WA
Kbalasubramanian@
scmmicro.co.in
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04/10/04 06:35 AM Subject
Is it a common mode problem?
Please respond to
Kbalasubramanian@
scmmicro.co.in
Dear EMC experts,
Radiated Emission @ 72 MHz in horizontal polarization from the EUT is about
34 db microvolts measured at 3 meters. The EUT is a USB device connected to
the USB port of the laptop computer through a 1-meter cable and the EUT is
in 'USB Suspend' state. There is no switching (crystal is stopped, the
processor is not executing any program code) current involved. The current
consumed by the EUT is about 300 micro amps. Since I do not have a high
precision current probe, I used 2 digital multi-meters one at the d.c +ve
line & the other at the d.c. –ve line of the EUT to measure the current and
the current is found to be exactly equal in magnitude in both the wires.
Same EUT when directly connected to the laptop computer without the cable,
the emission @ 72 MHz is below the floor level noise of the analyzer.
Is it still a common mode problem?
Sincerely
K.Balasubramanian
Sr.Engineer, Hardware.
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