Dear Experts,
We are running radiated emission test for a device that contain a 50
MHz oscillator. Trace length of this clock line from the 4 pin oscillator
output to the processor pin is about 20 mm. A 230 volts A.C to 5.0 volts
D.C adapter (it is a 2 pin a.c connection, no earth point) is being used
for powering the device while running the radiated emission testing. Length
of the 5.0 volts d.c power unshielded cable is about 1.8 meters. No other
cable involved in the testing.
We recorded the emission (QP) at all the harmonics (from the 3rd to
20th) by varying the position of the d.c power cable. 4 sets of reading
with 4 different cable position (no patterns followed in the cable
positioning, cable is placed at random fashion) have been recorded. Care is
taken not to change the position of the device.
Emission at certain harmonics are found to vary drastically with the
cable positioning. At 350 MHz the variation was maximum, at one position
the emission was 25 db and in another position it was 42 db (17 db
variation). At 50 MHz the variation was minimum and is about 0.8 db only
(maximum 6.7 db and minimum 5.9).
From the readings will it be possible to identify the radiating
element? The frequencies at which the variation in emission is less, then
the PCB trace is the radiating element and for the frequencies at which the
variation is more the cable is the radiating element - will such a
conclusion be meaningful? Then it works out for the 50 MHz emission PCB
trace is the element but the trace length is very much lesser while
compared to the wavelength. For that matter even for 1000 MHz, the trace
length (20 mm) is much lesser to the wavelength (30 cm). While thinking
>from the trace length - wavelength perspective cable seem to be only
radiating element, will this assumption be correct?
There are other data lines toggling at 4 MHz, 25 MHz and 125 MHz. It
is decided to run the same test (record the emission at all the 50 MHz
harmonics by randomly varying the cable positioning without changing the
device positioning) by disabling these data lines. What further testing can
be done in the anechoic chamber to identify the radiating element? Will it
be possible to identify the radiating element using a near field probe?
Sincerely
K.Balasubramanian
Project Leader - Hardware.
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