IEEE Geoscience and Remote Sensing Magazine - March 2015 - 16

small changes in the brightness temperature of natural
ground targets, the radi-23.45
ometer imposes stringent
radiated emissions require-23.46
ments in its frequency band
-23.47
on other components in
the SMAP observatory. The
-23.48
noise-equivalent sensitivity
for the radiometer is specified
-23.49
at 0.7 Kelvin. Any noise contribution from the powered
-23.50
radar electronics (excluding
the transmit duration, during
-23.51
which time the radiometer
-23.52
blanks out its measurement)
should be a small fraction of
-23.53
the 0.7 K limit. Specifically,
VV
a radiated emissions limit of
-23.54
VN
3 dBuV/m over 1370-1450
VH
MHz was derived for the
-23.55
1200 1210 1220 1230 1240 1250 1260 1270 1280 1290 1300
powered radar during the
Frequency (MHz)
receive window to meet instrument self-compatibility
(a)
requirements.
LR Mean Power of Receiver-V vs Frequency of RFI Tone
from 20130326T175754_FM_TTRFI1_ADPver35
The design approach for
-45.09
low radiated emissions is
VV
briefly discussed here, fol-45.11
VN
lowed with an EMC test
VH
-45.13
summary in the next sec-45.15
tion. Radiated emissions
is a multifaceted problem,
-45.17
limited by the weakest link
-45.19
in the system, and as such
must be approached from
-45.21
multiple aspects of the radar
-45.23
design, fabrication, and as-45.25
sembly-from board layout
to cable harness construc-45.27
tion. Countermeasures fall
-45.29
into two categories: minimizing radiating sources
-45.31
internal to the radar, and
-45.33
maximizing radar shield-45.35
ing ef fec tiveness to t he
1200 1210 1220 1230 1240 1250 1260 1270 1280 1290 1300
outside environment [11].
Frequency (MHz)
The digital electronics and
(b)
its external interfaces are
a chief area of concern for
Figure 5. Radar out-of-band RFI suppression response for: (a) maximum echo case, and (b) minimum
radiating sources due to the
echo case. A strong RFI tone (~70-dB above minimum echo) is simultaneously injected and swept from
fast (nanosecond scale) rise/
1200-1300 MHz. Outside of the 1250 ! 5 MHz susceptible band, the variation in detected echo power is
fall times of digital system
0.22 dB or less, which satisfies the radar 0.4-dB RFI error allocation.
clocks and drivers, which
can produce harmonics up
to
1-2
GHz
or
higher.
Techniques
applied to minimize
compatibility (EMC) between the radar electronics and
radiating sources include [12]:
the sensitive radiometer. As a passive sensor that detects
Power (dBm at OVRP)

Power (dBm at OVRP)

LR Mean Power of Receiver-V vs Frequency of RFI Tone
from 20130326T180930_FM_TTRFI1_ADPver35

16

ieee Geoscience and remote sensing magazine

march 2015



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