Biomedical Engineering Reference
In-Depth Information
summarized as follows. The pulse generator (PG) transmits, through the transmitting
antenna, electromagnetic pulses with time duration of few hundred picoseconds and
a pulse repetition frequency (
f
RP
). After a time delay, approximately equal to the
flight time of the pulse from the transmitter to the receiver, the echoes captured
by the receiving antenna are amplified by the low-noise amplifier (LNA), and then
multiplied with a delayed replica of the transmitted pulse generated locally (i.e.,
on-chip) by the shaper (see Fig.
1
a).
The output of the multiplier is then averaged by a high-gain, low-bandwidth (
B
)
low-pass filter (i.e., integrator, see Fig.
1
a), in order to increase the output signal to
noise ratio (SNR
out
) of the receiver and track the envelope of the output voltage of
the multiplier [
17
].
Since vital signs vary within a few hertz, an integrator with a 3-dB band (
B
3dB
)
of 100 Hz allows accurate detection of their variations.
To explain intuitively the operating principle, let us assume that the delay generator
(DG) provides a delay equal to the entire time-of-flight of the pulses, i.e., round trip,
from the transmitter to receiver. If the target is not moving (i.e., static target), the
local replica and the amplified echo are timely aligned and the multiplier provides
constant amplitude pulses, as shown in Fig.
1
b, case (a). Therefore, the signal at the
output of the integrator (a high-gain, low-pass filter) is an almost constant envelope,
as shown in Fig.
1
c.
Note that the integrator will provide an almost constant output voltage envelope
regardless of the relative shift (
δ
) between the local replica and amplified echo, for
any other constant
δ
, assuming
δ
is lower or equal to the duration time of the pulse. If
the target is moving, the movement causes a time-varying
δ
between the local replica
and the eco amplified by the LNA.
Therefore, the multiplier provides an output pulse that may be positive, negative,
or zero, depending on the time shift
δ
caused by time-varying distance between radar
and target and due to the target movements around its quiescent position.
In particular, this radar sensor is thought to operate in two operating modes:
ranging mode (RM) and tracking mode (TM). In RM, the DG provides a variable
delay in order to span the range of interest and identify the target (see Fig.
1
a). In
RM, the radar sensor allows us to identify the presence of the target and the time of
flight. When the target is detected, the radar can be switched to the TM, in which the
DG provides a fixed delay (i.e., equal to the time of flight identified in RM) in order to
monitor a fixed range gate (see Fig.
1
a) and tracking the target motion (displacement)
around its quiescent position. Therefore, the output voltage is directly sensitive to
the target movements, e.g., the chest movement due to the pulmonary activity in case
of respiratory rate monitoring. The output voltage can be expressed approximately
as follows:
t
≈
A
B
2
π
v
OUT
(
t
)
v
Q
(
τ
)
×
v
P
(
τ
)
dτ
2
π
B
t
−
t
(1)
A
B
2
π
≈
×
×
−
v
Q
(
τ
)
α
v
Q
(
τ
δ
)
dτ
2
π
B
t
−
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