Biomedical Engineering Reference
In-Depth Information
The narrowband interferer is modeled as a single-tone interferer, shown in
Eq. (
3
) for analytical simplicity. However, the results derived for a single-tone in-
terferer also apply for an interferer which can be modeled as a band-limited random
process. This will be validated by the measurement results.
i
(
t
)
=
A
i
cos (2
πf
i
t
)
(3)
Based on the input/output relationship of an ideal squarer, the squarer output
V
out
(t)
can be written as:
K
A
s
e
−
π
t
−
t
0
A
i
cos
(
2
πf
i
t)
2
2
τ
2
i (t)
]
2
V
out
(t)
=
K
[
s (t)
+
=
cos
(
2
πf
c
t)
+
KA
s
e
−
2
π
t
−
t
0
2
τ
2
1
+
cos
(
2
πf
c
t)
2
1
+
cos
(
2
πf
i
t)
2
KA
i
=
+
KA
s
A
i
e
−
π
t
−
t
0
2
τ
2
+
{
cos [2
π (f
c
−
f
i
) t
]
+
cos [2
π (f
c
+
f
i
) t
]
}
(4)
It can be observed from Eq. (
4
) that, after squarer operation,
s(t)
would be split
into two parts. The first part is a squared Gaussian pulse, down-converted to base-
band while the second part is an up-converted squared Gaussian pulse with carrier
frequency of
2f
c
. Similarly for NBI
i(t)
, it will generate a DC component and a high-
frequency component at
2f
i
. A band-pass filter can filter out the DC NBI signal and
the high-frequency components. The low-frequency component of squarer output
is used for energy detection because signal amplification in low-frequency region
consumes less power and requires simpler circuit implementation. However, there
is also a down-converted intermodulation component of
s(t)
and
i(t)
to
f
c
−
f
i
. This
component may fall in the baseband pass-band and affects the receiver performance,
depending on the frequency difference between the carrier and the interferer.
Key Circuit Blocks
Transmitter
Figure
4
shows the schematic of the proposed UWB transmitter. The LC VCO core
consists of a cross-coupled NMOS pair (NM
1
−
2
) and an on-chip LC tank (L
1
−
2
,C
1
−
2
)
[
10
]. The VCO oscillation frequency can cover the frequency range of 3.5-4.5 GHz
by tuning varactor control voltage V
tune
through SPI. The pulse generator consists of
a three-stage delay cell and an edge combiner to convert the standard non-return-to-
zero (NRZ) baseband data to return-to-zero (RZ) pulse to control the on/off of the
VCO through the tail transistor NM
3
. It should be noted that NM
3
does not function
exactly as a tail bias current transistor, but rather acts as a control switch to power
on/off the VCO. Its sizing is chosen to ensure sufficiently fast turn on time for the pulse
generation. Compared to conventional UWB pulse generator with pulse shaping filter
[
11
], high output swing, which is critical for robust communication, can be easily
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