Biomedical Engineering Reference
In-Depth Information
matching, we place a capacitor C
m
in parallel with the coil (Fig.
8.7
,left),whereC
m
resonates with the coil inductance at the NMR frequency !
0
. This network forms
a band-pass filter, whose frequency response peaks at !
0
. The filter may be viewed
as a preamplifier with “passive” voltage gain of Q,whereQ is the coil quality. As
C
m
has negligible loss compared to the coil, the passive amplification hardly adds
any noise and maintains the original signal-to-noise ratio from the coil. In other
words, the passive amplifier has an NF close to 0 dB but with the gain of Q,which
leads to a low receiver NF according to the Friis equation [
7
]. While this passive
amplification scheme is not applicable for wideband signals due to the frequency-
dependent transfer function, it suits well the NMR signal, which in general has a
very narrow bandwidth (about 1 kHz in our case). Nonetheless, nonoptimal coil-
LNA impedance matching at 50˝, instead of the optimum noise matching based
on the passive amplification, has been a conventional choice, as the former is
convenient in the conventional NMR electronics that have largely been realized at
the discrete level. This shows an advantage of the integrated NMR electronics.
The LNA-coil resonance matching for minimum noise figure corresponds to an
impedance mismatch between the LNA and the coil. In contrast, the PA and the
coil need to be impedance matched for maximum power delivery. In order to simul-
taneously achieve both the optimum LNA-coil noise matching and PA-coil power
matching, the palm system adopts an advanced matching network (Fig.
8.4
a, right).
In the 1-chip system, on the other hand, we provide only the LNA-coil resonance
matching using C
m
(Fig.
8.4
b, right) without using the advanced network; thus, the
PA and the coil are not impedance matched. This is because the components of
the advanced network are too large to be integrated, and using discrete components
defeats the purpose of constructing a 1-chip system. Nonetheless, the 1-chip system
manages to deliver a reasonable amount of power to the coil for proton excitation.
The VGA (Fig.
8.7
) is to handle both the palm and 1-chip systems, whose NMR
signal strengths are different. It is a differential common-source amplifier with
tunable loads. By tuning the load impedance through V
ctrl
, we can change the gain
of the VGA from 0.8 to 22. The mixer is a double-balanced Gilbert mixer with an
active load. It provides a voltage gain of 26 dB.
8.3
Transceiver Measurements
We implemented two variations of the NMR RF transceiver with essentially
the same architecture (Fig.
8.4
), one for the palm system and the other for the
1-chip system, in 0.18-m complementary-metal-oxide-semiconductor (CMOS)
technology. The transceiver IC for the palm system occupies an area of 1.4 by
1.4 mm (Fig.
8.2
) and is packaged in a 32-lead QFP package. The transceiver IC
with the on-chip planar coil for the 1-chip system occupies an area of 4.5 by 2.5 mm
(Fig.
8.3
) and is so packaged in a 56-lead TSSOP package that the coil part of the
