Biomedical Engineering Reference
In-Depth Information
enough to mitigate the 1=f noise and low enough to facilitate the rejection of out-
of-band noise by placing an off-chip band-pass filter at the outputs of the mixers.
Two mixers are used to perform the frequency down-conversion with quadrature
oscillator outputs. The outputs of the mixers, after the band-pass filtering, are
digitized by an off-chip analog-to-digital converter and subsequently undergo an
image rejection signal processing, to avoid the extra 3-dB noise figure degradation
brought by the frequency down-conversion.
NMR transceivers usually employ two separate clocks, one with the NMR
frequency for the proton excitation and the other as the local oscillator (LO) with
the frequency different than the NMR frequency by the target IF to produce the
correct IF. In contrast, in our transceiver, both the transmitter excitation signal and
the receiver local oscillator signal share the identical frequency, both derived from
the same clock (Fig.
8.4
). In this scheme the clock frequency is set at a value 3 kHz
larger than the NMR frequency !
0
so as to produce the target IF of 3 kHz. Therefore,
the excitation signal is 3 kHz off from the NMR frequency. Nonetheless, it can still
excite protons, for it has a nonzero bandwidth due to its finite duration and the
bandwidth can be made large enough to cover !
0
. The advantage of this single-clock
scheme is simplicity: we only need to tune one frequency in the NMR experiment,
instead of tuning two clock frequencies while maintaining their difference at 3 kHz.
8.2.2
Characteristics of the Coils
The off-chip solenoidal coil of the palm system (Figs.
8.2
and
8.4
a) has 14 turns
around a capillary tube (inner diameter, 0.75 mm; outer diameter, 1 mm). The
sample volume inside the coil is 2L. The coil has an inductance of 100 nH, a
resistance of 0:5˝,andaQ of 28, all measured at the NMR frequency 23.9 MHz
of the palm system.
The on-chip planar spiral coil of the 1-chip system (Figs.
8.3
and
8.4
b) has 25
turns and occupies an area of 2.5 by 2.5 mm. We use a package that exposes the coil
part while encapsulating the rest of the chip (Fig.
8.3
). The open part of the package
above the coil can hold a 5L of sample. To reduce the coil resistance, five metal
layers are connected in parallel. SONNET electromagnetic field solver is used in the
coil design. The coil has an inductance of 430 nH, a resistance of 31˝,andaQ of
1.9, all measured at the NMR frequency 20.9 MHz of the 1-chip system. The low Q
is due mainly to the coil's dc resistance, while the substrate and skin effect are less
pronounced at the NMR frequency.
8.2.3
Transmitter with Proton Filter
The power amplifier (PA) of the NMR transmitter in general needs to have a
large output power tuning capability in order to control the amount of energy
