Biomedical Engineering Reference
In-Depth Information
Fig. 8.5
Transmitter chain and power tuning scheme (From Ref. [ 3 ])
that goes into the protons during the NMR excitation phase. The transmitter in
the 2-kg portable system did not integrate a PA, since meeting the power tuning
requirement was not trivial with an integrated PA. The new transmitter in the
palm and 1-chip systems, which is shown in detail in Fig. 8.5 , integrates the entire
front-end transmitter chain, including a PA. We manage to tune the PA's output
power by exploiting the proton's natural high-Q (
10 4 ) filtering ability.
To start with, the PA is realized as a differential chain of cascaded four inverter
stages (Fig. 8.5 , bottom right). The inverters are consecutively quadrupled in size
to sequentially amplify power and ensure drivability at the output. This class-D
arrangement is simple to design and does not consume static power, but it produces
a square wave output with fixed voltage amplitude of the power supply V DD , thus,
calling for a technique to tune its output power.
To this end, we tune the duty cycle of the transmitted signal. A given transmitted
square wave (frequency: ! 0 ; amplitude: V DD / with a specific duty cycle (Fig. 8.5 ,
top) assumes a particular power distribution of the fundamental tone at ! 0 and
higher-order harmonics. The power distribution over the harmonics varies with the
duty cycle. Here we only need to look at the variation of the power at ! 0 with
the duty cycle, for higher-order harmonics lie outside the “proton filter” band:
protons are a high-Q (
10 4 ) band-pass filter centered at ! 0 , in the sense that they
are not excited by signals that lie outside the frequency band. As the duty cycle
is altered from 0 % to 50 %, the ! 0 -component changes its voltage from 0 to
.4=/ V DD (Fig. 8.5 , top right). This effectively corresponds to the output power
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