Multiple-Tuned Narrow Band Preamplifier
G.X. Shen
MR Research Center, Department of Radiology
University of Pittsburgh, Pittsburgh, USA
Introduction
Most MRI scanners equipped with spectroscopy
capability and MR spectrometers use wide band
preamplifiers to cover a wide range of different nuclear
resonant frequencies. However, the major disadvantage
of using a wide-band preamplifier is its low gain and
higher noise figure. We have designed a quadrupletuned narrow-band preamplifier (0.5 dB noise figure)
which can improve the SNR by about 20% compared to
the wide-band preamplifiers (1.2 dB noise figure).
Methods
One of the most important factors for the design of
a low noise figure preamplifier is the input resonant
circuit at the first stage. We designed a high Q and
low-loss matching circuit Due to the very small signal
(level (mV) at the input to the preamplifier, commercial
tuning diodes (varactors) with very high capacitance
ratio (M/A-COM, Inc.) can be used to design ,the
multiple-tuned preamplifier. The schematic diagram of
a multiple-tuned preamplifier with low noise figure is
shown in Fig. 1.
VariableDC
t
AC coupling
Fig. 1 Diagram of a multiple -tuned narrow band
preamplifier using varactor through different DC bias.
A GaAs tuning varactor with very high Q and
capacitanceratio, MA46H204 was chosen (M/A-COM,
Inc., Lowell, MA, USA). This varactor has a
capacitanceratio of 10.0 and Q of 1,500 at an operating
frequency of 50 MHz and reverse biased voltage of 4
V. A RF choke (4.7 mH) was used in the circuit for DC
voltage bias. The variable capacitor is used for
adjusting the frequency and is rated from 0.8-8 pF.
The bandwidth of this tuning circuit depends on the
Q of the varactor, the capacitor and the inductor at the
input of the preamplifier. Normally, the Q of the
capacitor is very high, thus improving the Q of the
inductor is critical for the design of this narrow band
preamplifier. The inductor is wound on a Teflon
cylinder that is 70mm in length and 17mm in diameter.
The inductor uses 16AWG wire and has 15 turns to
approach 1.2 PH inductance.
Because the SNR of a preamplifier is usually
dominated by the first stage (2), the second stage was
designed as a normal wide band amplifier to cover all
designed RF frequencies.
Results and Discussion:
The prototype of a quadruple-tuned preamplifier has
been implemented and tested in our electronic lab.
Each nuclear frequency can be pre-tuned through DC
bias and controlled by a modified pulse sequence,thus
easily optimizing their performance. The gain of the
preamplifier for all nuclei was measured as $1 by a
Network Analyzer, HP 8753B, and noise figures
measured by Noise Figure Meter, HP 8970B. The
measured data at 1.5 T are listed in Table 1.
1H
39.3
10.7
19F
7Li
31P
38.8
35.2
34.7
Gain (dB)
10.5
8.8
9.2
Bandwidth (MHz)
0.47
0.46
0.48
0.49
Noise Figure (dB)
Tablel. List of gain and noise figure of different
nuclei for the multiple pre-tuned narrow band
preamplifier.
For this quadruple-tuned application, the tuning of
preamplifier can be tirther simplified, because the
frequencies of 1H (63.7 MHZ) and 19F (60.2 MHz) are
so close, as are those of 7Li (24.8 MHz) and 31P (25.9
MHz); thus, 10 MHz bandwidth can cover both of them
without significantly loss of gain (less than 3%).
Summary:
A prototype of a multiple-tuned narrow band
preamplifier demonstrates high gain and low noise
figure over a wide range of frequencies. This design can
be used to improve SNR for high resolution MR and
multi-nuclear MRI/MRS
Reference:
1. RF and Microwave Semiconductors, p. 4-23, 1996,
MA-COM and AMP Company, MA 0 1853, USA.
2. Dorf R.C., Electrical Engineering Handbook, CRC
press, Inc., 1993.
Acknowledgments:
This work was supported by Whitaker Foundation,
Public Health Service grant POlNS35945 and GE
Medical Systems.