Biomedical Engineering Reference
In-Depth Information
V
maximum V/
Φ
(b)
(a)
Reference voltage
Bias Current
-V
r
Integrator
SQUID
Pre-Amp.
+
V
r
+
-
V
-
Output
-
+
+
L
f
R
f
(n+1/4)
Φ
0
flux
Φ
Fig. 3.13.
A schematic diagram of a DOIT circuit (
a
), and reference voltage ad-
justment on a
Φ
-
V
curve (
b
)
(
n
+1
/
4)
Φ
0
, as shown in Fig. 3.13b. The slew rate of a DOIT circuit is
limited by the time constant of the integrator.
The advantage of a DOIT is that the electronics is quite simple compared
to that of a flux modulation type. There is, however, a disadvantage, in that
that amplifier noise affects a SQUID directly. Especially at low frequency, the
1
/f
noise of the amplifier degrades the performance of the SQUID. DOIT
is useful for SQUIDs with a large
∂V/∂Φ
, which eliminates the amplifier-
attributable noise.
Increase of
∂V /∂Φ
: Additional Positive Feedback (APF).
As men-
tined in a previous section, the resolution of a practical SQUID is limited
mainly by preamplifier noise rather than the intrinsic noise of the SQUID.
However, if the transfer coecient
∂V/∂Φ
is increased, the amplifier-attrib-
utable noise
S
Φ
a
given by (3.46) can be reduced. Especially for SQUIDs ope-
rated with DOIT, amplifier noise is dominant - if
∂V/∂Φ
is small. D. Drung
et al. proposed that additional positive feedback (APF) is effective to obtain
Φ
-
V
characteristics with high
∂V/∂Φ
[13]. An equivalent circuit of APF is
shown in Fig. 3.14, where a resistor
R
a
and a inductor
L
a
are connected in
parallel to the SQUID loop as a APF circuit. Because a part of the bias cur-
rent
I
b
flows in the APF circuit, the inductor
L
a
generates a magnetic flux,
which couples to the SQUID through the mutual inductance
M
a
.
The flux induced in the APF circuit makes the
Φ
-
V
curve of the SQUID
asymmetric. The asymmetric
Φ
-
V
curve is calculated numerically, adding
the flux by APF
Φ
a
to the sets of equations indicated in a previous section,
where
Φ
a
is given by
V
R
a
.
Φ
a
=
M
a
(3.50)
Figure 3.15 shows the calculation results of the
Φ
-
V
curve of a SQUID with
an APF effect for various bias currents. The calculation parameters are
I
c0
=
0
.
1
μ
A,
L
sq
=20pH,
R
s
=1
Ω
,
R
a
= 100
Ω
,and
M
a
= 300 pH. The bias
