US2012166117A1PendingUtilityA1
Method and apparatus for evaluating superconducting tunnel junction detector noise versus bias voltage
Est. expiryOct 29, 2030(~4.3 yrs left)· nominal 20-yr term from priority
G01R 29/26
47
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Claims
Abstract
A technique for characterizing the noise behavior of a superconducting tunnel junction (STJ) detector as a function of its applied bias voltage V b by stepping the STJ's bias voltage across a predetermined range and, at each applied bias, making multiple measurements of the detector's current, calculating their mean and their standard deviation from their mean, and using this standard deviation as a measure of the STJ detector's noise at that applied bias. Because the method is readily executed under computer control, it is particularly useful when large numbers of STJ detectors require biasing, as in STJ detector arrays In a preferred implementation, the STJ is measured under computer control by attaching it to a digital spectrometer comprising a digital x-ray processor (DXP) coupled to a preamplifier that can set the STJ's bias voltage V b using a digital-to-analog converter (DAC) controlled by the DXP. An on-board digital-signal-processor in the DXP is then programmed to implement the technique by stepping through a sequence of bias voltages using the DAC and, for each voltage, capturing 1000 values of the detector current I d from the DXP's baseline filter and then computing their mean <I d > and standard deviation σI d . Minima in the plot of σI d . vs V b are shown to correspond to minima in the plot of detector energy resolution vs V b , allowing a plot or table of σI d . vs V b values to be used to locate an optimum value of V b for the detector operating point.
Claims
exact text as granted — not AI-modified1 . A method for generating a noise curve to characterize the noise in a superconducting tunnel junction (STJ) detector as a function of its applied bias voltage by:
stepping the STJ's bias voltage V b across a range of bias voltages, and, at each step i,
making multiple measurements of the current I d flowing through the STJ detector,
calculating their mean, <I d >,
calculating their standard deviation σ d from the mean <I d >, and
recording at least some of the pairs of values {V b , σI d } i so determined, as the said noise curve.
2 . The method of claim 1 where, with each pair of values {V b , σI d } i recorded, the associated value <I d > i is also recorded.
3 . The method of claim 2 where the measurement instrument is DC coupled to the STJ detector so that the recorded {V b , <I d >} i pairs constitute a representation of the STJ's I-V curve.
4 . The method of claim 1 where the noise curve is examined to locate regions of low noise or local minima and one of these is selected as a operating point for using the STJ as a detector.
5 . The method of claim 4 where the noise curve is compared to a curve of detector resolution versus applied bias voltage in order to determine which of said located regions will provide the best operating point.
6 . The method of claim 1 wherein making a measurement of the current I d includes applying a filter to I d .
7 . The method of claim 6 , wherein the filter is a digital filter of the form
F
i
(
I
)
=
∑
j
=
i
-
K
i
I
j
-
α
∑
j
=
i
-
2
K
i
-
K
-
1
I
j
where F i (I) is the filter output at time step i, α is a constant, and the I j are digitized values of the current I d at earlier time steps j.
8 . The method of claim 1 wherein the measurements of the current I d are made using electronics attached to the STJ that include a preamplifier that sets the bias voltage V b on the STJ using a digital-to-analog converter.
9 . The method of claim 8 wherein the said electronics also include a digital processor that can control the bias voltage V b on the STJ by sending data to the said digital-to-analog converter.
10 . The method of claim 1 wherein a test is added to assure that said measurements of I d are only made when the STJ is in quiescent mode and not emitting a current signal pulse due to absorption of energy from a photon or particle.
11 . A method for generating a noise curve to characterize the noise in a superconducting tunnel junction (STJ) detector as a function of its applied bias voltage by:
attaching to the STJ a preamplifier that can adjust the STJ's bias voltage V b by means of a digital-to-analog converter; attaching to the preamplifier a digital x-ray processor (DXP) that receives the preamplifier's output and can transmit data to its digital-to-analog converter; providing processing logic in the DXP that, on command, can capture a digitized value FI of the preamplifier's output I d ; providing control logic in the DXP that, for each member V b,i in a sequence of bias voltage values: sets the bias voltage value V b,i on the STJ using the analog-to-digital converter; causes a set of values of FI to be captured; averages the values of FI to obtain their mean <FI>; computes the standard deviation σFI of the set of FI values about <FI>; and stores the values of V b and σFI to produce said noise curve.
12 . The method of claim 11 wherein, when the DXP stores the values of V b and σFI, it also stores the value of <FI>.
13 . The method of claim 12 wherein both the preamplifier and DXP are DC coupled, so that the stored sets of V b and <FI> values constitute a representation of the STJ's I-V curve.
14 . The method of claim 11 wherein capturing said value FI includes applying a digital filter to the preamplifier's output.
15 . The method of claim 14 wherein the digital filter has the form
F
i
(
I
)
=
∑
j
=
i
-
K
i
I
j
-
α
∑
j
=
i
-
2
K
i
-
K
-
1
I
j
where F i (I) is the filter output at time step i, α is a constant, and the I j are digitized values of the preamplifier's output at earlier time steps j.
16 . The method of claim 11 wherein said DXP control logic is invoked by an external computer and generated pairs of V b and σFI values are transmitted to the external computer.
17 . The method of claim 11 where the DXP processing logic also assures that the DXP is not also processing an STJ output signal pulse at the time of the capture of the value FI.
18 . Apparatus for generating a noise curve to characterize the noise in a superconducting tunnel junction (STJ) detector as a function of its applied bias voltage, the apparatus comprising:
a preamplifier, for connection to the STJ, that can adjust the STJ's bias voltage V b by means of an associated digital-to-analog converter; a digital x-ray processor (DXP), connected to the preamplifier, that receives the preamplifier's output and can transmit data to its associated digital-to-analog converter; processing logic in the DXP that, on command, can capture a digitized value FI of the preamplifier's output I d ; control logic in the DXP that, for each member V b,i in a sequence of bias voltage values:
sets the bias voltage value V b,i on the STJ using the analog-to-digital converter;
causes a set of values of FI to be captured;
averages the values of FI to obtain their mean <FI>;
computes the standard deviation σFI of the set of FI values about <FI>; and
stores the values of V b and σFI to produce said noise curve.
19 . The apparatus of claim 18 wherein, when the DXP stores the values of V b and σFI, it also stores the value of <FI>.
20 . The apparatus of claim 19 wherein both the preamplifier and DXP are DC coupled, so that the stored sets of V b and <FI> values constitute a representation of the STJ's I-V curve.
21 . The apparatus of claim 18 wherein capturing said value FI includes applying a digital filter to the preamplifier's output.
22 . The apparatus of claim 21 wherein the digital filter has the form
F
i
(
I
)
=
∑
j
=
i
-
K
i
I
j
-
α
∑
j
=
i
-
2
K
i
-
K
-
1
I
j
where F i (I) is the filter output at time step i, α is a constant, and the I j are digitized values of the preamplifier's output at earlier time steps j.
23 . The apparatus of claim 18 wherein said DXP control logic is invoked by an external computer and generated pairs of V b and σFI values are transmitted to the external computer.
24 . The apparatus of claim 18 where the DXP processing logic also assures that the DXP is not also processing an STJ output signal pulse at the time of the capture of the value FI.Join the waitlist — get patent alerts
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