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7), with three ground lines radiating from that central location: one to the TPW maintenance system vacuum Dewar, one to the battery box, and one to the top of the QVNS cryoprobe.
Recent precision JNT measurements using a QVNS reference have concentrated on determining the value of the Boltzmann constant by measuring the voltage-noise power spectral density of a resistor at the TPW.
We then apply this approach to measure the temperature of a sense resistor that is collocated with the JJ arrays on the QVNS chip and use the symmetry of the measurement channels to directly determine the undesired correlated noise in our JNT system, which is not related to Johnson noise.
1), and we similarly define voltage transfer functions [H.sub.A,Q](f) and [H.sub.B,Q](f) between the QVNS voltage source [V.sub.Q] and the input to the amplifiers.
It is also possible to include a similar term,[ .sub.Cn,Q], in the QVNS cross-correlation measurement, but we assume[ .sub.Cn,Q] = 0 because the JJ array acts as a low-inductance short that isolates the two measurement channels; removing this assumption results in more complicated equations but does not significantly change the conclusions herein.
In the case of the QVNS source, there is no information about the QVNS tones in the gaps between the tones, and therefore the bandwidth of evaluation is a single frequency bin.
To directly measure the presence of correlated noise, we modified the NIST JNT system to measure Johnson noise at 4 K from resistors collocated with the QVNS source (see Fig.
(4), requires subtraction of the measured Johnson noise [C.sub.R] from the measured cross-correlator at the QVNS tone frequencies [C.sub.Q] before taking the ratio
In practice, the inherent differences in the output impedance of the JJ arrays in the QVNS (which are purely inductive) and the Johnson noise source (which is purely resistive and 200 [ohms] in this experiment) and the required physical locations at cryogenic temperatures and in a TPW cell, respectively, make matching the transfer functions difficult.
, is to fabricate resistors on the QVNS chip to partly match the output impedance of the 200 [ohms] Johnson noise resistor.
The resistance of the on-chip QVNS resistors is slightly smaller than the Johnson noise resistor.
We then tuned the parallel matching capacitors on the QVNS leads.
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