Study on Noise Matching Between SQUID Sensor and Its Readout Electronics

Noise matching between superconducting quantum interference device (SQUID) sensor and readout electronics has always been an important problem. Conventionally, in order to avoid the hysteresis, SQUIDs should be operated in the range of β
_c
<; 1, where β
_c
is the Stewart-McCumber parameter. Recently, Liu et al. extended the SQUID operating area to βc > 1 due to a large noise parameter Γ*. When β
_c
> 1, SQUID exhibits a large ∂V/∂Φ, which is beneficial in reducing the noise contribution of the readout electronics, δΦ
_e
. In this work, we compare the SQUID system noise δΦ
_s
measured with four different readout electronics. Here, the four readout electronics employed are as follows: 1) the flux modulation scheme (FMS); 2) the direct readout scheme (DRS), which uses six parallel connected bipolar transistor as its preamplifier (DRS-PCBT); 3) the DRS, which uses a single preamplifier AD797 (DRS-AD797); and 4) the two-stage scheme. The preamplifier voltage noise of DRS-PCBT and DRS-AD797 are about 0.4 nV/√Hz and 0.9 nV/√Hz, respectively. For a two-stage scheme, the equivalent flux noise reaches about 0.25 μΦ
₀
/√Hz, the measured SQUID intrinsic noise, δΦ
_i
, will dominate δΦ
_s
. In other readout schemes, however, δΦ
_s
depends on the value of β
_c
. Finally, we found that FMS is suitable to match SQUIDs with β
_c
<; 1. DRS-PCBT is a good choice for SQUIDs with 1 <; β
_c
<; 3 whereas DRS-AD797 suffices for SQUIDs with β
_c
> 3.