32 Bit Digital SQUID Electronics - STL Systemtechnik - #1

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32 Bit Digital SQUID Electronics DSE32-3 ® GradMag The digital SQUID electronics control system DSE32 is a new development allowing for multi-channel SQUID data collection. The electronics in combination with the software offers a powerful and easy SQUID control including autocalibration for both high-T and low-T SQUIDs. The c c combination of a large 5 MHz bandwidth, an extraordinary 20 M /sec slew rate, a basically 32 bit 0 dynamic range, and a high >140 dB linearity grants a wide spectrum of system applications especially in unshielded environment. STL's bidirectional low-noise fiber-optic interface (BiFOC) with a data rate of 2 MBytes/s offers sampling rates of more than 100 kHz with 3 channels simultaneously running. In special cases like logarithmic sampling a maximum sampling rate of 10 MHz can be used. The low power consumption results in a low noise contribution and long lifetime battery operation. Due to the phase synchronized fiber-optic network a multi-channel system can be organized with up to several hundred simultaneously triggered channels. The electronics may be run by a notebook using the unique fiber-optic PCMCIA card under various operating systems. ® GradMag F Principle The digital SQUID electronics uses the SQUID itself as an integral part of the analog-to-digital conversion. The SQUID's transfer function being exactly periodic in the physical constant allows to increase the reliable number of bits compared to 0 conventional ADCs without increasing the noise: -Within one -period of the transfer function the point of 0 operation of the electronics is stabilized by a flux-locked loop (FLL). Conventional ADCs are used to digitize the feedback voltage to the relevant bits inside the -period. 0 - Controlled jumping between different points of operation and up or down counting of the -steps yield the additional 0 theoretically error free bits. The combination of both bit sections forms the digital output signal which is valid up to 32 bits. The linearity, however, depends strongly on the SQUID used . F F F F low-T SQUIDs. Both the typical high-T bias-reversal and the c c low-T modulation mode are implemented and can even be used c simultaneously. The built-in autocalibration mode finds all necessary parameters of the SQUIDs automatically. Only one button away are the noise optimized bias current and bias voltage, the phase shift in the bias-reversal mode and the reverse-gain-error describing the scaling error between the physically scale and the electronically defined flux scale 0 within the period. Manual adjustment is possible in an expert 0 mode. Temperature drifts are minimized by an intrinsic digital temperature compensation based on temperature measurement. Currents of the power supply or LED used for the fiber-optic data link are very critical potential sources of magnetic noise. Therefore, the power consumption has been reduced to less than 1W per channel thus minimizing magnetic noise and making battery operation possible. Special care has been taken to avoid the fiber-optic link to become a noise generator by implementing a sophisticated transfer protocol which is inactive in the frequency band of interest. A baud rate of 20 MHz is used to get a data rate of 2 MB/s. Multi-channel systems can easily be built up using a cascaded network of fiber-optic routers with four inputs and one output. These routers, available with STL’s DLM24 data monitoring system, may perform data reduction and include up to 2 GB of memory. Exactly simultaneous triggering is vital for software gradient formation with fast signals. It is implemented by using the 20 MHz clock of the fiber-optic network which is phase synchronized by a special master / slave concept to derive the trigger signal. F F Implementation Large bandwidth paired with high sensitivity was the principle design goal. Hence the preamplifier forming the first stage of a combined analogue and digital FLL features a bandwidth larger than 5 MHz and an input noise of approximately 400 pV/ÖHz. Every time when the flux fed back to the SQUID exceeds 1 , 0 the FLL is opened for a short moment to permit the point of operation to slip one or more -periods. This happens 0 extremely fast and allows for a slewrate of more than 20 million /sec. 0 The DSE32 SQUID electronics may be used with High-T or c F F F STL electronics: noise measurement on a High-Tc SQUID (Forschungszentrum Jülich, Faley). The measurement was performed with a bias reversal frequency of 100 kHz. White noise: 6.5 fT/ÖHz, noise @ 1 Hz: 30 fT/ÖHz. in the new Berlin Magnetically shielded room field amplitude rms · ÖHz 100 fT 10 fT 2 fT 5 fT 20 fT 50 fT 1 Hz 10 Hz 100 Hz 1 kHz frequency white noise: 6.5 fT/ÖHz High-Tc SQUID fiber- optic plastic cable DC-power cable RF - tight housing ® GradMag channel 1 channel 2 channel 3