Multiplexing read out for SPICA TES microcalorimeter: FLL with Double Loop and Cryogenic feedback
Date Issued
2009
Author(s)
Abstract
The concept of the “Double Loop FLL” (DL-FLL) arises from the necessity to multiplex tens of
pixels, to properly work in the MHz range by applying the Frequency Division Multiplexing (FDM)
technique.
In the standard FDM technique the position of each pixel in a column is coded by biasing the pixels
at different frequencies. It is therefore clear that, for a given signal bandwidth, the number of pixels
will increase with the maximum bias frequency allowed by the system. On the other hand, the
utilization of SQUID as readout requires a feedback with substantial gain, in order to cope with the
intrinsic non linearity of these devices. The feedback return path through cables suffers from delays
(phase rotation), that have to be kept within safe margins, to avoid instability, i.e. negative feedback
turning into a positive feedback. In practice, for a given cable length, this effect limits the
maximum frequency, hence the number of pixels that can be multiplexed.
To circumvent this limitation, the Double Loop FLL implements a loop at cryogenic temperatures
at the typical frequencies of the signal. In this way, the cable length can be kept very short,
therefore recovering a large maximum frequency and number of pixels. A second feedback loop is
implemented at low frequencies (DC) for keeping the SQUID working point. This second loop can
therefore adopt long cables (warm) and its gain can be independently chosen at high values, for
maximizing the SQUID stability.
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