# The Cryo-Harness Technology for the Athena X-IFU Detection Chain

H Geoffray, J Adams, H Fossecave, E Cucchetti, E Bellouard, P Peille, C Daniel, O Maisonnave, E Guilhem, A Jolly, et al.

► To cite this version:

H Geoffray, J Adams, H Fossecave, E Cucchetti, E Bellouard, et al. The Cryo-Harness Technology for the Athena X-IFU Detection Chain. Journal of Low Temperature Physics, 2024, 215 (3-4), pp.177-183. 10.1007/s10909-024-03082-5. http://dx.doi.org/10.1007/s10909-024-03084-9

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# The cryo-harness technology for the Athena X-IFU detection chain

**H. Geoffray**

herve.geoffray@cnes.fr

Centre National d'Etudes Spatiales (CNES)

**J. Adams**

NASA-GSFC

**H. Fossecave**

Cap Gemini

**E. Cucchetti**

Centre National d'Etudes Spatiales (CNES)

**E. Bellouard**

Centre National d'Etudes Spatiales (CNES)

**P. Peille**

Centre National d'Etudes Spatiales (CNES)

**C. Daniel**

Centre National d'Etudes Spatiales (CNES)

**O. Maisonnave**

Centre National d'Etudes Spatiales (CNES)

**E. Guilhem**

Cap Gemini

**A. Jolly**

Centre National d'Etudes Spatiales (CNES)

**I. Maussang**

Centre National d'Etudes Spatiales (CNES)

**B. Jackon**

SRON Netherlands Institute for Space Research

**S. Bandler**

NASA-GSFC

**B. Doriese**

NIST Boulder

**M. Durkin**

NIST Boulder

**J. Kuur**
**Research Article**

**Keywords:** X-ray TES readout, cryogenic harness, TDM readout

**Posted Date:** December 13th, 2023

**DOI:** https://doi.org/10.21203/rs.3.rs-3547355/v1

**License:** © This work is licensed under a Creative Commons Attribution 4.0 International License.

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**Additional Declarations:** No competing interests reported.

**Version of Record:** A version of this preprint was published at Journal of Low Temperature Physics on April 27th, 2024. See the published version at https://doi.org/10.1007/s10909-024-03082-5.
## Abstract

CNES (French Space Agency) is in charge of the development of the X-ray Integral Field Unit (X-IFU) instrument for Athena, the high resolution X-ray spectrometer of the ESA Athena X-ray Observatory. X-IFU will deliver spectra from 0.2 to 12 keV with a spectral resolution in the range of 2.5 eV up to 7 keV on a 5" pixels, with a field of view > 4' equivalent diameter. The main sensor array detection chain is a key part of the instrument, being by far the main contributor to its performance. It involves major partner: NASA GFSC, SRON, VTT, APC, and IRAP. The cryo-harness interconnecting the Focal Plane Assembly (FPA) cold interface to the Warm Front End Electronics (WFEE) is under CNES responsibility. The different technical solutions are the loom technology and the shielded twisted pair technology. Electrical characterizations have been performed on breadboards to assess the crosstalk performances for each solution. The results of these analysis are a driver to perform the trade-off between the cryo-harness technologies.

## 1 Introduction

The X-ray Integral Field Unit (X-IFU) instrument is the high resolution X-ray spectrometer of the ESA Athena X-ray Observatory. X-IFU will deliver spectra from 0.2 to 12 keV with a spectral resolution in the range of 2.5 eV up to 7 keV on a 5" pixels, with a field of view > 4' equivalent diameter [1] [2]. The detection chain conceptual design for X-IFU discussed in a previous paper [3] have evolved in the past few years in order to secure the performances and development costs. New TES pixels will be implemented with slower time constant and a reduced sensitivity to magnetic field. This directly leads to a reduction of the number of channels and increase of the MUX factor. The number of close electronics boxes, or warm front end electronics (WFEE) has also decreased, as the number of close electronics boxes has reduced to 2 electronics boxes. The sub-kelvin cooler interface, which handles cold electronics, the second stage SQUID, is a 4 K interface. The cryostat outer vessel is a 50 K thermal interface, cooled passively thanks to L-shaped thermal shield (L-grooves).

The cryo-harness is a key contributor the X-IFU detection chain performance. The cryo-harness is implemented between the 4 K interface of the Focal Plane Assembly (FPA) (see [4] for a detailed description) and the warm interface (around 300K), at WFEE side (see [5] for a detailed description). The new Athena concept implements two scientific harnesses, each of those handling 32 channels. For each channel, different type of signal transit in the harness: the bias signals and the "clocking" or dynamic signal necessary to operate the cold electronics in Time-Domain Multiplexing (TDM). The output signal is also part of the bundle. The dynamic signal are set at 6.25 MHz, with rising/falling edges with 10-20 ns settling time. Those signals, together with the scientific signal, are very sensitive to electrical crosstalk.

The new Athena concept also induced a potential increase of harness length, increasing the risk of electrical crosstalk. It also requires specific EMI/EMC design implementation, in order to prevent any performance degradation, as the harnesses are directly exposed to electromagnetic fields between the 4K and the 300 K interfaces.
## 2 The cryo-harness technology trade-off

The cryo-harness technology aimed for X-IFU should combines very stringent requirements. Thermalization stages are implemented between the 4K interfaces and the 300 K interface. The cryo-harness technical solution choice is strongly related to the thermalization capability, while preserving signal integrity and EMI/EMC susceptibility. Signal integrity is of primary concern, as TDM architecture translates in fast signals with high frequency components due to fast rising and falling edges. This in turns translates in strong electrical requirements. Assembly constraints and testability need to be taken into account for the cryo-harness technology choice. Two main technologies have been considered for the X-IFU detection chain. The first technology is the twisted shielded pair, which has been used for the Planck instrument. The second technology is based on loom technology. Although the first one is basically more immune to crosstalk effect and provide a potential efficient shielding, the second one presents the advantage of being a potential better candidate for integration and thermalization aspects, as the number of readout signal (directly proportional to the number of readout channels) is high given the X-IFU detection chain number of pixels. As we have identified the drivers for cryo-harness technology choice, we need to perform a trade-off between the two solutions, bases on electrical characterizations on breadboards to assess the crosstalk performance for each solution, the demonstration of the thermalization performance, and an EMI/EMC characterization and design optimization.

## 3 The electronics breadboarding

### 3.1 Laboratory set-up

Electronics breadboarding combines the three critical elements. The focal plane cold electronics interface simulator, which implements both passive components and active components to amplify (with a factor ~1000) the signals. Amplification is necessary to simulate output (scientific) signal. It also allows to perform the crosstalk measurement. The WFEE electronics simulator, with representative interface including buffers, based on the RHF200 component. The cryo-harness simulator between the focal plane interface and the WFEE interface. This set-up has several purposes. The primary purpose is to assess the crosstalk performance of the cryo-harness solution, by measuring crosstalk between wires. The crosstalk is measured between a signal line (active) and a passive signal line. Measurements have been performed on both sides, on the focal plane simulator board and on the WFEE simulator board. Both loom technical solution and shielded twisted pair have been tested. The results obtained with the loom technology are presented in this paper. Secondary purposes of the electronics set-up is to validate the design choices and definition of the electrical interfaces on the WFEE side. Buffer implementation and associated performances can be assessed. Finally, focal plane passive electronics breadboarding will also allow to establish a definition for a tooling interface with representative load, in the frame of the warm electronics testing. The last objective is to correlate obtained results with simulations.

Electronics breadboarding is displayed on Fig. 1. Specific care has been taken for the design of the electronics boards. The leads geometry obeys to specific topology to limit crosstalk between PCB leads
and implementation for small connectors to prevent crosstalk at connectors level, in order to be able to measure intrinsic loom crosstalk. The loom definition is compliant with the current instrument design (Gauge xx, distance between pairs around xx). An isolation signal pair is implemented between each signal pair (twisted pairs). Constantan loom have been selected in order to have a serial resistance close to the final flight harness. Two looms variant have been tested with total length of 2 m or 3 m, with impedance characteristics $Z_0 = 40 \Omega / Z_{series} = 66 \Omega/m$. The electrical interface definition is co compliant with specifications (differential architecture, representative interfaces on both side at focal plan level and WFEE level),

Test plan has included tests with practical objectives, from RHF200 buffer characterization, electronics board tuning, impact of the interface on the signals, constantan loom characterization (sensitivity to harness length, loom stacking performance, ...).

Criteria to assess the performance rely on signal integrity. Signal integrity parameters translate into rising/falling edge time characterization, characterization of the crosstalk of the signals, damping of over/undershoots transients. Electronics definition includes impedance matching to avoid signal propagation associated signal degradations.

## 3.2 Main results

Measured signal are displayed on Fig. 2. Waveforms, amplitude and delay are consistent with the simulation. The breadboard provides a very stable and reproducible results, which assess the compliance with respect to the requirements. As a worst case, falling and rising edge have been set to 8 ns at WFEE board input. The bandwidth of the signal is measured at different levels and is consistent with simulations (~ 20 MHz). The set-up show no signal oscillations, which confirms that the resistive harness provides an efficient damping. In the 160 ns time period, the end of the settling transient needs to be stable, as readout requirements lead to sample the signal in the last 40 ns. The signal integrity results validates the interface choices at WFEE output.

Figure 3 shows the signal on an adjacent line to the signal line after amplification. The ratio between the two signals provide an estimate of the crosstalk at FPA input, of the order of 66dB. At WFEE input, the obtained crosstalk figure is ~ 77dB.

Measurements have been performed in the frequency domain, to corroborate the results with the crosstalk figure obtained in the time domain. Calculations have been conducted to verify the consistency of both set of data, e.g data acquired in the time and frequency domain. This was performed through a dedicated campaign, by injecting sine waves at selected frequencies (see Fig. 5). The crosstalk level has been measured both by comparing the peak amplitude of the victim line in time domain (orange line) and by comparing the ratio of the power spectrum densities (PSDs, blue line) of the input and victim signal at the investigated frequency value. A further comparison has been performed using a spectrum analyser on the set-up (green line). All three methods show excellent consistency over the 0.1–100 MHz range,
confirming the results obtained through simulation. Finally, a correlation with LTspice simulations have been performed which allows a rescaling of the parasitic capacitance with realistic values

# 4 Conclusions

The signal integrity has been assessed and provides results in line with requirements. The crosstalk between adjacent lines is compliant with the requirements (< - 70 dB or less according the definition). Vertical crosstalk between looms when stacked is of the same order of magnitude. Loom are a credible solution for X-IFU detection chain. Future work will focus on the definition of the interconnect between 4K and 300K, the implementation of grouping strategy for critical signals at each stage, and the definition of the implementation of EMI/EMC covers (tombacs, ...). Consistent results have been obtained in the time domain and frequency domain, which demonstrates that both test conditions can be used to assess the performance at electronics boxes subsystem level.

# References

1. D. Barret et al., Proc. SPIE. 9905 (2016), DOI

2. J.-W.A. den Herder et al., Proc. SPIE 12181 (2022), DOI

3. H. Geoffray et al., Proc. SPIE 12181 (2022), DOI

4. H.J. van Weers et al., Proc. SPIE 12181 (2022), DOI

5. M. Gonzalez et al., Proc. SPIE 12181 (2022), DOI

# Figures
<figure><img src="image_2.png" /><figcaption>Figure 1</figcaption></figure>

Implementation or the electronics breadboard. Left, FPA board and Right WFFE board. (Color figure online)

<figure><img src="image_3.png" /></figure>
Figure 2

Yellow, input signals. Blue, WFEE output. Pink, FPA input. Green, reinjected input at WFEE side. (Color figure online)

<figure><img src="image_4.png" /></figure>

Figure 3

Yellow, signals at WFEE board input. Blue, WFEE board output. Pink, crosstalk (first neighbour). Green, reinjected input at WFEE side. (Color figure online)

Image not available with this version

Figure 4

Legend not included with this version
<figure><img src="image_5.png" /><figcaption>Figure 5</figcaption></figure>

Orange, measurement of the crosstalk in adjacent line as a function of frequency (same as Fig. 4).
Blue, crosstalk rejection obtained through PSD ratio of the input measurement. Green, spectrum analyser
measurements of the crosstalk rejection. (Color figure online). Violet (dashed lines), LTspice simulations