Does a coil also have a capaciotance?

In sci.electronics.design Okkim Atnarivik wrote: : In sci.electronics.design Spehro Pefhany wrote: : : On Fri, 13 May 2011 12:04:32 -0700 (PDT), the renowned Bill Sloman : : wrote: : : >No. Did have an interesting conversation with a couple of guys at an : : >Analog Devices presentation earlier in the week - they are developing : : >an infra-red sensing array which basically runs on the edge of super- : : >conduction, with super-conducting quantum interference devices to : : >amplify the sensor output. The whole thing is going into a satellite : : >with a closed-circuit liquid helium refrigeration system.

: : Some nasty vibrations from that cryocooler, I'll bet.

: Sounds like the SAFARI instrument for the SPICA mission. Actually, : EMI causes more headache than mechanical vibrations, currently.

To elaborate a bit: the final SQUID stage at 4K must drive several metres of cable before there is the first location in the spacecraft with enough cooling power to accommodate a decent semiconductor based amplifier. The 4K cooling budget does not allow very much signal power to be generated there, hence fighting to get enough SNR for the cable stretch with the presence of EMI is indeed a headache. I have been considering a 4K amplifier operating more deeply in D-class, or something with higher efficiency than a standard SQUID anyway. An intersting twitch is that the standard SQUID in a way

*does* resemble the D-class amplifiers: the signal is effectively pulse width modulated Josephson oscillation.

However, the schedule is so tight that there is not much opportunity to develop anything novel or unproven, so perhaps we'll go by the brute force and just shave the margins where-ever possible. A D-class CMOS amplifier

*might* do the job, but the handful off-the-shelf micropower CMOS comparators I tried failed a quick test at 4K. A test would require a full custom chip design it seems.

Regards, Mikko

Did you end up using the SiGe option?

Best regards, Spehro Pefhany

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By EMI I assume you are talking about 'cr@p' leaking in through the cable shield. This is probably silly, but could an active shield driven from the high temperature end help with EMI? I was playing around with a driven shield last year to help get the signal out. (reduce cable capacitance) So shield was driven by the signal. To shield from EMI can you drive the shield with a signal from the outside? Some sort of antenna? SQUIDS are pretty high frequency though, I was only doing

1MHz stuff.

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Yes. In the fine tradition going all the way back to Sophocles.

-- Paul Hovnanian mailto: snipped-for-privacy@Hovnanian.com

------------------------------------------------------------------ Human beings were created by water to transport it uphill.

In sci.electronics.design Spehro Pefhany wrote: : Did you end up using the SiGe option?

SiGe is the obvious choice for the cryogenic LNA at the 'warm' 110K end of the cable, but its efficiency when operated class-A causes too much dissipation at 4K. Making a class-B or class-D amplifier out of packaged SiGe discretes is probably too tedious, especially dealing with the stability issues and the dissipation budget when there are many active devices per an amp. Custom transistors or an ASIC would be conceivable, e.g. SiGe devices from the AMS SiGe BiCMOS process (available via MOSIS or Europractice) are known to function at 4K. Weinreb and his colleagues have recently fabbed cryogenic SiGe amps for the SKA using (I think) IBM SiGe process. A SiGe ASIC is probably a slow path, however, which is likely to take several design iterations. We have a superconducting foundry in-house, hence I'd prefer to keep the control in our own hands and try to manage with SQUID-related devices.

Regards, Mikko

In sci.electronics.design George Herold wrote: : By EMI I assume you are talking about 'cr@p' leaking in through the : cable shield. : This is probably silly, but could an active shield driven from the : high temperature end help with EMI? I was playing around with a : driven shield last year to help get the signal out. (reduce cable : capacitance) So shield was driven by the signal. To shield from EMI

We did consider guarding, but that becomes tough at high frequencies where the cable represents a (small) fraction of the wavelengths. Already a small phase shift between the EMI and the guard signal would cause untolerably strong coupling.

Regards, Mikko