[time-nuts] Data collection cycle -- multiple sources to one counter?

Dr. David Kirkby drkirkby at medphys.ucl.ac.uk
Tue Jun 21 06:46:56 EDT 2005

Brooke Clarke wrote:
> HI David:
> A PIN diode is great for switching RF frequencies above the diode's 
> lifetime.  But not so good at switching DC coupled signals.
> The FET type switches, like the MAX458 are designed for switching DC 
> coupled signals.
> Have Fun,
> Brooke Clarke, N6GCE

Minicicurits have GaAs switches too, which are DC coupled. Some of them have an minimum isolation of 
80 dB (100dB typical) in the range DC-10MHz, but work to 5GHz. But they only have SPDT. The SP4T ones 
are DC-3 GHz and have a somewhat lower isolation. To get more than 4 positions, you would need to 
combine them.

I would have *thought* something that works to a high frequency (3~5GHz), must have a small area, and 
so the delays repeatable. The MAX458 is speced to only 100MHz, and has internal buffers, so my guess 
is that it would be less repeatable than a GaAs switch that works from DC to 3GHz. I'm only basing 
this on eductated gueses, not practical measurements.

Many of the High Electron Mobility Transistors (or whatever you want to call them, as different 
manufacturers call them different things), have maximum frequencies of operation well in excess of 
20GHz. These are tiny so its hard to see how you could get much variation of time delay through the 
channel. However, since they have high gain from DC to light, they can be difficult to keep stable in 
low frequency applications.

I don't have shares in Minicircuits.

Dr. David Kirkby PhD CEng MIEE,
Senior Research Fellow,
Department of Medical Physics,
Mallet Place Engineering Building,
Gower St,
University College London,
London WC1E 6BT.

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