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Gas-tube spark arrestor as RF pulse-modulator?!

I find some SMT mystery components on a few-watts RF pulse board for an NMR probe driver. Little white ceramic cylinders with gold end caps, maybe 0.1" diameter. But they're wired *across* a 50-ohm pcb trace, and driven by some 500V FETs via a resistor chain and one HV capacitor. WTF?

Huh. What if these are spark gaps? If the designer chose several different voltages, then one DC pulse could switch the 800MHz signal through and back out. A higher DC pulse would short the RF to ground through a different spark gap. But why not just modulate the RF at signal level using diode switching?

Reply to
Bill Beaty
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What's the current rise-time for a GDT spark arrestor? Sub-nanosec?

Found it! The mystery SMT components look just like this:

Surge Arrestor 90V SMD

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Reply to
Bill Beaty

A single coil NMR? (Where the same coil does the drive and detection.)

I so then they could be to protect the detctor amp during the RF pulse. You can get some wicked voltages during the pulse. An early NMR coil/ amp that I made would sometimes arc over to a near by ground.

George H.

Reply to
George Herold

As I understand NMR, there are some really large magnetic fields involved. Induced across a wire, they can create a rather large voltage spike. If the input impedance needs to be fairly high, a spark gap is about the only way to protect the components against over-voltage. They may also be there to prevent the 500v FET's from generating a rather high voltage pulse, which might blow up whatever is connected downstream.

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Jeff Liebermann     jeffl@cruzio.com 
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Jeff Liebermann

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