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

Started by Bill Beaty March 15, 2013
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?
On Mar 15, 11:46=A0am, Bill Beaty <bi...@eskimo.com> wrote:
> I find some SMT mystery components on a few-watts RF pulse board for
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 http://www.digikey.com/product-detail/en/CG590MS/F2748CT-ND/950274
On Mar 15, 2:46=A0pm, Bill Beaty <bi...@eskimo.com> wrote:
> I find some SMT mystery components on a few-watts RF pulse board for > an NMR probe driver. =A0Little white ceramic cylinders with gold end > caps, maybe 0.1" diameter. =A0 But they're wired *across* a 50-ohm pcb > trace, and driven by some 500V FETs via a resistor chain and one HV > capacitor. =A0 WTF?
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.
> > Huh. =A0 What if these are spark gaps? =A0 If the designer chose several > different voltages, then one DC pulse could switch the 800MHz signal > through and back out. =A0A higher DC pulse would short the RF to ground > through a different spark gap. =A0But why not just modulate the RF at > signal level using diode switching?
On Fri, 15 Mar 2013 11:46:51 -0700 (PDT), Bill Beaty
<billb@eskimo.com> wrote:

>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?
<http://www.littelfuse.com/products/gas-discharge-tubes.aspx> 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. -- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558