RE: AMMRL: Gradient current leakage and shimming...

From: Richard Shoemaker <Richard.Shoemaker_at_Colorado.edu>
Date: Tue, 12 Aug 2008 13:00:45 -0600

Dear AMMRL folks,

Regarding the issue of gradient "off" current affecting shimming, just to
add a few more details on the subject based on Josh's comments:

This is often (if not always) a problem with gradient amplifiers. Varian
has provided a solution to that for many years, which is a little box with
diodes in it to cut-off current leakage after the output voltage drops below
the turn-on voltage of the diodes. Most Varian system w/ gradients shipped
with a little silver-box with XLR (micrphone/gradient) connectors on it....
that's what this box is used for.

Since the introduction of the cold-probe, the "new" version of this box has
two switches called "Deadband" and "Damping". The Deadband serves the
purpose mentioned here (to eliminate current leakage w/ the gradients off),
and the Damping is to slow/dampen the slew-rate of current rise to prevent
gradient ringing.

With such a passive diode-switch on the output of the gradient amplifier,
there should be no difference in shimming Z1 with the amplifier on or off,
provided the gradient amplifier is not too far out of balance.

Cheers,

-Rich Shoemaker

---
Richard K. Shoemaker, Ph.D., Director, NMR Spectroscopy Facility
University of Colorado at Boulder
Phone:  (303) 492-7062	Fax: (303) 492-5894
E-Mail: Richard.Shoemaker_at_Colorado.edu    
-----Original Message-----
From: Josh Kurutz [mailto:jkurutz_at_uchicago.edu] 
Sent: Tuesday, August 05, 2008 6:34 PM
To: Xi An Mao
Cc: <ammrl_at_ammrl.org>
Subject: Re: AMMRL: Cryoprobe tuning problem and gradient dirturbing problem
Xi-an Mao,
Regarding your lock instability, I suspect the gradient amplifier/ 
power supply is the problem. We had the same phenomenon on our Varian  
Inova a little while back.
When on, the gradient amp supplies a small current to the probe's  
gradients, which then act like shims. For fun, try shimming up a  
sample on a working system, then turn off the (z) gradient power. You  
should find that the lock levels falls and the lineshape gets  
distorted, but both can be restored by adjusting the z1 shim.
If your problem is like ours, your gradient's "off" current is  
unstable, effectively mimicking an unstable shim. Thus, your gradient  
amp needs service. If you're sending it in for repair, you'll need to  
be in close communication with your repair engineers. I got the  
impression this is a weird failure, and the amp may still pass all  
their normal tests.
Good luck!
Josh Kurutz, Ph.D.
Tech. Dir., Biomolecular NMR  Facility
University of Chicago
On Jul 29, 2008, at 10:09 AM, "Xi An Mao" <xmao_at_wfubmc.edu> wrote:
>
> Dear All:
>
> We have problems with our two Bruker cryoprobes. One probe does not  
> response to proton tuning/matching, but the other channels (13C, 15N  
> and 2H) work fine. The other probe has unstable lock level. One when  
> the gradient cable is disconnected can the lock level remain calm.
>
> I am wondering if anyone of you had similar problem and how did you  
> fix it. I appreciate all feedbacks.
>
> Best regards.
>
> Xi-an Mao
> Cleveland Center for Structural Biology
>

Received on Tue Aug 12 2008 - 09:01:07 MST