Re: [AMMRL] Frequency ocillation in wobb curve on Prodigy TCI probe

From: Michael Groves via groups.io <mgrovesnmr=yahoo.com_at_groups.io>
Date: Thu, 30 May 2024 21:45:45 +0000 (UTC)

If it shows up with both the RF cables disconnected from the probe then I'm
less likely to suspect the cold preamps. The next thing I might look at is
one of the lock blanking signals--there's a blanking signal that goes from
the L-TRX through the preamps to the probe and is not passed through the
RF cables--so when the input was disconnected from the 2H preamp and the
signal persisted it may point to this.  If I recall correctly, this is
the pulse that switches the cold 2H preamp from transmit to receive mode.

I would reconnect the RF cables to the probe and preamp and then disconnect
the TP-F0 out from the L-TRX or the input from the PSM-A in the back of the
AQS rack.  That blanking pulse goes from the L-TRX to one the PSM-A
board in the back of the AQS rack and then through one of the cables that
goes to the preamp.  If I remember correctly.  If that pulse is
causing the issues then disconnecting the TP-F0 cable should stop the wiggles
in the tuning signal as well.  From there it might be a question of
tracking where exactly the cross-talk is happening.  

Cheers,Mike

On Thursday, May 30, 2024 at 01:58:09 PM MDT, Bob Berno <bberno_at_mcmaster.ca> wrote:

Hi Ken,

Okay, just grasping at straws, but now I think that you might be getting
some crosstalk within the preamp stack itself.  I don't know how this
can happen, but maybe a cable is loose or isn't properly grounded or something
like that. At the side of the preamp stack, there is a plastic cover on each
of the slices which you can easily pop-off.  Once you have popped off
those pieces of plastic, that will expose a ribbon cable and several other
wires connecting each preamp slice to each other. 

Take a photo before doing anything, so that you have a record of which cables
go where (it should be obvious, but you never know).  Carefully check
that all of the cables are properly seated.  Also, as you trace the
"daisychain" of cables, when you get to the last preamp slice you will probably
see 50 ohm dummy load resisters.  Make sure that these are properly seated. 

If it isn't a problem with the preamp stack next to the magnet, then maybe
it is a problem with the cold preamps in the probe.Let's hope that is not the
case.

Bob.

Dr. Bob Berno
Manager: McMaster NMR Facility
Department of Chemistry & Chemical Biology
McMaster University,  ABB-156
1280 Main Street West
Hamilton, ON   L8S 4L9
(905) 525-9140 ext:24158
Fax: (905) 522-2509
http://macs.mcmaster.ca

> From: main_at_ammrl.groups.io on behalf of Kenneth Sharp-Knott via groups.io
> Sent: Thursday, May 30, 2024 3:07 PM
> To: main_at_ammrl.groups.io <main_at_ammrl.groups.io>
> Subject: Re: [AMMRL] Frequency ocillation in wobb curve on Prodigy TCI probe


This last time I disconnected both cables from the front AND the cable going
into the back of the preamp.  No change.

Only when I set 2H lock to OFF in the acquisition parameters did the oscillation
disappear.

It is also present in the 15N wobb curve which is a separate channel on this
probe.  

At this point I'm not sure how the lock signal is getting mixed up with the
other channels.

Ken Sharp-Knott
Manager of Analytical Services and the NMR Facility
Department of Chemistry 
Virginia Tech
(540)267-6502 (Cell)
(540)231-0885 (Office)

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Received on Thu May 30 2024 - 14:45:57 MST

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