Dear Colleagues,
here come the collated, explicit responses we got for our inquire about
"Experiment with 2H decoupling and locking" on our INOVA spectrometer.
Thanks again to Steve Philson, Ghirmai H. Meresi, Rajan K. Paranji, Greg
Young, Karl M. Koshlap, Eugene F. DeRose, Toshi Nishida, and Jack W.
Howarth.
In our case, even after setting ampmode to 'dddp' we found a gating
hardware problem, which left the second decoupler open and did not gate
it even in the 'p' mode. Eventually, we tracked the problem down to a
bad contact at one of the chips on the the XMTR Control board.
Thanks again for all the help we received.
Ulli
________________________________________________________________
Original questions:
we are trying to do biochemical experiments with deuterium decoupling
and locking on our INOVA 500. We are using the diplexer provided by
Varian to combine the deuterium lock and decoupling rf (from a 3rd
transmitter channel) and the lock channel on the probe. If someone is
doing these experiments we would appreciate if you could get in touch
with us.
Our problem seems to be that already the noise level of the amplifiers
just being put into cw mode (not even putting out the decoupling rf)
makes the lock very noisy and decreases our lock level to a useless
level. Measuring on a scope we find that the noise level coming out of
the amplifiers to be about 87-90mV when initiated to cw mode, and about
nothing when set to idle or pulsed mode. The noise seems to be random
noise, but it has quite a component at the lock frequency (judged by
inserting the lock filter). Connecting the amplifier cable to the
diplexer, about 5mV survives back into the lock (receiver) side which
seems to be sufficient to saturate the lock receiver.
Specific questions therefore are:
- What noise level do you expect out of the amplifiers, when they are in
cw mode (no sequence running, no decoupling on)?
- How can we prevent the lock receiver to get saturated for this
experiment.
Again, if you are doing such experiments on a Varian System, we would
appreciate talking to you.
Thank you very much
Ulrike Werner-Zwanziger
_______________________________________________________________
From:
Steve Philson <philson_at_nmr.chem.umn.edu>
You might try a pair of in-line crossed diodes at the output
of the decoupler amp.
_______________________________________________________________
From:
Greg Young <cheech_at_med.unc.edu>
Greetings,
We do most of our deuterium decoupling with a deuterium decoupler
accessory, but one of
the users needs more deuterium power than the decoupler provides for
deuterium relaxation
studies. For that user we put deuterium on the third channel and cable
it to the lock
diplexer. If the third channel gets into cw mode it causes the system
to lose lock. The
deuterium relaxation sequences use explicit blanking and unblanking of
the third channel
during the sequence. The lock correction is held constant (with
lk_hold) before the
deuterium amplifier is unblanked and held until the acquisition is
finished.
The linear amplifiers are noisy because of the way they are biased for
linear operation.
The old Class-C amplifiers had almost no output when there was no signal
input. The
linear amplifiers are amplifying the noise that appears at their input
unless they are
blanked.
The only way I know of to maintain lock is to use lk_hold before
unblanking the amplifier
and reactivate the lock with lk_sample after the amplifier is blanked
and the lock
circuitry has settled.
If you need more information, contact me by phone or email.
Greg
_______________________________________________________________
From:
Eugene DeRose <derose_at_niehs.nih.gov>
Dear Ulrike,
I have run 2H decoupling experiments using our INOVA 600 and 500
spectrometers. Our 600 uses the lock transmitter and Varian's
2H decoupler amplifier for 2H decoupling. (Make sure the 2H decoupler
amplifier is installed correctly if this is your hardware
configuration.) Our 500 uses a 4th BB RF channel for 2H decoupling. In
both cases the diplexer should be connected with J1 to an
2H bandpass filter connected to the probe; J2 to the 2H decoupler
transmitter/amplifier; and J3 to the LOCK preamp. If you do not
have an 2H bandpass filter, you may see a significant increase in the
noise level of the lock. When using the lock transmitter/decoupler
amplifier for 2H decoupling set dn3='H2' and ampmode='dddd', i.e., all
channels to default. When using a 4th BB RF channel for 2H
decoupling set dn3='H2' and ampmode='dddp', i.e., 1H, 13C, 15N to
default and 2H to pulsed mode. Do not use CW mode. This
can be found in the instructions for ProteinPack.
Hope this helps.
Best regards,
Eugene
_______________________________________________________________
From:
Jack Howarth <howarth_at_nitro.med.uc.edu>
Ulrike,
I don't have any specific numbers for you, however when
we had the deuterium decoupling duplexer box added to our
800 MHz system we had a similiar issue. It turned out that
the 400 watt ampifilier in the console was adjusted to be
overdriven to provide more power but at a higher noise level.
Fortunately, we were slated to get a 1 Kw amplifier upgrade
on that channel. If I recall correctly, the ampilier can be
adjusted to provide lower noise although with less power output.
Jack
_______________________________________________________________
From:
nmr.asktac_at_varianinc.com
Did you ever try setting dn3='H2' and the fourth flag to p and see what
happens to the noise yet?
Cheers,
Ghirmai H. Meresi, Ph.D.
_______________________________________________________________
From:
Rajan K Paranji <rajan_at_cabm.rutgers.edu>
Hello Ulrich
We too use 2H deocupling, by using our fourth channel amplifier.
Let me add, at the risk of sounding a bit vicarious, that is
reassuring to know that someone else also faces a very similar problem
like ours :). I too observe that, when I just hook up the AMT output
into one of the inputs of the diplexer, without any pulsing going on,
the lock level just drops by 70% or so. At first, I suspected the
diplexer. Basically it is a diode based switch and it is supposed to do
a good job in isolating one path from the other. But, it was kind of
wierd that two of our INOVAs had faulty diplexers on them and I gave up
that idea and contacted Varian. In my opinion, Varian does not have a
viable solution to this. They only suggested that we better use the
"lkhld" function within the pulse sequence to latch the lock level
before the decoupling begins. But, as is clear from your account as
well, this problem is much before the pulsing/decoupling starts. Using
"lkhld" does not seem to alleviate the problem.
I am sorry that I am not able to suggest any solution to the problem
but I thought it may be a good idea to share the notes when we face the
same kind of issue. Thank you for bringing this up. Now I feel that
this problem is of general nature, than being a localized hardware
fault. I will be grateful if you can keep me posted, if you come across
any solution to this.
Best Regards
rajan
_______________________________________________________________
From:
Karl Koshlap <koshlap_at_tamu.edu>
Dear Dr. Werner,
We have had a little experience doing deuterium decoupling on
our Inova
500 and 600. I cannot answer your specific questions; however, when
doing
deuterium decoupling on channel 4, we set ampmode='ddpp' (i.e., if can't
be
cw). Hope that this helps...
Good luck,
Karl Koshlap
_______________________________________________________________
From:
Toshi Nishida <toshi_at_desmond.biovitrum.com>
Dear Ulrika;
You probably have sorted out the problem. I assume your INOVA 500 is
three RF channels with two AMT boxes. Unless Varian has changed
something, ampmode is a bit confusing. For ordinary experiments the
ampmode is 'cccc' if ampmode is used. The second AMT box has only one
amplifier (low band) and this is the forth letter of 'ampmode'. The
third letter has no meaning because that place is empty. To do H2
decoupling ampmode should be 'cccp'. The last 'p' is for the AMT of the
third channel. If you leave it as 'cccp' then other experiments such as
gNhsqc may have problem to decouple 15N. The ampmode should be then '
cccc'.
I haven't done H2 doecoupling on INOVA 500 (ours is 3 channel machine)
but done on INOVA 600 and INOVA 800 (they are 4 channel machine,
respectively). However I had hard time to figure out ampmode things on
INOVA 500 in past.
Hope this is of any help.
Kindest regards,
Toshi
_______________________________________________________________
--
Ulrike Werner-Zwanziger
Dep. of Chemistry Tel.: (812) 856-4629
Indiana University Fax.: (812) 855-8300
800 E. Kirkwood Ave
Bloomington, IN 47405
Received on Mon Oct 14 2002 - 12:27:53 MST