I take this question to be: is the signal to noise problem you are
experiencing due to
(1) a bad preamp noise figure at 1H frequency and okay at 13C frequency OR
(2) noise from the AMT power amplifier adding more noise at the proton
frequency than at the 13C frequency.
So the question is do you have too little signal or too much noise? It
should be easy to tell the difference. Take a scan at 1H frequency with
zero pulse width. Print out the FID (noise) with the noise at least 2 to 5
cm tall. Now disconnect the power out cable from the AMT and repeat the
experiment with the same spectral and plot parameters so the scaling is the
same. Compare the two plots to look for the any additional noise leaking
into the system at the 1H frequency. This process would be easier if the
manufacturers put speakers on their audio output to the digitizer. Then
with the speaker on you could disconnect and reconnect the AMT output cable
hear the difference. I think speakers are one of the best trouble shooting
tools available.
If the AMT output is adding noise to your system then you could add a pair
or tuned diodes in the transmitter line to get around the problem.
Other sources of noise which might need eliminating are:
* VT temperature sensing device ( thermocouple or other). Here just unplug
the VT temperature sensing cable to check for a difference. If it is a
problem then work on the bypassing at the base of the probe.
* Tachometer cable. This can sometimes bring noise down to the probe area
if it is not bypassed correctly.
* Sometimes the upper and lower stacks fail to make good electrical
connection. If this happens each stack is bring noise from the outside down
to the sample area. If they make good electrical connection then the
continuous cylinder shield the sample area from the outside world.
Good luck with the finger pointing exercises.
woody@acornnmr.com
> -----Original Message-----
> From: RDUDLEY [mailto:rdudley@ARSERRC.Gov]
> Sent: Monday, May 11, 1998 6:37 AM
> To: ammrl@wwitch.unl.edu
> Cc: JBROUILLETTE; RDUDLEY
> Subject: Varain Pre-amp.
> Sensitivity: Confidential
>
>
> X400-Mts-Identifier: [;4656360911051998/A15263/CERES]
> Hop-Count: 0
>
> We have a Varian Unity+400 NMR spectrometer. It has a single preamp
> for both proton and x-nucleus. Recently we had our 5mm id BB probe
> repaired and upon return we checked the specs. We were unable to meet
> the proton signal to noise spec (350/1). We thought that it might be
> the probe. On the 5 mm BB probe we were able to meet the C13 signal
> to noise spec (150/1). Varian thinks that we have a bad pre-amp. We
> have a third party service contract and they think it is the AMT
> amplifier. I find this had to believe, but I am troubled by thinking
> that the pre-amp is bad. Has anyone else had a similiar experience on
> a Varian Unity, Unity + 400 Mhz and less? Please e-mail me directly
>
> Bob Dudley
> rdudley@arserrc.gov
>