I have been trying to figure out why we can't meet the S/N spec with our
Nalorac 5mm 3-res Z-axis gradient probe on our Bruker DRX-600 for some
time. The Nalorac installer came close to the S/N spec of 1150:1 (1H
sensitivity on Et benzene) but was unable to meet it. He suggested that I
measure the Noise Figure of the spectrometer to see if there is too much
noise. I did this measurement using a 50 ohm metal film resistor in place
of the probe, comparing the noise in a "spectrum" acquired with the
resistor at room temp. with the noise in a spectrum acquired with the
resistor immersed in liquid nitrogen. The NF is calculated as follows:
NF(dB) = -1.279 - 10 log (1 - rms_cold^2 / rms_warm^2)
I measured in three separate determinations:
NF = 3.66, 3.26, 3.52
Bruker told me that they don't consider Noise Figure to be a reliable
measurement. Brian Marsden at Varian-Nalorac told me that anything over 3
is way too high, that I should be getting around 1.8 for an "older" system
(more than 5 years old). Our system was purchased used from MR resources
in 1999, and at that time it was about 3 years old. He also said that a
Noise Figure of less than 1 was assumed in setting their S/N specifications
for the probe. Finally, he felt that our measured S/N value of 500-600:1
for the older Bruker TXI probe that was "original equipment" with the DRX
is "way low".
I swapped the 1H preamp slice with an exchange part, of the "older style",
from Bruker and got the following values:
4.01, 3.71
Finally, I checked the NF using the same method on a newer (2001?) DRX-600
in our School of Pharmacy, which has the "newer style" preamp. I got the
following value:
1.55
All of this is somewhat confusing. I think there is a problem with S/N and
with Noise Figure, but my comparison with another DRX is somewhat ambiguous
because they have a newer style preamp. I hope that some people out there
have experience with NF and can give me typical values for this type and
vintage of system, also I wonder if people trust these
measurements? Finally, if anyone can suggest where to look for a noise
problem in the receiver I would be most grateful.
Neil
Neil E. Jacobsen, Ph.D.
NMR Facility Manager
Department of Chemistry
119 Old Chemistry
1306 E. University
University of Arizona
Tucson, AZ 85721
520-621-8146
FAX 520-621-8407
Received on Wed Jun 18 2003 - 12:20:29 MST