Howdy,
Noise figures are a useful tool when looking at an NMR system
performance. All modern NMR systems should have a noise figure no higher
than 3. In general, lowering the noise figure from 3 to 2 will increase
S/N about 10%.
When doing a noise figure measurements there are some important things
to consider. Many times a poor noise figure can be traced to some other
unexpected source of noise. Common ones are:
1. Transmitter leakage into the receiver.
2. Decoupler leakage into the receiver.
3. Trans-coupler losses.
So if you can, disconnect the transmitter and decoulper cables before
they get to the preamp / trans-coupler. Then make the same measurements.
If they are better than with the cables connected, then you have leakage
from these transmitters into the receiver. This can sometimes be
improved with back to back diodes (tuned diodes are better).
Also try to connect the cold resistor to the preamp behind the
trans-coupler. This allows you to measure the loss in your trans-coupler
and, sometimes, discover a broken trans-coupler.
By the way, connecting an audio amplifier to the audio output of the
receiver (the signal going to the digitizer) allows you to hear when the
noise level goes down when you connect and disconnect cables. It also
allows you to hear when the noise is not white noise. When the noise is
not white noise something is contributing systematic noise to the NMR
receiver.
woody_at_acornnmr.com
> -----Original Message-----
> From: Neil Jacobsen [mailto:neil_at_u.arizona.edu]
> Sent: Wednesday, June 18, 2003 9:26 AM
> To: ammrl_at_chemnmr.colorado.edu
> Subject: Noise Figure Measurement on DRX-600
>
> 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 Thu Jun 19 2003 - 12:21:22 MST