System Noise Figure

One important parameter which indicates how well a spectrometer is working is the noise figure of the preamp and total system. An inexpensive method of determining the system noise performance is the "hot/cold resistor" test. Done with care, this can provide valuable insight into spectrometer performance.
The first step is constructing the noise source. Take a piece of RG58 coax cable and carefully strip the cover back about 2 inches. Now slide the shield back as far as possible (about 1 inch), cut the exposed center conductor to a 1/2 inch in length and strip off the insulation on a 1/4 inch of the conductor. Solder a 1% 50 Ohm metal film resistor to the center lead. Cover with shrink tubing and shrink the cover tight leaving the unconnected end of the resistor exposed. Slide the shield down over the resistor and solder the shield and the unconnected end of the resistor together to make a shielded noise source. Take care to make all leads as short as possible. In most environments, it is very important to use a shielded noise source to help reduce outside RF noise. Interference renders this measurement useless.
Carbon resistors increase resistance as much as 20% when cooled in liquid nitrogen. For this reason, two sources need to be prepared if carbon resistors are to be used. One which measures 50 Ohm at 20oC and one which measures 50 Ohm in liquid nitrogen. Determine the values with an Ohm meter. The metal film resistors change only a few percent when cooled by liquid nitrogen and are therefore preferred.
Connect the shielded 50 Ohm noise source to the input of the preamp in place of the probe. If the entire system noise figure is to be determined, leave all devices for transmitter coupling in place. Set the spectrometer to operate on the frequency of interest with a large sweep width (+/- 10,000 Hz) and a gain level set to fill the digitizer more than half way when acquiring a single scan. Collect a scan and determine the RMS of the noise after fourier transform with no line-broadening. This can be done with computerized routines or by plotting the noise and drawing a line on top and bottom of the noise such that about 10% of the maximum noise excursions fall outside the lines. Any obvious spikes in the noise spectrum can be ignored, but their presence indicates interference in the NMR system.
Repeat the above process with the 50 Ohm liquid nitrogen noise source in place of the probe while the source is being cooled by liquid nitrogen. Process the data such that the same normalization constants are used and determine the RMS value of the noise as above.
These two RMS noise values can be used to calculate the instrument noise figure as indicated by the equation below:

NF(dB) = -1.279 - 10log[1 - { (RMSc/RMSw) }**2]

where RMSc is the value of the RMS noise in liquid Nitrogen and RMSw is the value at room temperature (20 degC).
A table relating noise figure to the ratio of the cold RMS noise value to the warm RMS noise value (RMSc/RMSw) as calculated by the above equation is shown below

	Ratio	NF(dB)		Ratio	NF(dB)
	0.99	15.73		0.90	5.93
	0.98	12.74		0.85	4.29
	0.97	11.01		0.80	3.16
	0.96	9.78		0.75	2.31
	0.95	8.83		0.70	1.65
	0.94	8.06		0.65	1.11
	0.93	7.41		0.60	0.66
	0.92	6.86		0.55	0.29
	0.91	6.37

This test is easy to do, but requires careful experimental technique. If a noise level difference cannot be observed between a hot and cold resistor, make sure the system can observe an NMR signal from a standard sample. If you are unable to obtain a reproducible system noise figure, problems in the system noise profile could be presenting limitations on spectrometer performance. A good procedure is to strip the spectrometer receiving system to the bare minimum. If possible take out any unnecessary lock filters (turn off the lock) and decoupler filters from the receiving system. Remove the transmitter coupler if possible. With this minimum system determine the system noise figure. Anything more than 3.0 dB needs to be improved. Now add back the removed components one by one. Repeat the test after the addition of each component to determine how they affect the system noise figure. Today's spectrometers typically give overall system noise figure of less than 2.0 dB.

To be most useful, these noise figure tests should be done routinely as part of preventive maintenance on the NMR instrument. This history of performance makes it easy to see when something goes wrong. They are still useful without this history, since most working NMR systems have noise figures in the range range 1.0-2.5 dB. If the NMR system is performing outside this range, other noise tests can help determine which module of the system is at fault.
The system noise figure is an important factor in determining the NMR instrument's overall signal to noise. The NMR instrument is designed to have the preamplifier gain and noise figure determine the total system noise figure. As a rule of thumb, a 1 dB increase in the noise figure decreases the signal to noise by 10-15%. For adequate signal to noise, the overall system noise figure needs to be in the 1.0 to 2.5 dB range. If the system noise figure is outside that range, further tests are needed. Remember, if the signal to noise is low, then there can be at least three problems:

Not enough signal from the probe
Poor system noise figure
Noise from other sources

Typical Problem Areas:

Frequency dependent?
Change to another nucleus and determine the noise figure. It is best if the other nucleus uses a different preamp. If the noise figure then falls within the desirable range the preamp is suspect, but not proven guilty. Check the other tests described.
Transmitter Power Amplifier
In some NMR spectrometers the power amplifier is linear. These amplifiers can often emits RF noise at the observe frequency. If the noise blanking circuitry is defective or inadequate this will add noise to the system. With the noise meter measuring the system noise level, disconnect the transmitter cable. If the noise level (noise level not noise figure) drops there is noise coming from the power amplifier. This needs to be fixed before the NMR system will deliver optimal signal to noise.
Decoupler Power Amplifier
The phenomenon is the same as above except the amplifier to be checked is the decoupler power amplifier.
Lock
The lock transmitter can also add noise to the system. Observe the noise level on the noise meter with the preamp connected to the probe. If the noise level increases or decreases when the lock cable is disconnected from the probe, then the lock is adding noise (meter increases) to the overall system or the lock may be overloading the preamp (meter increases or decreases). Either way is not desirable. Further filtering of the lock and/or receiver system is required. Remember the lock can be putting noise in at the observe frequency AND/OR the lock transmitter can be overloading the preamp.
Transmitter Coupler and Directional Couplers
Most NMR systems have some circuitry in front of the preamp to couple on the transmitter and/or decoupler. Measure the system noise figure with and without this circuitry. The difference is the loss in this circuitry. It should be less than 0.5 dB and not more than 1.0 dB.
Filters
Most NMR systems have some circuitry in front of the preamp to filter out the lock transmitter and/or decoupler. Once again, measure the system noise figure with and without this circuitry. The difference is the loss in this circuitry. It should be between 0.5 and 1.0 dB.
Gain Level
If the preamp's gain is not much larger than the console noise figure, then the system noise figure is not determined by the preamp. In many systems, the dynamic range of the NMR signal is very large. This is especially true for biological samples in water. With large signals the operating spectrometer gain settings are sometimes set low to keep the preamp and console from overloading. Overloading produces artifacts, lineshape distortion and baseline distortion in the NMR spectrum. Determine the NMR system noise figure at several setting ranging from very high to very low. If your system is forced to operate at a gain setting at which it has a poor noise figure, you need to improve the dynamic range of the preamp.
RF Interference
Sometimes the probe or console picks up RF interference. A good way to detect this interference is to connect an amplified speaker to the audio channel. You can hear the interference. The noise meter can also indicate the interference by a changing level reading. The speaker and noise level test needs to be done with and without the probe connected to determine the source of interference.

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Last updated: 01/22/03