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Thursday, September 11, 2008

Output Power Expressed in decibels (dB)

It is often very confusing for students to know exactly how much RF power (in Watts) they are using in their NMR experiments, especially since the two major instrument manufacturers express the output power differently. It is very expensive to use a power level too high as NMR probes can be damaged if proper safety measures are not taken. Both Varian and Bruker express the output power of their instruments in decibels (dB). The maximum power level (pl1, pl2, pl3 .... etc) on a Bruker spectrometer is -6 dB and the minimum power is 120 dB. On Varian spectrometers (at least our INOVA), the maximum power (tpwr, dpwr etc) is 63 dB and the minimum power is 0 dB. Because of amplifier non-linearity, one may get maximum power below 63 dB on a Varian instrument (perhaps 55 dB to 60 dB) with no further gain at higher settings. On Bruker instruments, the non-linearity can be taken into account by calibrating with known attenuators and creating a correction table (CORTAB). If one is using an amplifier with a maximum output power of 100 Watts and the effects of amplifier non-linearity are neglected, the output power will be approximately:

100 W @ 63 dB (Varian) and -6 dB (Bruker)
1 W @ 43 dB (Varian) and 14 dB (Bruker)
1 mW @ 13 dB (Varian) and 44 dB (Bruker)

The figure below shows a plot of the power level in dB vs. the fraction of total amplifier power. The expressions in the figure do not take into account amplifier saturation.

CAUTION : Exact output power on any particular instrument must always be measured and calibrated.

Thank you to Victor Terskikh of the National Ultra-high Field NMR Facility for Solids for suggesting this post.

3 comments:

patarroyo said...

It would be useful if manufacturers would list power in dBm, which is a standardized scale that would not vary from system to system (though you would still need to calibrate to know for sure).

Glenn Facey said...

patarroyo,

Thank you for your comment. I couldn't agree more. For those readers not familliar with a dBm, it is the power expressed in dB with respect to a 1 mW reference.

Glenn

Bernie OHare said...

Bruker Avance (Series III) now list powers in dB notation as well as in Watts, such as pl1 = 1.5 dB and pl1w = 33.25 watts