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Here our 1H amplifier of NMR spectrometer is damaged twice for long time or high power RF irradiation. Why the duty cycle is important for amplifier or probe? If the damage happens to amplifier, which part of it is broken?

edit: The type of our spectrometer is Varian InfinityPlus for solids bought in 2003. I have no idea about the sort of amplifiers, just know the amplifier is for power output. Someone used about 80kHz 1H power for 1H decoupling during 100-200ms acquisition. The efficiency of the amplifier output droped gradually. The efficiency of it reduced to one-third at one time. After an interruption of power supply, the amplifier is found no power output any more. Another event is that the electric capacity and resistance of the proton amplifier are burnt suddenly.

Thanks!

asked Jun 05 '10 at 18:32

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Frey
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updated Jun 10 '10 at 10:31

Evgeny%20Fadeev's gravatar image

Evgeny Fadeev
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Hi Zhang, sorry for the intrusion into your posts. I've merged the follow-up answer into your original question. That way the whole thing is more understandable.Best regards, Evgeny. - Evgeny Fadeev (Jun 10 '10 at 10:33)

Evegeny - I have some RF power amplifier background and will be happy to help you and others in that regard. I am not a chemist or NMR user. Lowell Beezley info@bptec.com - lbeezley (Mar 15 '11 at 08:37)


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Hello Zhang,

I can answer part of your question. Hopefully someone familiar with amplifiers will tell you more.

Duty cycle (average fraction of time the device is on or is receiving a signal) multiplied by power of signal will give the average amount of power deposited into the probe or put out by the amplifier.

Some part of that power will have to be dissipated as heat. In the case of probe - almost all of that power goes into heat - if the probe is well tuned and matched.

Since each device has a limit in the amount of heat that can be dissipated, you can break the device by overheating it. So at any given level of power there is certain value of maximum safe duty cycle. If you lower power you can increase the duty cycle.

This post by Kirk Marat has a good explanation of how probes are damaged by high power irradiation.

Regards, Evgeny.

link

answered Jun 05 '10 at 19:22

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Evgeny Fadeev
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updated Jun 05 '10 at 19:25

Hi Evgeny, Thanks for your reply! Two experiments A: experiment with 100ms continuous RF irradiation and 10s recycle delay B: 40ms irradiation and 2s recycle delay The duty cycle of B is larger than that of A, so which is more dangerous to probe or other hardwares? - Frey (Jun 10 '10 at 04:00)

I recently posted much about these amplifiers on the AMMRL list here: http://www.ammrl.org/archives/July-2010/11.html - Jerry Hirschinger (Sep 28 '10 at 12:34)

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In addition to Evgeny's good and accurate answer:

Zhang,

It is interesting that you report the 1H amplifier to be damaged (twice, no less) rather than a probe getting damaged. That in itself is unusual (but certainly not impossible) because more typically it is the probe that gets damaged rather than the amplifier.

You also note "...long time or high power..." It does make a difference so please pick one or the other. ;)

It would also be helpful to tell the Wiki a bit more about the spectrometer where this amplifier is located: age, solid or liquid, 1H frequency, driver or final amplifier, if appropriate and so on.

In a properly designed RF power amplifier that is not pushing the boundaries of design, for example, extreme high power or frequency, perhaps the most significant failure mode is from excess heat (sometimes from clogged air passageways or weakening fan). Again, perhaps second in line is deteriorating electrolytic power supply capacitors (for aging units). Most vacuum tube amplifiers (used in high power instrumentation) are rather robust to so-called "bad loads" like NMR probes (tuned or untuned). Transistor amplifiers are generally a bit more twitchy but they are usually accompanied by fault detection circuitry which is intended to limit damage in bad situations.

If you can provide more background information then it might be able to help with the problem diagnosis and then with a possible solution.

Joseph

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answered Jun 08 '10 at 14:37

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Joseph DiVerdi
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If the amplifier is connected to the probe, you may well check whther the output impedance of the amplifier and the input impedance of the Probe are matched well. If there are mismatches there could be reflecting power and this might add to the demand on the tolerance limits of the amplifier.

S.Aravamudhan

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answered Oct 16 '10 at 06:01

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SankarampadiAravamudhan
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