Re: Visualing RF pulses 'online'
Jim Breeyear (jbreeyea@moose.uvm.edu)
Wed, 27 Mar 1996 08:26:12 -0800
Paul Driscoll wrote:
>
> Hi. I run an NMR lab aimed at protein structures but I am no RF expert.
> This may then sound like a dumb question but I am going to ask it
> anyway.
>
> It seems a number of NMR labs like to stick a digital oscilloscope
> online (so to speak) by using a line section/plug-in element (from Bird
> Electronic Corp. USA, for exmaple) sitting in the transmitter line. My
> understanding is that this component picks up the reflected power from
> the probe and thereby visualise the input pulses and waveforms on the
> scope, giving a realtime check on what the pulse programmer is
> producing. I understand the line section/plug-in element hardly affects
> the RF power at all.
>
> I have seen this in operation in a couple of laboratories - it is really
> a nice thing to have to confirm that the pulse program is really doing
> the right thing - but it's not a cheap thing to do if you don't have a
> high frequency digital scope (eg. 500MHz or 1GHz). Can it be done less
> expensively if all you want to visualise is the presence or absence of a
> pulse rather than the high frequency oscillations within the pulse? What
> I have in mind (and this may be the mad part) is that you could
> integrate or rectify the input to a (lower frequency) scope and thereby
> simply observe the pulse envelope? If so, what sort of device could be
> to achieve this the integration step? I am obviously looking for an
> off-the-shelf component or (very) simple homemade device design.
>
> If this is a stupid idea, please let me know. We are hoping to get a
> high frequency scope soon, but we feel we need a stop-gap solution.
>
> Any help/advice with this idea would be appreciated. I am looking to get
> more involved with issues of this kind. If there is any more general
> advice which books/magazines to consult for NMR/RF/elctronics ideas
> coulkd you let me know.
>
> Cheers,
> Paul
> ---------------------------------------------------------------------
> Paul Driscoll | Office (answer)phone: (44)-171 380 7035
> Dept. Biochem. & Mol. Biol. | NMR lab phone: (44)-171 391 1354
> University College London | driscoll@biochem.ucl.ac.ukHi Paul. Yes I know what you mean when you talk about demodulating the RF
pulses and only look at the pulse scheme. I would prefer to go into the
console and look for a lower level source rather than stick something in
the probe input circuit. I would think that doing that would cause some
loss and change the 90's etc. I prefer to use the power meter to measure
power and that is all. Reflectometers are good for a check of what the
probe is doing but that is about all I use that for. Are you looking for
distortion due to the RF amplifier/probe or are you just curious as to
what the pulses look like at the business end? If I wanted to look at the
pulse train I would look at a point in the console before the RF. I also
look at the end result at the work staion more than test equipment. Maybe
some labs have trouble with arcing and like to monitor for that. I
probably am not much help but nice chatting with you. Jim.