Spectrum Recorder PCI-E Card?

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Harald_Consul
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Spectrum Recorder PCI-E Card?

Post by Harald_Consul »

In the thread "Integrated Lab Measuring and Controlling (Labview, Labjack, ...)" we have already figured out, that high-frequency analysis is pretty costly.

For cost effectiveness I am interested in recording only (without any analyzing features) of a raw high-frequency signal from a scintillation tube.

Is there some kind of spectrum recorder PCI-E card (ideally with 16 bit A/D converter), that could ship round about 90 Gbit/s (5 Gsample/s * 16 bit) directly to an NVMe SSD (NVMe solid state disk)? (I mean like a spectrum analyzer, however recording only.)

Other computer interfaces like USB or Ethernet are much to slow for 90 Gbit/s. The next best popular interface would be 10Gbit Ethernet. However, there are no 10Gbit Ethernet spectrum analyzer or oscilloscopes on Ebay, either.
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Re: Spectrum Recorder PCI-E Card?

Post by Chris Mullins »

5 Gsamples/sec at 16 bits is extremely high performance - I'm not sure you'd find something like that off the shelf at any price.

Here's one source of more "reasonable" PCIe boards: http://ultraviewcorp.com/displayproduct.php?cat_id=1, they have a 12-bit 2Gsample/sec board: http://ultraviewcorp.com/displayproduct ... 1&sub_id=8, at only $19,000.

If you can settle for 12 bits, you might be able to roll your own by combining the AD9213 eval board (https://www.analog.com/en/design-center ... d9213.html) and the ADS8-V1EBZ data acq. board: https://wiki.analog.com/resources/eval/ads8-v1

More info on that AD9213 a/d converter: https://www.electronicsweekly.com/news/ ... s-2018-05/. Apparently the chip itself is over $3,600, so you're still in the 5 digit price range.

I don't think the PC interface is the (most) limiting technology here.
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Re: Spectrum Recorder PCI-E Card?

Post by Harald_Consul »

Thanks Chris.

As you gave me the right key words, I did another Ebay search now:

(A/D, data-acquisition) (card, board) (PCIe, PCI-E, PCI-Express) @ Ebay

However, I did not find a 16 bit 5GSample Card in the range of 1000 USD.

So, how about a forum maker project "Spectrum Recorder PCI-E Card"? ;-)

I guess, a core metal 16 bit 5GSample A/D converter chip would be about 10 USD. Why wasting money, if we could do ourselves even better?
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Re: Spectrum Recorder PCI-E Card?

Post by Chris Mullins »

Harald,

Most of the devices from your ebay search are audio A/D devices, sampling at 192 kHz.

I don't think you understand how extreme 16 bits at 5 Gsamples/sec is with the current state of the art. I doubt ANYONE makes something with those specs, at ANY price, at least not as an off-the-shelf product.

Your guess of $10 for an A/D chip with those specs is WAY off. The link I gave above for the new AD9213 chip (which is only 12 bits) mentions a $3,600 price tag just for the chip.

Here's an article about some A/D chips with more "relaxed specs" - 14 bits, and 625 to 1300 Msample/sec: https://www.eenewsembedded.com/news/14- ... iondefence

Note that prices JUST FOR THE CHIP range from $406 to $11,108 each, in thousand piece quantities.

If you want a quick survey of A/D chips available for easy purchase, try this Digikey link: https://preview.tinyurl.com/y95k5x2o. I narrowed it down to A/D converters in stock, sampling at 1Gsample/s and above. You can see the number of bits ranges from 7 to 16, with prices ranging from $168 to $4400.

A complete A/D system built around one of these will be very expensive, and a real engineering challenge to design (or even fully test). There's a reason why those A/D boards are so costly.
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Re: Spectrum Recorder PCI-E Card?

Post by Rich Feldman »

Harald, the term Spectrum Recorder in your topic title leaves me confused.

Do you mean Spectrum in the sense of an electronic signal waveform (e.g. Fourier transform), as hinted by your link to a spectrum analyzer page? Why would that kind of spectrum at output of a scintillator-coupled PMT be of interest, unless the intensity of radiation is varying rapidly and periodically?
Or Spectrum in the sense of energy distribution in ionizing radiation? To get the latter from a scintillator-and-PMT signal, each signal pulse is reduced to one number, that has nothing to do with the frequency content of the PMT output waveform.
Either way, there's no "spectrum" you can capture with an ADC.

Please excuse me for guessing that you mean the radiation energy spectrum. Sounds like you want to digitize the pulses as if you were viewing them with an oscilloscope, with plenty of samples for each pulse. Then compute the pulse areas. That's doing it the hard way, unless you already have the oscilloscope -- and oscilloscopes don't digitize to 16 bits or even, generally, 12 bits.

Unless I've overlooked details of your application, why not quantize PMT pulses the way most people do it? Analog front end converts the PMT pulse to one whose height can be detected, or shape sampled, with much slower electronics. Without loss of "energy" resolution. Doesn't Gamma Spectacular (among others) send the scaled pulses to a PC audio input for digitization?
scint.JPG
image from http://amptek.com/products/dp5g-oem-dig ... -and-pmts/

I just learned that sometimes the PMT pulse shape, not just its area, is different for different kinds of radiation. Paying attention to that would put you ahead of 99% of the applications for ionizing-radiation spectral measurement, in my unpracticed opinion.
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Re: Spectrum Recorder PCI-E Card?

Post by Harald_Consul »

Yes. That's exactly the inaccuracy problem.

A multi-channel-analyzer for a scintillator (consisting out of a transformation crystall and a photo multiplier tube inside) counts each bandwith of volt amplitudes (e.g. 10,5 mV - 11,5 V) separatly. Each amplitude corresponds to a certain energy of a particle.

However, when 2 particles hit the crystall at the same time, the sum of the resulting voltage falls in a different volt bandwith is counted like as it would have been from one particle with higher energy.

Thus, I want to capture the raw data to resolve for the particles much more sophisticated by a statistical software. (I am a statistician, remember?)
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Re: Spectrum Recorder PCI-E Card?

Post by John Futter »

Harald
this is the bane of nuclear physics
cured by backing away to keep deadtime very low - inverse square law
(deadtime = rejected counts due to two or more arriving within the integration time)
You can also use differing scintillators that are much faster in decay time so shaping time is coresspondingly much shorter such as LYSO and BGO.
Note that most MCAs use PGA's to get the data in an almost parrallel fashion into memory something that a processor cannot do

We use these at work https://www.fastcomtec.com/products/mpa/mpa4/ and these are more than fast enough for beam on target work such as proton microprobe, PIXE, PIGE, RBS, and NRA with LYSO BGO GeLi SiLi detectors note we tend not to use NaI detectors for this work due to afterglow - these are better suited to low background counting where afterglow does not happen
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Re: Spectrum Recorder PCI-E Card?

Post by Steven Sesselmann »

Hello Harald,

At the recent IEEE in Sydney I was talking with one of the exhibitors from Hawaii who explained that they design and make ADC's with Giga Bit sampling, might be something.

http://www.naluscientific.com


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Re: Spectrum Recorder PCI-E Card?

Post by John Futter »

As Steven has pointed out yet another offering using gate arrays to get gigabit equivalent input
And looking at thier enduse I would say they are eye wateringly expensive almost custom tfor the LHC
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Re: Spectrum Recorder PCI-E Card?

Post by Harald_Consul »

I see,measurement technology is unbelievable expensive in comprehension to other electronic equipment.

Maybe I could use some nerd raw data acqusition equipment like a software defined radio.

The features of the offered software defined radio are:
  • 1 MHz to 6 GHz operating frequency
  • up to 20 million samples per second
  • 8-bit quadrature samples (8-bit I and 8-bit Q)


Meanwhile I have figured out, that the signal amplitude of a (negative voltage) scintillation tube typically is in the range of
~20 mV with a pulse length of ~20 ns.

1/0.00000002s = 50'000'000 Hz

So, 6 GHz would be fine, but 20 million samples per second is too slow, right?

What would be a good sampling rate (#samples/second) to measure a 20 ns signal, in order to separate the mixed combined voltage amplitudes from multiple particle events into true single particle events (respectively their voltage amplitudes)?

P.S.: If you are not telling me, I have to buy the software defined radio and figure out myself, I guess. ;-)
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Re: Spectrum Recorder PCI-E Card?

Post by Rich Feldman »

>> I see,measurement technology is unbelievable expensive in comprehension to other electronic equipment.

Did you overlook digital multimeters that cost less than $5, and are often given away free for buying something at a Harbor Freight store?
Light bulbs and magnetrons are relatively inexpensive, because the technology is mature and millions are made every day.
Same goes for smart phones and large TV sets. Technically not too challenging, and the makers get revenue from tracking you long after the original purchase.
Super digitizers, like you originally asked for, are expensive because they are technically cutting-edge, and the quantities are too low to justify high volume production methods and cost reduction. I bet the $3500 IC's that Chris pointed to don't use a plain old xx-nanometer digital CMOS process, on plain silicon, through mainstream 12-inch wafer fab lines.

First exercise for you, Harald:
Find out and explain the meaning of 6 GHz and 20 Msps in the same software-defined radio spec. Front-end downconverter? Undersampling, with sampler aperture size and jitter very small compared to repetition rate?

As for how to separate nearly-coincident pairs of 20 ns analog-voltage pulses,
here are some guesses (without research) about what might be good enough for your interest.
How about one good sample every 2 nanoseconds? (500 Msps) You know the pulse shape, and should be able to fit the data samples no matter how the sample clock phase is aligned to any given pulse location.
Analog 3db bandwidth 200 MHz? (sub 2 ns risetime)
Digitized to render 10 or 12 bit numbers for your largest expected coincident-pulse voltage?
I think this is getting into mainstream oscilloscope territory, except maybe for the bit count.
Another critical consideration is voltage noise.

What fraction of the pulses will overlap, if they have ordinary random statistics and the average rate is 60,000 cpm?
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Re: Spectrum Recorder PCI-E Card?

Post by Steven Sesselmann »

Harald_Consul wrote: Tue Jan 29, 2019 9:33 am Meanwhile I have figured out, that the signal amplitude of a (negative voltage) scintillation tube typically is in the range of
~20 mV with a pulse length of ~20 ns.
What exactly are you trying to achieve with GHz sampling?

Can you amplify and stretch the pulse so you can sample at a lower rate?

This is how I achieve pulse height analysis from PMT's with only 48 Khz sampling and a few software tricks.

Steven
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Re: Spectrum Recorder PCI-E Card?

Post by Harald_Consul »

Well, yes I thought it would require 100 or maybe even 1000 samples to disassemble a multiple signal voltage amplitude (of 20 ns in this case) into its originals properly. This is to some degree also a statistical/ mathematical question I have not researched fully, this time.

However I can say,
  • the more single event amplitudes are contained in a cumulated measured amplitude,
  • the more easily the amplitudes of two or more low energy events do exactly pile up to the amplitude of a higher energy event and
  • the more individual the curvature of each single event amplitude is
the (much) more samples will be required to properly decompose a multiple signal into its origins.

List above last edited on Thu 2019-01-31 12:06 pm

Richard, I will "do my homework" and come back.

Steven, is there a turn key solution for pulse stretching?
Last edited by Harald_Consul on Thu Jan 31, 2019 5:12 am, edited 4 times in total.
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Re: Spectrum Recorder PCI-E Card?

Post by John Myers »

A rule of thumb is to sample at a rate of 2.5 times the frequency of the signal. In order to avoid aliasing you need to sample above the Nyquist rate (2x the freq).
So to accurately reproduce a 20ns pulse you would sample at 125Msps.

It may be possible to use a slower rate (undersampling) if there were no other longer pulses that would alias with the faster ones.
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Re: Spectrum Recorder PCI-E Card?

Post by Andrew Seltzman »

Some things to think about:

You are trying to digitize a stochastic, non-repetitive signal. Frequency domain methods (eg a spectrum analyzer) will not be of much use.
The Nyquist rate is the BARE MINIMUM frequency that is theoretically required to reconstruct a REPETITIVE signal.
20 points per period of a sine wave will give you nice looking plot.
10 points per pulse will generally be ok to fit a gaussian pulse nicely if you have a shaping amplifier.
If you don't have a shaping amplifier, the best you can localize the time location of the edge is on the order of sampling period of the digitizer.
In this case, if you are looking to capture pulse height, capturing the peak value with an analog peak detector circuit and then using a slower ADC will be much more useful. As Steven said, using a shaping amplifier to slow down the pulse is another valid alternative.

Also look into something called a "baseline restoration circuit" which can help at high count rates.
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Re: Spectrum Recorder PCI-E Card?

Post by Steven Sesselmann »

Harald_Consul wrote: Wed Jan 30, 2019 8:48 am Steven, is there a turn key solution for pulse stretching?
What I use is just a fast rail to rail opamp as a current amplifier with a high impedance drain, so when a pulse comes in via the coupling, it takes time to filter through.

Obviously this only works when the pulse count rate is slow, otherwise you get pulse pile up (PPU).

Thousands of people around the world are now doing gamma spectrometry with sound card sampling which everyone here said would be impossible when it was first suggested 10 years ago.

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Re: Spectrum Recorder PCI-E Card?

Post by Rich Feldman »

Bit of clarification about continuous-time and discrete-time functions, and the Nyquist rate.

Consider a continuous-time function x(t) that has the important property of being bandlimited. It has no frequency content at or above the limiting frequency Fb. Periodicity doesn't matter. Frequencies that we don't care about do matter.

Suppose we generate a discrete time signal X(n) by sampling x(t) at regular intervals -- times separated by exactly 1/2Fb.
Then x(t) can be perfectly reconstructed from X(n)
, using the interpolation method called sin(x)/x or "sinc" or Whittaker-Shannon.
https://en.wikipedia.org/wiki/Whittaker ... on_formula

Digital storage oscilloscopes often support horizontal scale factors that have more pixels than samples. Then they render the waveforms using sin(x)/x interpolation, which is not fakery. It actually eliminates real errors that would appear if they drew straight lines to connect the sample dots.

Of course the key to lossless sampling is having a bandlimited signal to start with.
If you start with an arbitrary function y(t), you can generate x(t) with a low-pass filter ("anti aliasing filter") that passes no frequencies at or above Fb. Information lost at that point is gone. Whatever gets through into x(t), there's no fidelity benefit from using a sample rate higher than 2Fb.
In practice, designers need some room for the anti-alising filter to roll off between 100% passing with constant delay (in frequency range of interest) to 100% stopping (at and above half the sampling rate).
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Re: Spectrum Recorder PCI-E Card?

Post by Harald_Consul »

For my understanding. Do you or don't you agree with:
Harald_Consul wrote: Wed Jan 30, 2019 8:48 am However I can say,
  • the more single event amplitudes are contained in a cumulated measured amplitude,
  • the more easily the amplitudes of two or more low energy events do exactly pile up to the amplitude of a higher energy event and
  • the more individual the curvature of each single event amplitude is
the (much) more samples will be required to properly decompose a multiple signal into its origins.

List above last edited on Thu 2019-01-31 12:06 pm
?

Especially did I forget a characteristic of the mathematical problem, which determines the necessary number of samples (data points) to decompose properly?

Further questions to the mathematical problem:
  • Is the amplitude/duration relation among the amplitudes of singular high energy particles and singular low energy particles approximately constant?

Intermediate result of mathematical part of the problem:

From the first research into this topic it looks like mixed distribution models aka mixture models from the free statistical software R might be suitable to decompose complex amplitudes.
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Re: Spectrum Recorder PCI-E Card?

Post by Rich Feldman »

Harald,
Some of your particle detector questions need to be answered before we can properly tackle the pile-up problem.
If you can't find them by reading, or asking on forums, how about finding them by experiment?

First please help your helpers stay engaged.
* Clarify whether the goal is real-world spectrum measurements by you, or thought experiments, or something else.
* What is the maximum average count rate you need to deal with?
* What's the approximate numerical probability that any event will have another event close enough to make a combined pulse?
* How low must the probability be, to not interfere with your application?

* Why don't you reduce the overlap probability by using a detector that makes shorter pulses, as suggested by others in this thread? Or by moving the detector to reduce the maximum count rate?

Before speculating on samplers, you need to declare your expectations about the PMT output when mixed radiation is present.
* What is the pulse waveform for one representative particle? Please draw or point to a picture, with x-axis time units given.
* What is the range of pulse shapes for particles of the same energy, interacting at different places within the scintillator?
* How about the range of pulse shapes for all particles of a different energy? Repeat for other energies in the range of interest.
* Do you trust that the detector system is linear? That means that when two events happen at about the same time, their respective output voltages for each instant are simply added.

Each pulse shape (the real continuous-time waveform) has a frequency content, and that drives the required sampling rate.
If the typical pulses are smooth and well-behaved, they may be naturally sort of bandlimited. If faster-edge risetime were on the order of 2 ns, that suggests not much going on above 200 MHz, and high fidelity sampling might be done at 500 Msps.

The key to reconstruction (to get pulse locations much more precisely than the sample interval, or to resolve overlapping pulses)
is to do the interpolation properly. Connecting sample dots with straight lines is simplistic, and significantly wrong, unless you oversample by an unnecessarily large ratio. Fine if oversampling is cheap. Foolish if your money is better spent on good samples at a lower rate, with proper front end filter, as found in practical oscilloscopes.
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Re: Spectrum Recorder PCI-E Card?

Post by Richard Hull »

Rich makes a fabulous point. GM counters have a dead time that must be accounted for with 60 usec being one of the best and 200 usec being the norm in many older tubes. True random events like radioactivity have issues with detection just not being counted due to a GM tube's dead time.

Moving the detector farther away with a GM set up might not allow for the inverse square law to compute back to the actual radiation emission rate!
This is especially true with mica windowed tubes as a tremendous amount of alphas just will not be counted due to their MFP in air! This can amount to, often, 50% of the total radiation. Add to this, the fact that many low energy betas can scatter before being counted in a more distant mica windowed detector.

For neutron tubes, especially like the 3He tubes and PMT gamma detectors, the pulses are very short compared to the gas amplification scenario within a GM detector. Moving a neutron detector can result in fairly accurate back figuring using the inverse square law provided you are not so far away from the source that scattering and reflections from nearby moderators do not significantly interfere.

It all comes back to......

1. Knowing what you are measuring and all of its oddball characteristics and gotchas.
2. Knowing what you are measuring with (detector and all of its electronics), and its limitations and gotchas.
3. Trying like the devil to stay out of statistics that grasp at straws where the statistics becomes like a drunk at a lamp post at night....Using it more for support that illumination.

Rich touched on all of this in a wise and steady manner above.

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Re: Spectrum Recorder PCI-E Card?

Post by Harald_Consul »

Thank you guys. All those pitfalls in the electronic pathway of the analog signal beginning with the PMT and ending with analog preprocessor part before the A/D converter have been very insightful.

I have figured out, that 50 samples/ observations may be a good start for applying mixed distribution models, as 30 observations is the bottom to test a normal distribution. As the amplitude is about 20 nano-seconds long, that would make 2.5G Sample/s.

However, at the moment 2.5 MSample/s is the maximum of data acquisition cards (DAQ cards) at Ebay. Thus, I am postponing the acquisition of the DAQ card to wait for higher sampling rates in 1 or 2 years. For bridging I have to use my 200MHZ digi osci, meanwhile, which however is pretty limited in the total number of samples. But I am not willing to pay 5000 USD for a 2.5 GS/s card.
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Re: Spectrum Recorder PCI-E Card?

Post by Rich Feldman »

Sorry, can't stay away.
Harald, I think you are missing the main point. Let's try again using one specific example.

Suppose your 20 ns pulses (with arbitrary alignment to sampling clock, and arbitrarily overlapping neighbors) are smooth and have minimum risetimes on the order of 2 ns, hypothetically. So that after a linear, continuous-time lowpass filter that stops everything above 250 MHz, very little was lost. Example continues with the assumption that in continuous-time filtered waveform, you could accurately get the location and size of each pulse, even when they partly overlap.

Now you want to do the processing using discrete-time methods (and discrete voltage resolution, but that's an independent detail).
You want 50 points per 20 ns pulse, which is 2.5 Gs/s (0.4 ns sample spacing). OK so far.
I claim that capturing 10 points per pulse ( 0.5 Gs/s, 2.0 ns spacing) in the filtered signal is enough.
Because you can compute the missing 40 points, without error, by proper multi-point interpolation from the sparser samples.
You could interpolate to a time resolution of 500 points per pulse if you wanted -- practically continuous-time. It works because of the properties of bandlimited signals. Wish there were time to make an illustrated demonstration, to overcome intuitive resistance to the concept.
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Re: Spectrum Recorder PCI-E Card?

Post by Harald_Consul »

Thanks Rich!

First of all it is absolutely common when two or more scientists from different disciplines talk together in some kind of interdisciplinary communication, that they do not understand each other fully, simply due to different education and different wording. This sometimes requires a lot of patience on both sides. ;-)

When a mixed distribution model is applied to distinct between a multiple particle amplitude and single particle amplitude this is only based on the shape respectively the curvature of the signal amplitude.

Any interpolation between two samples will not exactly meet the true curvature of the amplitude. On the contrary the interpolation always pushes the curvature towards the one, that has been assumed for the interpolation. As any interpolation must be based on some curvature assumption, the interpolation can produce further data points, but no additional information about the true distribution/ true curvature of the signal.

Thus, the only possibility to decompose a piled up multi particle signal into singular particle events would be a better mathematical approach than the one I mentioned, that would require less than 50 samples/ data points per amplitude.
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Re: Spectrum Recorder PCI-E Card?

Post by Rich Feldman »

Harald,
Would you mind sending, as lists of numbers, one or more realistic-looking PMT pulses with 0.4 ns between samples? You can draw them yourself, and they could even be piecewise linear if that simplifies your task.

I want to whip up, or at least threaten to whip up, a numerical demonstration.
We will resample at 1/5 of the rate, then see how sin(x)/x interpolation perfectly reconstructs all points along curves between the "sparse" samples.

Of course we will put an anti-aliasing filter before the sparse sampling. All interpolated points will exactly match the full-rate signal after the filter.
You need to be content with the fidelity of the full-rate filtered pulses
, which is like having front end electronics with a useful but not excessive analog bandwidth. Let's see how that looks when the original pulses are designed by you. Piecewise-linear shapes would get their sharp corners rounded by the filter, and excessively fast risetimes would be knocked down a bit.

The demo will have knobs to choose the time shift between two superimposed PMT pulses, and the phase of the sparse sampling.
Addition of the two time-shifted pulses can happen before or after the filtering, before or after the sparse sampling, and before or after the interpolation. We will see that they all give the same answer.

Respectfully,
Rich
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Re: Spectrum Recorder PCI-E Card?

Post by Harald_Consul »

Rich, currently I do not have any real scintillation data, as I am still working on figuring out the electrical pin allocation of my "brand new" second hand scintillator tube.

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