Nanosecond pulse stretching
Nanosecond pulse stretching
(OP)
I work for a small fiber-optics company. We build nanosecond pulsed lasers. The problem that I have been presented with is: if you have a nanosecond pulse from a photodiode, how can you determine its power, energy or even amplitude with a 10MSPS ADC? More specifically, how can I stretch and amplify a 1ns pulse to something greater than 1us while still preserving information about the original pulse like amplitude and duration?
-Paul
-Paul





RE: Nanosecond pulse stretching
Buffer and amplify the signal then feed it to a magnitude detector and a timer or counter.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Nanosecond pulse stretching
RE: Nanosecond pulse stretching
How fast is the rep rate. If it's fast enough, you can still do sampling, but I would guess that a 10MSPS ADC has too low a bandwidth. I would think that you'd need something more like a 250MSPS ADC, but it would have to have at least 5 GHz bandwidth to even come close.
Why don't you just rent a scope?
TTFN
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RE: Nanosecond pulse stretching
Our company needs an instrument that we can keep for the express purpose of pulsed laser spectroscopy, and we can only afford to spend a thousand dollars. My specialty is embedded systems, so my experience with high frequency stuff is very limited.
RE: Nanosecond pulse stretching
Cited part is a 3GSPS, 3GHz full power bandwidth ADC. You can get by, probably, with something like:
http://www.national.com/pf/AD/ADC08D500.html, which has a 1.7GHz bandwidth at 500 MSPS.
As for power/energy, your existing meters should already be able to do one or the other.
TTFN
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RE: Nanosecond pulse stretching
RE: Nanosecond pulse stretching
I wonder if you could get one of the high speed comparator chips to have sufficient rise time to construct an analog sample and hold circuit?
I've been fighting firmware fires (not real fires Keith) all day and my head hurts, so I'm not going to think though this.
But, maybe some of the aforementioned high speed comparators, surface mount, with a well laid out/shielded PCB *MIGHT* be in order.
Since the rep rate is low (1KHz mentioned by the OP) A PIC can certianly be used for the control of the circuit, but I's sure watch my grounding, etc.
Right now, I wouldn't even hazard a guess as to a suitable dialectric for the cap at this point.
Maybe a chat with an RF or microwave buddy for the front end of this critter might be in order. Maybe going as far as having him design the front end out of microwave capable chip transistors.
It would sure eat up that $1000.00 budget pretty quick though. Best of luck and keep us posted!
Cheers,
Rich S.
RE: Nanosecond pulse stretching
If you can use a series of pulses, you can reconstruct a pulse with a high speed sample and hold and vary the timing of each sample, (trigger on start of pulse, trigger + 10 pS, trigger + 20 pS, up to the width of your pulse...) digitize each sample fairly slowly (1 kHz rate), and analyze the results in software.
The timing circuit and sample and hold that are going to be operating at gigahertz frequencies are the difficult, expensive bits, and you don't want to cut corners there.
Another realtime approach is to have a small bank of comparators with various thresholds, each with an integrator to get a pulse width measurement.
Basically, you're making an AD converter with good amplitude accuracy but lousy resolution, and pretty good time resolution.
Develop some a priori information about the pulses, rent a high speed DSO for a month and make some measurements. Then you should be able to correlate the results from your ADC to some decent results.
RE: Nanosecond pulse stretching
You essentially need to do a bunch of analog domain work before hitting the digital realm.
At these speeds you need excellent layout quality. I'm hard pressed to see you doing this for under $1k.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Nanosecond pulse stretching
Just a thought.
Otherwise, to measure the pulse width and amplitude in a manner sufficient for a processor without extreme cost, you will need to convert it using an equivalent time detector method. Most TDR-type position and fluid level sensors use equivalent time detection means that take pico-second pulses and equivalent-time detect them over many samples such that the standard timer-counter input of a small micro can measure the time-converted pulse width. Tektronix does this with many of their super-fast scopes. The 11801 series could display 20 GHz signals, yet the equivalent-sample time base in the scope is 250 kHz (basically the 20 GHz signal is sampled over many repeated periods, and is reproduced exactly in shape as a 250 kHz waveform on the screen).
RE: Nanosecond pulse stretching
RE: Nanosecond pulse stretching