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Old 04-05-2010, 05:56 PM
Nano001 Nano001 is offline
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Default nanosecond pulse generator

Hi everyone. I was wondering if it is possible to use a 555 timer in Astable mode to generate a pulse train of 16 pulses, each of width of 40ns and period of 80ns, total of 1.280 microseconds. Will the 555 timer operate at this high of frequency? How can I have the timer stop after my 16 pulses? Thank you.
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Old 04-05-2010, 06:40 PM
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Not a 555 timer; it's not fast enough.

Look at using something like Linear Technology's LT1016 ultrafast comparator as an astable multivibrator. You could gate it using a timer.

You can download LTSpice from Linear Technology's website; it's free and good. It has the LT1016 in the library.

Google is your friend.
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Old 04-06-2010, 02:44 PM
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Ok thanks. I actually have an LT1016. I will look at how to configure it as a multivibrator and post my circuit.

Will the 555 timer be ok to use as generating single pulses of 40ns pulse width and 80ns periods? Using proper RC combinations?
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Old 04-06-2010, 04:07 PM
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The 555 is touted as a "precision" timer. It's about as precise as watering flowers with a garden hose.

It's not quite that bad, but it's difficult to get a really accurate output.

If you want to have a pretty accurate signal, use a crystal oscillator and a counter, perhaps something in the 74VHC series.
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Old 04-12-2010, 03:40 PM
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Thanks for the help. I am currently designing my own AStable 555 configured in astable mode using the LT1016 comparator in LTSpice. I will use the DFlipFlop that is in their library. Will the flip flop component have any impact on generating 13MHz frequency? Is it just the comparator that needs to acheive high switching speeds?
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Old 04-12-2010, 03:52 PM
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Join the Yahoo! LTSpice User's Group.
Link: http://tech.groups.yahoo.com/group/LTspice/

There are quite a few models available there, including a large number of 74HCxx models.

While simulations can give you a reasonable idea of circuit performance, your mileage may vary when you build the actual circuit. Obtaining and using models that more closely simulate actual devices is a step in the right direction.

While 13MHz itself isn't that high of a frequency, you are dealing with square waves, which require a great deal of bandwidth to properly reproduce. Keep in mind that a perfect square wave is the sum of ALL of the odd harmonics of the fundamental frequency, requiring infinite bandwidth, which is why perfect square waves only exist in perfect simulations.
See this Wiki entry: http://en.wikipedia.org/wiki/Square_wave
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Old 04-13-2010, 08:12 PM
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I simulated a 555-like timer circuit using LT1016 for the comparators (attached). The output gave me my desired pulses, however I was wondering how stable and precise this will be when I actually build it. Should I look into using a crystal oscillator like was suggested?
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Old 04-13-2010, 09:22 PM
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If you want accuracy, use a crystal.

If you want better accuracy, use an OCXO (Oven Controlled Crystal Oscillator).

[eta]
Roman Black has an interesting and cheap way to make your own OCXO, here:
http://www.romanblack.com/xoven.htm
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