Help with finding a right configuration

Discussion in 'General Electronics Chat' started by Garurumon, Nov 12, 2013.

Mar 17, 2013
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So, I need a way to convert 2-15V amplitude 1Hz-4MHz signal into TTL level (5v). I wanted to do a schmitt trigger, but I read that those are not really good on such high frequencies.

I googled a lot, and all of the circuits I found didn't prove to be very good on simulations when the high frequency signal is applied.

Is there any other way to shape the signal into 5V square wave?

2. tshuck Well-Known Member

Oct 18, 2012
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What form is the 2-15V signal to begin with?

Mar 17, 2013
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sine wave, square wave, triangular wave, I want to be able to measure any form...

Oh yeah, it's for the input of the frequency counter.

4. tshuck Well-Known Member

Oct 18, 2012
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A quick search gave me several 7400 series ICs with operational ranges well within your requirements, here is one...looking at the parametric tab lists its max frequency at 35MHz...

Mar 17, 2013
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What does it mean if it's "NAND" schmitt trigger? Is it just how it is realized? Does it have anything to do with it's performance/setup?

hey, is resistor attenuator good at reducing signal amplitude at high frequencies? cause if it is, i could make it easily

6. JohnInTX Moderator

Jun 26, 2012
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The front end of this frequency counter may be of interest. It was referred to in another thread here at AAC.

7400 series logic is fast enough but won't work at 2-15Vin. How about a fast comparator with a little hysteresis?

Mar 17, 2013
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I tried to simulate this, but it never works in multisim, so I'm a bit skeptical...

like which one?

Could I just reduce the amplitude with resistive attenuator and then feed it into 74 series IC...? I could make two inputs, or a switch, for two ranges?

8. ronv AAC Fanatic!

Nov 12, 2008
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Take a look at the LT1720.

9. JohnInTX Moderator

Jun 26, 2012
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Re: mulitsim, don't know. It looks like the circuit issues short, sharp pulses for each cycle. Maybe multisim can't do it...

Re: 74xx and resistive divider - maybe. Consider 74ALSxx. It needs 100uA pulled out of an input through a resistance low enough to not exceed .4V for a logic low.. Turning the crank says it will just pull low at 4Meg to ground. Call it 1Meg. to be sure. That's the lower R in the divider.

Now, it needs 1.44V min at the input to make a logic high. Call it 2V. That makes the top R about 388K. So swinging from 0-2V is a legit logic swing (for 'ALS).

Now run the input to 15V. The input voltage on the 'ALS is ~10V, way more than it wants. You can handle this by putting a zener diode across the 1M R to clip the voltage to something safe..

The problem with using TTL of any type is that the input impedance varies with the input voltage. Not as bad with 'ALS but still there. Meh. May work for you.

My thought about a comparator was one with an open-collector output that ran at 15V from the get go. The 1720 ronv suggests is fine. It would require a different resistive divider on the input is all. Since its inputZ is high, this divider could be high resistance. Set the ref voltage at 1V and have at it.