# astable oscillator question

Discussion in 'Homework Help' started by bug13, Jul 18, 2012.

1. ### bug13 Thread Starter Well-Known Member

Feb 13, 2012
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as seen in picture below, when i hook up the oscilloscope to outA and gnd, I can get about 1Hz, but when I connect it to OutB, I somehow get a Sine wave, what could be wrong? I am guessing one of the NAND gate is tossed?

I should get a 22kHz square + 1HZ square wave on OutB.

note: In is always high, and the OutA on the second one should be labeled OutB

2. ### Wendy Moderator

Mar 24, 2008
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My guess is you are beating the two frequencies together, which would create a sine wave. Just a guess though. Try each oscillator separately and see what it does.

3. ### mlog Member

Feb 11, 2012
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What's the value of the resistor at the bottom? 220 or 22k or something else?

4. ### bug13 Thread Starter Well-Known Member

Feb 13, 2012
1,214
38
sorry about my writing, it's 22k

5. ### t_n_k AAC Fanatic!

Mar 6, 2009
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783
What's the sinewave frequency you see on the CRO - maybe it's 50Hz due to CRO signal grounding oversight.

I'm assuming you have actually built the circuit.

The output should be a gated pulse train at the nominal conditions - if the primary astable is indeed 1Hz then you would expect approx 500ms bursts at the second astable frequency [22kHz?]. However, I'd check your values do actually give those stated frequencies.

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6. ### bug13 Thread Starter Well-Known Member

Feb 13, 2012
1,214
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Sorry, the second should be around 2kHz, my mistake.

the circuit was built, I will need to go back to school to check the frequency tomorrow.

And can you explain a bit more on this one, I think I get your idea, but I am not sure if I got right idea tho.

And another thing, how robust is a NAND gate, I am thinking I might tossed one of the NAND gate, as connect the positive to the wrong when I first hooked it up to the scope.

7. ### absf Senior Member

Dec 29, 2010
1,493
374
Then the wave should look something like the attached....

Top = Out_A
bottom = Out_B

Allen

• ###### astable osc_2.PNG
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85.5 KB
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8. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
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If you leave the signal ground off the circuit connection [by mistake or due to a connection fault] often you will see 50Hz [or 60Hz] mains 'noise' picked up on the CRO active. I would see 50Hz here in Australia.

If you have a spare chip it might be worth checking the problem is elsewhere rather than with a damaged gate(s).

9. ### bug13 Thread Starter Well-Known Member

Feb 13, 2012
1,214
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I can only get 1Hz with my physical connection, but I can't even get 1Hz in multisim, is it the right connection below???

10. ### t_n_k AAC Fanatic!

Mar 6, 2009
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I've not used Multisim but I note your sim schematic has no ground node.

11. ### bug13 Thread Starter Well-Known Member

Feb 13, 2012
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I have checked it, it is exactly what you said, thanks