NAND gate oscillator frequency calculation

Thread Starter

ukiceman

Joined Nov 22, 2009
9
Hi Guys

I’m in the middle of doing an NAND gate oscillator tutorial and, as usual, i’m stuck on the maths. Could somebody help me with the frequency calculation please?

C2 = .1uf
R4 = 2 megohms

Is it the same as the 555 astable calculation?

1.44/ ((200000000) x .1x10-6) = 7.2 hertz

The book is using

C2 = .1uf
R4 = 2.2 megohms, and suggests I should be seeing around 1Hz

When I tried 1.44/ ((200000000) x 01x10-6) = 0.72 hertz

This seems closer but, I’m not sure.

Kind Regards
Shaun
 

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SgtWookie

Joined Jul 17, 2007
22,220
Just a FYI,
If using CMOS logic ICs for oscillators, you're much better off using those with Schmitt-trigger inputs.
Examples of such ICs are:
4093 - Quad Schmitt-trigger NAND gates
4106 - Hex Schmitt-trigger inverters
74HC14 - Hex Schmitt-trigger inverters
74HC132 - Quad Schmitt-trigger NAND gates

Using just one of these gates, you can make a simple RC oscillator that can't stall. Some of the oscillators using non-Schmitt trigger gates can get "stuck".

If you need a gated oscillator, use a NAND gate. NAND gates can also be used for free-running oscillators by wiring the inputs together.
 

Audioguru

Joined Dec 20, 2007
11,249
A Cmos Nand gate makes a much better inverter with its threshold voltage closer to half the supply voltage when only one input is used. Connect the spare input to the positive supply. A Nor gate needs its unused input connected to 0V.

Here is the calculation for the corrected circuit that uses only single inputs:
 

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Thread Starter

ukiceman

Joined Nov 22, 2009
9
Thanks Guys

I’ve had a look at the link Jony130 posted and, the first formula I came across I crunched into a calculator giving me the same answer

( 2 x 3 ) x ( 17 x 10-9) = 1 / answer = 9.8 Mhz

If the first bit of the calculation is the number of gates, the second part is the propagation delay, then what are the other parts in his formula for?

I’m just working my way through the “Electronic Circuits for the Evil Genius”, only been doing this for a little over 3 month, the book doesn’t mention the formula it’s just saying what I’m seeing on my scope which is 1hz using a .1uf cap and a 2.2megohm

I just try and get a little bit more than what the book says each time I do something. So I’m just looking for a calculator friendly way that gives me the answer I’m looking for :) but when i put my numbers in i don't get 1hz

Regards
Shaun
 

Thread Starter

ukiceman

Joined Nov 22, 2009
9
Read this
http://www.fairchildsemi.com/an/AN/AN-118.pdf

And general equation
\(F=\frac{1 }{2.1....2.4*R*C}\)
what does the 2.1 part of this equation mean ?

i've included my datasheet, it looks like the propergation is 14ns, do i times that by 4

And, why does the equation in the link above on page one, give the answer without anything to with the RxC in the equation?

i'm proper lost, rubbish at this maths stuff :(
 

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Jony130

Joined Feb 17, 2009
5,186
Well, this equation F=1/(2nTp) apply to this circuit


For your circuit equation looks like
\(F=\frac{1 }{2.1....2.4*R*C}\)
or look at picture that Audioguru has attached. (1/0.455=2.19)
And no one will give you the exact formula, because there are too many variables.
A main reason is we don't now exact value of gate threshold voltage.
 
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