Oscillator question

on

In order for an oscillator to work there needs to be:

1. More gain than losses around the loop
2. The phase must be correctly lined up so that the feed back signal is correctly phased.

One or the other or both is your problem

You switched the RC.

Analogous to the frequency-domain statement made above, you need positive feedback over a short period, and negative feedback over a long period (DC).

As shown, you have two negative feedback loops. At best, you'll get oscillation due to the propagation delay of the inverter (at a frequency other than intended).

The canonical circuit is:

U1 has DC negative feedback through R, and positive AC feedback through U2 and C. Note Ri prevents overloading the input protection diodes on U1, important for large C.

Other circuits are common, using different connections of resistors or more inverters (often 3 total). None of these make any sense when two works.

If you don't mind depending on a device parameter, you can also make one with a single schmitt trigger inverter. The time constant corresponds to the input hysteresis band, which will generally be smaller than the approx. half supply worth in the above circuit.

Tim

on

might be worth reading.

-- Bill Sloman, Nijmegen

Not to mention that there are far better configurations, such as...

--- My guess would be that the output impedance of the rightmost inverter, coupled through the voltage divider formed by the 1.88K and the

This beauty is from TI's 1989 High Speed CMOS Data Book and has _always_ worked for me:

. +-------+--[C]--+ . | | | . | [R] | . | | \ | | \ | . +-| >O-+-O| >-+ . | / | /

A caveat, but what makes it always work, is that the voltage swing across C is 2Vcc.

Another caveat is that, like with most oscillators of this type, on power-up the first cycle will be longer than the rest.

After that, t ~ 1.8 RC

But, if you're using tiny logic, why don't you just implement your oscillator with a single Schmitt trigger inverter?

--- JF

In what respect?

It has a lot of words that Slowman doesn't understand... thus he thinks, "...might be worth reading." ;-) ...Jim Thompson

ion

Did you ever have problems with the input voltage to the left inverter going above Vcc or below ground? Some circuits I see use a high value resistor in series with the inverter input to limit discharge current through the protection diodes. Maybe it depends on the size of the capacitor? How much current can those protection diodes take?

-Bill

I dunno - last time I needed a clock, I made a pierce oscillator:

Cheers! Rich

A naked gate input would be best, since the protection diodes sink current which messes up the time constant.

Analysis is simple: assume the switching threshold is in the middle, so whichever state it's in, when the capacitor charges or discharges to +V/2 (which apparently occurs in t_1/2 = R*C*ln(2), assuming an initial voltage of +V), it switches. That puts +V*3/2 on the input, so the cap is forced down by +V/2 due to input diodes.

Input diodes typically handle

garyr:

Warning: SPAM! ;-)

What about downloading a single Java program (no installation required, if you don't like it, you simply delete it) that runs on any PC for free?

Thanks to all for your replies to my post.

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

Perfect! I just happen to have a triple-inverter IC in a SOT23-8 package, a (believe it or not) 74V2G04.

nd

rsion

It makes the simple point that an odd number of inverters will always oscillate, something that the OP might find useful.

-- Bill Sloman, Nijmegen

My

ond

ersion

The vocabulary isn't demanding - though a hill-billy red-neck like Jim might find it difficult. One wonders why John Fields couldn't have read the note - it isn't long - and found out for himself.

-- Bill Sloman, Nijmegen

nd

rsion

The amount of current the protection diodes can take isn't the only question you should be asking. The protection diodes dump charge carriers into the substrate, and they have been known to create odd effects in other devices on the same substrate.

-- Bill Sloman, Nijmegen

version

version

Slowman is such a f*ck-head. Probably doesn't even know the real derivation of "red-neck".

I have one of the ORIGINAL RCA CMOS manuals, the oscillator section of which was re-posted on my website years ago...

IIRC, There is a minor derivation error in that Application Note.

Wonder if Slowman can properly derive the correct frequency?

We already know the answer, don't we? No chance. ...Jim Thompson

n in

). My

second

C version

Haven't we all? So why the stupid question?

-- Bill Sloman, Nijmegen