need suggestion of a circuit

[jhill]

I need to generate a TTL level squarewave (duty cycle not important) that is
stable and adjustable at a frequency just slightly different from 3 times
the power-line frequency, for example 180.1 cps. I have tried a simple 555
oscillator using the most stable components I could find, but it still
drifts too much. Is there any circuit I could add to the 555, or build anew
to insure generation a constant small difference in frequency to the
3*60cps.
Can you point me in the right direction?
Can a PLL be used somehow?

TIA

 

[mike]

I need to generate a TTL level squarewave (duty cycle not important) that is
stable and adjustable at a frequency just slightly different from 3 times
the power-line frequency, for example 180.1 cps. I have tried a simple 555
oscillator using the most stable components I could find, but it still
drifts too much. Is there any circuit I could add to the 555, or build anew
to insure generation a constant small difference in frequency to the
3*60cps.
Can you point me in the right direction?
Can a PLL be used somehow?

TIA

Start with a frequency F1 where F1/m = 60 and F1/n = 180.1.
Phase lock it to 60.
Depending on how much jitter you can stand, you might be able
to use a microcontroller and a phase accumulation technique
(See DDS).
mike

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[JURB6006]

If you want it stable and timed well, go high and divide it down. Also, do you
want it to track the power line frquency or be stable on it's own ? Power
companies do make corrections so that a synchronous clock will work.

One wonders just what this is for, out of curiousity could you divulge that, or
is it some kind of secret ?

Realistically, if you go up to 16 X 180.1 it's about 2900 Hz. Up there you will
get alot less drift, and the drift you do get will be divided by 16. You're
only four flip-flops away and lots of them come two to a chip, and there are
programmable ones available. Tracking the power line frequency is also easier,
if that's what you want to do.

If you actually want to track the power line frequency you might be able to use
a TV countdown chip. They start out at 503Khz and to get 180.1Hz at that point
you could PLL an offset frquency of about 840Hz at the front end.

I'm still really curious about what exactly this is for.

JURB

 

[jhill]

Ok, here is what I am trying to do, and I have it all built except for
getting stable timing pulses:

I want to make a little different xmas tree lights display: I am putting on
three strands of lights, each strand has all the same color bulbs, say an
all red strand, an all green strand and all blue strand. I want the lights
on the tree to appear to slowly change colors, something like those old
silver metal trees did with the color wheel over the spotlight. Each strand
goes up in brightness and back down, but they are 120 degrees out of sync
with each other, so as one goes up the other is coming back down.
I have three triacs, one in series with each strand, and am feeding them
approx 60 hz turn-on timing pulses, but each triac gets a trigger that is
120 degrees out of phase with the other. I get these timing pulses from
dividing the oscillator pulses by 3 and decoding the counts: 0, 1, 2. 0
count goes to one triac, 1 to the other and 2 to the third. If the
frequency of the pulses is slightly different from 60cps then the brightness
goes up and down at the difference frequency, so I want the difference freq
to be stable. So yes, I would like to track the powerline frequency with a
constant difference.

 

[JURB6006]

Interesting. I'll have to give it some thought. Of course you could just phase
control the whole thing and be done with it, but the challenge is nice too.

Interesting approach, I must say.

JURB

 

[jhill]

I would appreciate any thoughts you have. Putting a divide by 16 between
the 555 and the div by 3 & increasing the 555 ocs freq did help some on the
stability, but it still drifts. Whether it is the osc freq or the line freq
that is drifting I can't determine, since it takes so little drift to change
the difference freq.