oscillation stability

Discussion in 'Electronic Basics' started by steve, Jul 30, 2007.

1. steveGuest

I am using 74hc14 to produce a fixed tone.

However, the output keeps varying the low values around 1 khz.

Could anybody please refer to a similar simple circuit that will keep
the hz value stable.

Thanks.

2. Tom BiasiGuest

Hi Steve,
It would help to see how you are using this device to make the tone, and the
values of components that you selected.

Tom

3. Tim WescottGuest

How can anyone present you with a similar circuit when you aren't
telling us what circuit you're using? All we know is that there's a
74hc14 in it.

What frequency are you trying to produce?
How much frequency stability do you need?
Under what circumstances does your frequency vary?

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

4. BobGuest

What's the frequency, Kenneth?

5. Guest

Try replacing it with a 74HC04

Robin

6. ChrisGuest

Hi, Steve. I'd guess you're using this ultra-simple digital square
wave oscillator (view in fixed font or M\$ Notepad):
___ ___
| .---|___|--|___|----.
| | R A |
| | | |
| | |\ | |
| o----| >O----o------o-----o
| | |/
| |
| +|
| ---
| C ---
| |
| ===
| GND
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

This circuit will work with an inverter with schmitt trigger input
like the HC14. The input voltage will oscillate between the upper and
lower switching points, and the output will be a sqare wave with 50%
duty cycle.

You're experiencing one of the biggest limitations of this circuit.
The upper and lower switching points of the schmitt trigger are only
specified within a wide range, and they tend to drift with
temperature. That means your frequency will temperature drift far
more than you would expect from just component drift.

A second consideration is whether your supply is poorly regulated or
unregulated. The switching points of the schmitt triger can be seen
as a percentage of Vcc. Those percentages will change with varying
power supply voltage.

What to do? You haven't described much about what you're doing, and
you also didn't mention how much drift is acceptable.

Start out by keeping total R (including your pot) in the range of 33K
to 1Meg. Less means higher output current and chip heating, and more
means leakage currents and other problems predominate. Second, don't
use the HC14 to drive a piezo or another high current load, for the
same reason. Replace your cap with a ceramic NPO or other cap with a
tolerance of 5% or less. Make sure the circuit board is clean of flux
or other contaminants that can cause leakage currents which can bollix
up the works. If your pot is remote, put a 100pF ceramic cap in
parallel with the timing cap to help squelch higher frequency noise
that might be affecting your timing. Make this oscillator at the pin
5-6 inverter, and tie the negative end of the cap directly to the GND
at pin 7. Try to get your supply as well-regulated as possible. And
most significantly, use a multi-turn pot if you can to set frequency
-- the single turn ones are pretty miserable about holding a precise
value sometimes, especially at the ends of the wiper.

If these don't do it, you might want to try a 555. An LM555 is
specified with temperature drift of .015% per degree C in astable
mode, and drifts .3% per volt change in power supply. You can be
pretty much guaranteed nearly all of the frequency drift will be due
to your cap and pot, if you use a regulated supply.

And if you're running on a supply of less than 5V or are running on
batteries, try the LMC555. It has similar specs, and can run on the
full range of HC supply voltages without problem.

http://cache.national.com/ds/LM/LM555.pdf
http://cache.national.com/ds/LM/LMC555.pdf

Look in the data sheet for the typical astable circuit.

Hope this has been responsive. If not, please take a little time to
describe more fully what you're doing and what you need --
specifically:

* What's your supply voltage? Is it regulated?
* Do you just need a frequency, or do you need a 50% duty cycle square
wave?
* What kind of frequency stability do you really nned?
* Can you afford a separate chip for this?

Cheers
Chris

7. Skip saysGuest

Use this circuit and scale the values up for higher frequencies. It
works very well and it's pretty darn stable.

cheers,
s.

: I am using 74hc14 to produce a fixed tone.

: However, the output keeps varying the low values around 1 khz.

: Could anybody please refer to a similar simple circuit that will keep
: the hz value stable.

: Thanks.

8. steveGuest

Thanks to everyone for their replies.

I'm new to electronics, so circuit simplicity is essential!

It's a very simple HC14 circuit, with 1k pot and 10 uf cap. These keep
the hz values around the requirement 1khz.

Its probably the only chip with a 50 % duty cycle and square output..

The supply is around 6v dc from cells.

9. John PopelishGuest

If the cells also power any other digital things, those
things are probably causing the cell voltage to have brief
notches each time those things draw a spike of current.
These notches can cause the oscillator to switch output
states. For this reason, if you want a cleaner frequency,
you will need to filter the supply voltage. At the very
least, add a 0.1 uF of ceramic or film capacitor across the
supply pins of (and very close to) the oscillator chip. If
require similar but proportional (to the load current)
capacitors, close to them.

10. TolstoyGuest

The very same circuit the OP used on the HC14 will work as well with a
555 to give a 50% duty cycle. You just won't see it on the data
sheets.
Tie pins 2, 6 and the output pin to the + end of the cap. You can
leave pin 7 (discharge) open. The cap will cycle between 1/3 Vcc and
2/3 Vcc. Frequency independent of supply voltage.

11. neon

1,325
0
Oct 21, 2006
if you are not carefull some of these spice people will analyze this thng to death without conclusion. A smidth trigger is a very fast device meaning when it passes trough the switching point bias if there is any noise present during the transision you will get all those little fuzz input as an actual square output. where does the fuzz come from who knows probably layout gnd a slow input you are the problem not much explanation of what you got.

12. steveGuest

John is right because the dry cells used are the problem here.

The cells are not powering anything else, however there is stability
improvement by keeping a 500 ohm resistance with parallel diode, at
the +ve supply.