oscillation stability

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

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's your schematic? What're your component values? 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

What's the frequency, Kenneth?

Try replacing it with a 74HC04

Robin

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

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.

Let's look first at what to do to to help your existing oscillator. 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.

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

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

cheers, s.

: steve wrote:

: 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.

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.

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 the cells supply higher current loads, those loads will require similar but proportional (to the load current) capacitors, close to them.

[snip]

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.

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.