arod said:
I am building a digital voltmeter at school and my design so far relies on 2 clocks at about 10kHz and 19kHz.
Do you really need two different frequencies, and how accurate do they really need to be?
arod said:
It seems as if it would be hard to achieve these exact frequencies with a simple 555 timer because of the tolerances of resistors and capacitors. We have a device which measures capacitance very accurately in the lab. I was thinking I could get pretty close to the desired frequencies with a couple of pot resistors that can be manually tweaked until you get the desired frequency on the oscilloscope.
There is little point in selecting an exact capacitance and the adjusting the resistor. If you want to go that route just accept the value of the capacitor as it is and adjust the resistor.
R-C oscillators are not the best thing for accurate and stable frequency generation, unless you use high stability resistors and capacitors, the frequency will drift.
A oscilloscope is not the best thing for measuring exact frequency, about 2 or 3% accuracy at the best.
arod said:
I have read that crystal oscillators have exact frequencies and are used in many clocks and microcontrollers, but I am not sure how (if at all possible) one could adjust its frequency since they are all in the MHz range.
Yes crystals are quite accurate. The accuracy and stability does vary, depending on how much money you want to spend!
But the cheapest crystals are orders of magnitude more stable than an RC oscillator.
The frequency of a crystal can be "pulled", either by a capacitor or an inductor, but the effect is limited to 10 parts per million (a rough ball park figure).
arod said:
I am open to any suggestions on how to get an accurate clock for the 2 counters I am using in a digital voltmeter.
I would suggest deriving th two frequencies from a higher frequency crystal using dividers.
But first, do the frequencies have to be 10kHz and 19kHz?
JimB