# Measuring milliseconds.

1. Mar 16, 2007

### swain1

1. The problem statement, all variables and given/known data
Hi,
I have been asked to research two types of clock. The first to measure periods of nanoseconds and the second, milliseconds. Obviously I have talked about atomic clocks first but what clock can measure periods of milliseconds?

Cheers

2. Relevant equations

3. The attempt at a solution

2. Mar 16, 2007

### Staff: Mentor

Millisecond clocks are easier than nanosecond clocks. You would typically have an accurate crystal oscillator running in the 10MHz-100MHz range, and enable a counter with the signal that you want to measure the period of. Does that make sense?

3. Mar 16, 2007

### swain1

Yes it makes perfect sense but what is the type of clock called? Could a stopwatch working under this principle measure milliseconds?

4. Mar 16, 2007

### Staff: Mentor

No, a stopwatch is too slow. You need a clock that is many times the signal to be measured, in order to get good accuracy on the measurement. You need approximately 100x the signal frequency, in order to get 1% accuracy in the period measurement, right?

So a 100x clock for a 1ms signal (with a frequency of 1kHz) would be 100kHz, which is easy. More likely you would use a crystal clock oscillator module to generate a 10MHz clock, and feed that into the digital counter string that I mentioned.

Instruments called "frequency counters" are used to make the frequency and period measurements that you are asking about. Here is a wikipedia.org page with basic info on them. You can google frequency counter for more links and info.

http://en.wikipedia.org/wiki/Frequency_counter