| ??? 07/22/09 20:30 Read: times |
#167713 - Remember resolution != precision :) Responding to: ???'s previous message |
With a 1ppm crystal, the timing error will represent 2500Hz out of 2.5GHz.
A 1s read with a 1/128 prescaler will result a resolution of 128Hz. That is a resolution 20 times higher than the precision of the 1ppm crystal, in which case the read error in the prescaler is small compared to the timing error. The big advantage of being able to read the prescaler is that it reduces the measurement errors for lower input frequencies, where the prescaler will stand for a bigger error. For a 128MHz input signal, a 1ppm crystal will represent a worst-case of 128Hz - similar to one tick error from the prescaler. That is also why a good frequency meter should support selectable prescalers - for low frequencies, the prescaler should be reduced or completely removed. |
| Topic | Author | Date |
| Frequency Counter using prescaler | 01/01/70 00:00 | |
| Find prescaler. Then find time period. Then multiply count. | 01/01/70 00:00 | |
| Think about crystal stability | 01/01/70 00:00 | |
| Oscillator candidate | 01/01/70 00:00 | |
| i wish that would become clear to the majority | 01/01/70 00:00 | |
| 0.1 % | 01/01/70 00:00 | |
| I'm sorry? | 01/01/70 00:00 | |
| i'm sorry | 01/01/70 00:00 | |
| Clearer now | 01/01/70 00:00 | |
| Dividers increase the error, but... | 01/01/70 00:00 | |
| if extreme precision is required ... | 01/01/70 00:00 | |
| or count more seconds | 01/01/70 00:00 | |
| Remember resolution != precision :) | 01/01/70 00:00 | |
Excellent - responses thanks so much | 01/01/70 00:00 |