| ??? 07/22/09 14:47 Modified: 07/22/09 14:49 Read: times |
#167697 - Dividers increase the error, but... Responding to: ???'s previous message |
Assume you have a perfect counter and you count the low to high edges of your RF over exactly 1 second and you get a reading of
2 500 000 000 then your RF can actually be in the range of 2.5GHz +-1Hz. If you divide the RF by 128 before counting and you get a reading of 19 531 250 then you suffer from the same absolute uncertainty of +-1Hz, so the divided frequency can actually be in the range of 19.531250MHz +-1Hz. This corresponds to a RF of 2.5GHz +-128Hz! So, the error has increased by the dividing factor. But: +-128Hz compared to 2.5GHz is only an error of 128 x 1000000 / 2.5GHz = 0.05ppm. Usually, your time base used to fabricate the 1 second counting window will show a much bigger error, so you won't actually suffer from the additional error from the divider. Kai |
| 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 |