RE: Radio: frequency deviation & drift, vs Bluetooth requirements

AmbystomaLabs — Fri, 24 Feb 2017 15:41:29 GMT

Phase and frequency locked synthesis is one of those cornerstones of RF.

A separate oscillator generates the 2.4GHz signal, through a series of digital dividers the 2.4GHz signal is reduced down to 16MHz or some other local clock.

In this manner the 2.4GHz is regulated such that a fixed number of beats (or fractional number) of the 2.4GHz signal are seen for every beat of the local clock (ie, 16MHz).

By changing the numeric value of the division, the RF can be shifted to any one of the bluetooth channels yet still stay aligned with the local clock.

In this type of synthesis, the RF signal will have the same ppm error as the local clock. This is why the error of the clock is expressed as a ratio (part per million) and not in Hertz.

So a 10PPM local clock will give an error of 10 * 2,400,000,000/1,000,000 or +/- 24KHz at 2.4GHz.

Similarly, a 40PPM clock which I think is specified on some of the Nordic reference designs will give +/- 96KHz error at 2.4GHz.