VoIP Voice and Fax Signal Processing

12.4 MEASURING CLOCK PPM

Hardware designers and manufacturers should ensure clock PPM. The simplest option is to measure the PCM interface clock or system clock used in deriving the PCM clock under planned environmental conditions.

Example. Expected PCM clock frequency = 2.048 MHz. Measured PCM clock frequency = 2.0475 MHz. Clock drift in PPM = (2.0475 − 2.048) (1 million)/2.048 = −244 PPM. If measured frequency is more than 2.048 MHz, it will give positive PPM. A 50 PPM on a 2.048-MHz PCM clock is 102.4-Hz drift. Frequency counters of 1-Hz resolution are required to get a measurement of 1-PPM resolution, and this translates to instruments with a minimum of 7 or 6.5 digit counters for PCM clock.

12.4.1 External Estimate from Frequency Transmission Measurements

Assuming adapter boxes are available in closed form and clock terminals are not available for direct measurement, frequency drift between transmit and receive telephones can be used for estimating approximate clock drifts or PPM. If Adapter-1 is sending frequency f1s, and adapter-2 is receiving frequency f2r, usually both frequencies f1s and f2r will be the same with a few Hz or a fraction of Hz difference. If the frequency measurement can be made up to 0.01-Hz resolution, then f1s, f2r, f2s, and f1r can be used to estimate PPM. PPM from adapter 1 to 2 = (f2r – f1s)/f1s; PPM from adapter 2 to 1 = (f1r – f2s)/f2s after multiplication by 1 million. From the two PPM values, each unit PPM can be estimated. To improve the measurement ...