MT9040

Data Sheet

8

Zarlink Semiconductor Inc.

Jitter Tolerance

Jitter tolerance is a measure of the ability of a PLL to operate properly (i.e., remain in lock and or regain lock in the

presence of large jitter magnitudes at various jitter frequencies) when jitter is applied to its reference. The applied

jitter magnitude and jitter frequency depends on the applicable standards.

Jitter Transfer

Jitter transfer or jitter attenuation refers to the magnitude of jitter at the output of a device for a given amount of jitter

at the input of the device. Input jitter is applied at various amplitudes and frequencies, and output jitter is measured

with various filters depending on the applicable standards.

For the MT9040, the jitter attenuation is determined by the 1.9Hz low pass loop filter.

The MT9040 has twelve outputs with three possible input frequencies (except for 19.44MHz, which is internally

divided to 8KHz) for a total of 36 possible jitter transfer functions. Since all outputs are derived from the same

signal, the jitter transfer values for the four cases, 8kHz to 8kHz, 1.544MHz to 1.544MHz and 2.048MHz to

2.048MHz can be applied to all outputs.

It should be noted that 1UI at 1.544MHz is 644ns, which is not equal to 1UI at 2.048MHz, which is 488ns.

Consequently, a transfer value using different input and output frequencies must be calculated in common units

(e.g., seconds) as shown in the following example.

What is the T1 and E1 output jitter when the T1 input jitter is 20UI (T1 UI Units) and the T1 to T1 jitter attenuation is

18dB?

Using the above method, the jitter attenuation can be calculated for all combinations of inputs and outputs based on

the three jitter transfer functions provided.

Note that the resulting jitter transfer functions for all combinations of inputs (8kHz, 1.544MHz, 2.048MHz) and

outputs (8kHz, 1.544MHz, 2.048MHz, 4.096MHz, 8.192MHz, 16.384MHz, 19.44MHz) for a given input signal (jitter

frequency and jitter amplitude) are the same.

Since intrinsic jitter is always present, jitter attenuation will appear to be lower for small input jitter signals than for

large ones. Consequently, accurate jitter transfer function measurements are usually made with large input jitter

signals (e.g., 75% of the specified maximum jitter tolerance).

Frequency Accuracy

Frequency accuracy is defined as the absolute tolerance of an output clock signal when it is not locked to an

external reference, but is operating in a free running mode. For the MT9040, the Freerun accuracy is equal to the

Master Clock (OSCi) accuracy.

OutputT1

InputT1

A

–

20

-------





×10

=

OutputT1

20

18

–

20

---------





×10

2.5UI T1

()

==

OutputE1

OutputT1

644ns

()

488ns

()

-------------------

3.3UI T1

()

=

×

=

OutputE1

OutputT1

1UIT1

()

1UIE1

()

----------------------

×

=