MT9046 Zarlink Semiconductor, MT9046 Datasheet - Page 14

• Manufacturer Part Number: MT9046
• Description: T1/E1 System Synchronizer with Holdover
• Manufacturer: Zarlink Semiconductor

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 MT9046, two internal elements determine the jitter attenuation. This includes the internal 1.9 Hz low pass
loop filter and the phase slope limiter. The phase slope limiter limits the output phase slope to 5 ns/125 us.
Therefore, if the input signal exceeds this rate, such as for very large amplitude low frequency input jitter, the
maximum output phase slope will be limited (i.e., attenuated) to 5 ns/125 us.
The MT9046 has twelve outputs with three possible input frequencies (except for 19.44 MHz, which is internally
divided to 8 KHz) 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, 8 kHz to 8 kHz, 1.544 MHz to 1.544 MHz and 2.048 MHz to
2.048 MHz can be applied to all outputs.
It should be noted that 1 UI at 1.544 MHz is 644 ns, which is not equal to 1 UI at 2.048 MHz, which is 488 ns.
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 20 UI (T1 UI Units) and the T1 to T1 jitter attenuation is
18 dB?
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 (8 kHz, 1.544 MHz, 2.048 MHz) and
outputs (8 kHz, 1.544 MHz, 2.048 MHz, 4.096 MHz, 8.192 MHz, 16.384 MHz, 19.44 MHz) 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 MT9046, the Freerun accuracy is equal to the
Master Clock (OSCi) accuracy.
OutputT1
OutputT1
OutputE1
OutputE1
=
=
=
=
Zarlink Semiconductor Inc.
InputT1
20
OutputT1
OutputT1
×10
MT9046


–
-------- -
20
×10
18
14
×


×


(
--------------------- -
(
=
–
------ -
1UIT1
1UIE1
(
-------------------
(
20
644ns
488ns
A
2.5UI T1


)
)
)
)
=
(
3.3UI T1
)
(
)
Data Sheet