Measures of Performance 17 February 6, 2009
IDT82V3012 T1/E1/OC3 WAN PLL WITH DUAL REFERENCE INPUTS
3 MEASURES OF PERFOR-
MANCE
The following are some synchronizer performance indicators and
their corresponding definitions.
3.1 INTRINSIC JITTER
Intrinsic jitter is the jitter produced by the synchronizing circuit and is
measured at its output. It is measured by applying a reference signal
with no jitter to the input of the device, and measuring its output jitter.
Intrinsic jitter may also be measured when the device is in a non-
synchronizing mode, such as free running or holdover, by measuring the
output jitter of the device. Intrinsic jitter is usually measured with various
band limiting filters depending on the applicable standards. For the
IDT82V3012, the intrinsic Jitter is limited to less than 0.02 UI on the
2.048 MHz and 1.544 MHz clocks.
3.2 JITTER TOLERANCE
Jitter tolerance is a measure of the ability of a DPLL 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.
3.3 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 IDT82V3012, two internal elements determine the jitter
attenuation. This includes the internal 2.1 Hz low pass loop filter and the
phase slope limiter. The phase slope limiter limits the output phase slope
to 5 ns per 125 µs. 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 per 125 µs.
The IDT82V3012 has 16 outputs with 4 possible input frequencies for
a total of 64 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, 2.048 MHz to 2.048 MHz and
19.44 MHz to 19.44 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).
Using the above method, the jitter attenuation can be calculated for
all combinations of inputs and outputs based on the four jitter transfer
functions provided. Note that the resulting jitter transfer functions for all
combinations of inputs (8 kHz, 1.544 MHz, 2.048 MHz, 19.44 MHz) and
outputs (8 kHz, 1.544 MHz, 3.088 MHz, 6.312 MHz, 2.048 MHz, 4.096
MHz, 8.192 MHz, 16.384 MHz, 19.44 MHz, 32.768 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).
3.4 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 IDT82V3012, the Freerun
accuracy is equal to the Master Clock (OSCi) accuracy.
3.5 HOLDOVER ACCURACY
Holdover accuracy is defined as the absolute tolerance of an output
clock signal, when it is not locked to an external reference signal, but is
operating using storage techniques. For the IDT82V3012, the storage
value is determined while the device is in Normal mode and locked to an
external reference signal.
The absolute Master Clock (OSCi) accuracy of the IDT82V3012 does
not affect Holdover accuracy, but the change in OSCi accuracy while in
Holdover mode does.
3.6 CAPTURE RANGE
Also referred to as pull-in range. This is the input frequency range
over which the synchronizer must be able to pull into synchronization.
The IDT82V3012 capture range is equal to ±230 ppm minus the
accuracy of the master clock (OSCi). For example, a 32 ppm master
clock results in a capture range of 198 ppm.
The Telcordia GR-1244-CORE standard, recommends that the DPLL
should be able to reject references that are off the nominal frequency by
more than ±12 ppm. The IDT82V3012 provides two pins, MON_out0
and MON_out1, to respectively indicate whether the reference inputs
Fref0 and Fref1 are within ±12 ppm of the nominal frequency.
3.7 LOCK RANGE
This is the input frequency range over which the synchronizer must
be able to maintain synchronization. The lock range is equal to the
capture range for the IDT82V3012.
3.8 PHASE SLOPE
Phase slope is measured in seconds per second and is the rate at
which a given signal changes phase with respect to an ideal signal. The
given signal is typically the output signal. The ideal signal is of constant
frequency and is nominally equal to the value of the final output signal or
final input signal.
3.9 TIME INTERVAL ERROR (TIE)
TIE is the time delay between a given timing signal and an ideal
timing signal.
3.10 MAXIMUM TIME INTERVAL ERROR (MTIE)
MTIE is the maximum peak to peak delay between a given timing
signal and an ideal timing signal within a particular observation period.
