Rainbow Electronics DS2151Q - Manual (Page 30)

DS2151Q
022697 30/46
TRANSFORMER SPECIFICATIONS Table 12–3
SPECIFICATION RECOMMENDED VALUE
Turns Ratio 1:1 (receive) and 1:1.15 or 1:1.36 (transmit) ± 5%
Primary Inductance 600 µ H minimum
Leakage Inductance 1.0 µ H maximum
Interwinding Capacitance 40 pF maximum
DC Resistance 1.2 ohms maximum
12.3 JITTER ATTENUATOR
The DS2151Q contains an onboard jitter attenuator that
can be set to a depth of either 32 or 128 bits via the
JABDS bit in the Line Interface Control Register (LICR).
The 128–bit mode is used in applications where large
excursions of wander are expected. The 32–bit mode is
used in delay sensitive applications. The characteris-
tics of the attenuation are shown in Figure 12–4. The jit-
ter attenuator can be placed in either the receive path or
t he transmit path by appropriately setting or clearing the
JAS bit in the LICR. Also, the jitter attenuator can be dis-
abled (in effect, removed) by setting the DJA–bit in the
LICR. In order for the jitter attenuator to operate prop-
erly, a crystal with the specifications listed in Table 12–4
below must be connected to the XTAL1 and XTAL2 pins.
The jitter attenuator divides the clock provided by the
6.176 MHz crystal at the XTAL1 and XTAL2 pins to
create an output clock that contains very little jitter.
Onboard circuitry will pull the crystal (by switching in or
out load capacitance) to keep it long term averaged to
the same frequency as the incoming T1 signal. If the
incoming jitter exceeds either 120UIpp (buffer depth is
128 bits) or 28 UIpp (buffer depth is 32 bits), then the
DS2151Q will divide the attached crystal by either 3.5 or
4.5 instead of the normal 4 to keep the buffer from over-
flowing. When the device divides by either 3.5 or 4.5, it
also sets the Jitter Attenuator Limit Trip (JALT) bit in the
Receive Information Register 2 (RIR2.2).
CRYSTAL SELECTION GUIDELINES Table 12–4
PARAMETER SPECIFICATION
Parallel Resonant Frequency 6.176 MHz
Mode Fundamental
Load Capacitance 18 pF to 20 pF (18.5 pF nominal)
Tolerance ± 50 ppm
Pullability CL=10 pF, delta frequency=+175 to +250 ppm
CL=45 pF, delta frequency=–175 to –250 ppm
Effective Series Resistance 40 ohms maximum
Crystal Cut AT