GS1522-CQR(2000) Ver la hoja de datos (PDF) - Gennum -> Semtech
Número de pieza
componentes Descripción
Lista de partido
GS1522-CQR Datasheet PDF : 20 Pages
| |||
During pathological signals, the amount of jitter that the
phase detector will add can be calculated. By choosing the
proper loop bandwidth, the amount of phase detector
induced jitter can also be limited. Typically, for a 1.41MHz
loop bandwidth at 0.2UI input jitter modulation, the phase
detector induced jitter is about 0.015UIp-p. This is not very
significant, even for the pathological signals.
9. CHARGE PUMP
The charge pump in a slew PLL is different from the charge
pump in a linear PLL. There are two main functions of the
charge pump. One function is to hold the frequency
information of the input data. This information is held by
CCP1, which is connected between LFS (82) and LFS (84).
The other capacitor, CCP2 between LFS and LFA_VEE (89) is
used to remove common mode noise. Both CCP1 and CCP2
should have the same value. The second function of the
charge pump is to provide a binary control voltage to the
VCO depending upon the phase detector output. The
output pin, LFA (90) controls the VCO. Internally there is a
500Ω pull-up resistor, which is driven with a 100µA current
called ΙP. Another analog current ΙF, with 5mA maximum
drive proportional to the voltage across the CCP1 is applied
at the same node. The voltage at the LFA node is
VLFA_VCC - 500(ΙP+ΙF) at any time.
Because of the integrator, ΙF changes very slowly, whereas
ΙP could change at the positive edge of the data transition
as often as a clock period. In the locked position, the
average voltage at LFA (VLFA_VCC – 500(ΙP/2+ΙF) is such that
VCO generates frequency ƒ, equal to the data rate clock
frequency. Since ΙP is changing all the time between 0A and
100µA, there will be two levels generated at the LFA output.
connect LBCONT through a pull-up resistor (RPULL-UP). For
low loop bandwidth, leave LBCONT floating. The formula
below shows the change in the loop bandwidth using
RPULL-UP.
LBW
=
LBWNOMINAL
×
(---2---5----k---Ω------+-----R-----P---U----L--L----–----U---P----)
(5kΩ + RPULL – UP)
where LBWNOMINAL is the loop bandwidth when LBCONT is
left floating.
12. LOOP BANDWIDTH OPTIMIZATION
Since the feed back loop has only digital circuits, the small
signal analysis does not apply to the system. The effective
loop bandwidth scales with the amount of input jitter
modulation index. The following table summarizes the
relationship between input jitter modulation index and
bandwidth when RCP1 and CCP3 are not used. See the
Typical Application Circuit artwork for the location of RCP1
and CCP3.
TABLE 1: Relationship Between Input Jitter Modulation Index and
Bandwidth
INPUT JITTER
MODULATION
INDEX
BANDWIDTH
BW JITTER FACTOR
(jitter modulation x BW)
0.05
5.657MHz
282.9kHzUI
0.10
2.828MHz
282.9kHzUI
0.20
1.414MHz
282.9kHzUI
0.50
565.7kHz
282.9kHzUI
10. VCO
The GO1515 is an external hybrid VCO, which has a centre
frequency of 1.485GHz and is also guaranteed to provide
1.485/1.001GHz within the control voltage (3.1V – 4.65V) of
the GS1522 over process, power supply and temperature.
The GO1515 is a very clean frequency source and,
because of the internal high Q resonator, it is an order of
magnitude more immune to external noise as compared to
on-chip VCOs.
The VCO gain, Kƒ, is nominally 16MHz/V. The control
voltage around the average LFA voltage will be 500 x ΙP/2.
This will produce two frequencies off from the centre by
ƒ=Kƒ x 500 x ΙP/2.
11. LBCONT
The LBCONT pin (91) is used to adjust the loop bandwidth
by externally changing the internal charge pump current.
For maximum loop bandwidth, connect LBCONT to the
most positive power supply. For medium loop bandwidth,
The product of the input jitter modulation (IJM) and the
bandwidth (BW) is a constant. In this case, it is 282.9kHzUI.
The loop bandwidth automatically reduces with increasing
input jitter, which helps in cleaning up the signal as much
as possible.
Using a series combination of RCP1 and CCP3 in parallel to
an on-chip resistor (as shown in the Typical Application
Circuit) can reduce the loop bandwidth of the GS1522. The
parallel combination of the resistor is directly proportional to
the bandwidth factor. For example, the on-chip 500Ω
resistor yields 282.9kHzUI. If a 50Ω resistor is connected in
parallel, the effective resistance will be (50:500) 45.45Ω.
This resistance yields a bandwidth factor of
[282.9 x (45.45/500)] = 25.72kHzUI. The capacitance CCP3
in series with the RCP1 should be chosen such that the RC
factor is 50µF. For example, RCP1=50Ω would require
CCP3=1µF.
The synchronous lock time increases with reduced
bandwidth. Nominal synchronous lock time is equal to
[ 0.25 × 2 /Bandwidth factor]. That is, the default
bandwidth factor (282.9kHzUI) would yield 1.25µs. For
11
GENNUM CORPORATION
522 - 26 - 00
Share Link: 