SP8854E Ver la hoja de datos (PDF) - Mitel Networks
Número de pieza
componentes Descripción
Fabricante
SP8854E Datasheet PDF : 13 Pages
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SP8854E
VCC
296
296
VCC
40k
3k
3k
40k
296
24, 25 FPD, FREF
OUTPUTS
28
CRYSTAL
27
CAPACITOR
60k
60k
3·3mA
0V
Fig. 7g FPD and FREF outputs
50µA
50µA
100µA
100µA
0V
100µA
Fig. 7h Reference oscillator
APPLICATIONS
Fig. 7 Interface circuit diagrams (continued)
RF Layout
The SP8854E can operate with input frequencies up to
2·7GHz but to obtain optimum performance, good RF layout
practices should be used. A suitable layout technique is to use
double sided printed circuit board with through plated holes.
Wherever possible the top surface on which the SP8854E is
mounted should be left as a continuous sheet of copper to
form a low impedance ground plane. The ground pins 12 and
16 should be connected directly to the ground plane.
Pins such as VCC and the unused RF input should be
decoupled with chip capacitors mounted as close to the
device pin as possible, with a direct connection to the ground
plane; suitable values are 10nF for the power supplies and
<1nF for the RF input pin (a lower value should be used
sufficient to give good decoupling at the RF frequency of
operation). A larger decoupling capacitor mounted as close
as possible to pin 26 should be used to prevent modulation of
VCC by the charge pump pulses. The RSET resistor should also
be mounted close to the RSET pin to prevent noise pickup. The
capacitor connected from the charge pump output should be
a chip component with short connections to the SP8852E. All
signals such as the programming inputs, RF IN, REFERENCE
IN and the connections to the op-amp are best taken through
the pc board adjacent to the SP8852D with through plated
holes allowing connections to remote points without
fragmenting the ground plane.
Programming Bus
The input pins are designed to be compatible with TTL or
CMOS logic with a switching threshold set at about 2·4V by
three forward biased base-emitter diodes. The inputs will be
taken high by an internal pull up resistor if left open circuit but
for best noise immunity it is better to connect unused inputs
directly to VCC or ground.
RF Inputs
The prescaler has a differential input amplifier to improve
input sensitivity. Generally the input drive will be single ended
and the RF signal should be AC coupled to either of the inputs
using a chip capacitor.The remaining input should be
decoupled to ground, again using a chip capacitor. The inputs
can be driven differentially but the input circuit should not
provide a DC path between inputs or to ground.
Lock Detect Circuit
The lock detect circuit uses the up and down correction
pulses from the phase detector to determine whether the loop
is in or out of lock. When the loop is locked, both up and down
pulses are very narrow compared to the reference frequency,
but the pulse width in the out of lock condition continuously
varies, depending on the phase difference between the outputs
of the reference and RF counters. The logical AND of the up
and down pulses is used to switch a 20mA current sink to pin
18 and a 50kΩ resistor provides a load to VCC. The circuit is
shown in Fig. 7c.
When lock is established, the narrow pulses from the
phase detector ensure that the current source is off for the
majority of the time and so pin 18 will be pulled high by the
50kΩ resistor. A voltage comparator with a switching threshold
at about 4·7V monitors the voltage at pin 18 and switches pin
17 low when pin 18 is more positive than the 4·7V threshold.
When the loop is unlocked, the frequency difference at the
counter outputs will produce a cyclic change in pulse width
from the phase detector outputs with a frequency equal to the
difference at the reference and RF counter outputs. A small
capacitor connected to pin 18 prevents the indication of false
phase lock conditions at pin 17 for momentary phase
coincidence. Because of the variable width pulse nature of the
signal at pin 18 the calculation of a suitable capacitor value is
complex, but if an indication with a delay amounting to several
times the expected lock up time is acceptable, the delay will
be approximately equal to the time constant of the capacitor
on pin 18 and the internal 50kΩ resistor. If a faster indication
is required, comparable with the loop lock up time, the
capacitor will need to be 2 to 3 times smaller than the time
constant calculation suggests. The time to respond to an out
of lock condition is 2 to 3 times less than that required to
indicate lock.
Charge Pump Circuit
The charge pump circuit converts the variable width up and
down pulses from the phase detector into adjustable current
pulses which can be directly connected to the loop amplifier.
The magnitude of the current and therefore the phase detec-
tor gain can be modified when new frequency data is entered
to compensate for change in the VCO gain characteristic over
9
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