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SA8028 Datasheet(Fiches technique) 10 Page - NXP Semiconductors

Numéro de pièce SA8028
Description  2.5 GHz sigma delta fractional-N / 760 MHz IF integer frequency synthesizers
Télécharger  28 Pages
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Fabricant  PHILIPS [NXP Semiconductors]
Site Internet  http://www.nxp.com
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Philips Semiconductors
Product data
SA8028
2.5 GHz sigma delta fractional-N /
760 MHz IF integer frequency synthesizers
2002 Feb 22
10
1.5
RF and IF Charge Pumps
The RF phase detector drives the charge pumps on the PHP and
PHI pins, while the IF phase detector drives the charge pump on the
PHA pin. Both the RF and IF charge pump current values are
determined by the current generated at the RSET pin1. The current
gain can be further programmed by the CP0, CP1 bits in the C-word,
as seen in Table 1.
Table 1. RF and IF charge pump currents
CP1 2
CP0
IPHA
IPHP
IPHP–SU 3
IPHI
0
0
1.5xlSET
3xISET
15xlSET
36xlSET
0
1
0.5xlSET
1xlSET
5xlSET
12xlSET
1
0
1.5xlSET
3xlSET
15xlSET
0
1
1
0.5xlSET
1xlSET
5xlSET
0
NOTES
1. ISET = VSET/RSET: bias current for charge pumps.
2. CP1 = 1 is used to disable the PHI pump.
3. IPHP–SU is the total current at pin PHP during speed up condition.
1.6
Charge Pumps Speed-up Mode
The RF charge pumps will enter speed-up mode when STROBE
goes high after A-word has been sent. They will exit speed-up mode
on the next falling edge of STROBE. There is no speed-up mode for
the IF charge pump.
The charge pump, by default, will automatically go into speed-up
mode (which can deliver up to 15*ISET for PHP_SU, and 36*ISET for
PHI), based on the strobe pulse width following the A-word to
reduce switching speed for large tuning voltage steps (i.e., large
frequency steps). Figure 7 shows the recommended passive loop
filter configuration. Note: This charge pump architecture eliminates
the need for added active switches and reduces external component
count. Furthermore, the programmable charge pump gains provide
some programmability to the loop filter bandwidth.
The duration of speed-up mode is determined by the strobe pulse
that follows the A-word. Recommended optimal strobe width is equal
to the total loop filter capacitance charge time from VCO control
voltage level 1 to VCO control voltage level 2. The strobe width must
not exceed this charge time. An external data processing unit
controls the width of the strobe pulse (e.g.,
× number of clock
cycles).
In addition, charge pumps will stay in speed-up mode continuously
while Tspu = 1 (in D-word <D15>). The speed-up mode can also be
disabled by programming Tdis-spu = 1 (in D-word <D16>).
SR02356
VCO
C3
C2
R2
C1
R1
PHP[PHP–SU]
PHI
Figure 7.
Typical passive 3-pole loop filter.
1.7
Lock Detect
The output LOCK maintains a logic ‘1’ when the IF phase detector
(AND/ORed) with the RF phase detector indicates a lock condition.
The lock condition for the RF and IF synthesizers is defined as a
phase difference of less than
"1 period of the frequency at the input
REFin+, REFin–. One counter can fulfill the lock condition when the
other counter is powered down. Out of lock (logic ‘0’) is indicated
when both counters are powered down.
1.8
Power-down mode
With power applied to the chip, power-down mode can be entered
either by hardware (external signal on pin PON) or by software (by
programming the PD = Power Down bits (<B10, B9>) in the B-word).
The PON signal is exclusively ORed with the PD bits. If PON = 0,
then the part is powered up when PD = 1 (<B10, B9>). PON can be
used to invert the polarity of the software bits PD. Table 9 of section
2.4.2 illustrates how power-down mode can be implemented.
During power-down mode the 3-wire bus remains active and
programming-words may be pre-loaded before switching to
power-up mode. If the chip is programmed while in power-down
mode, the RF divider ratio NRF is internally presented to the RF
divider on the next falling edge of STROBE after STROBE has gone
high at the end of the A-word. Power-down mode does not reset the
sigma-delta modulator., i.e., power-down mode preserves the state
of the sigma-delta modulator (as long as power is applied to the
chip).
To take advantage of the register pre-loading capability while the
device is in power-down mode, the B-word needs to be sent a
second time (i.e., again, after the A-word), with the PD (<B10, B9>)
bits now programmed for power-up.
If power-up mode is to be controlled by hardware, the PON signal
must be toggled only after the A-word has been sent and STROBE
has gone high and then low.
When the synthesizer is reactivated after power-down mode, the IF
and reference dividers are synchronized to avoid random phase
errors on power-up. There is no power-up synchronization between
the RF divider and the reference clock. After power-up, there is a
delay of four edges (i.e. 1.5 cycles) of the output clock of the
reference divider before the RF phase detector is activated. That
means the reference divider must be powered up for the RF phase
detector to become active.
When initially applying or reapplying power to the chip, and internal
power-up reset pulse is generated which sets the programming-words
to their default values and also resets the sigma-delta modulator to
its “all-0” state. It is also recommended that the D-word be manually
reset to all zeros, following initial power-up, to avoid unknown states.




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