SP8858
controlling the dividers without disrupting the loop (and vice
versa).Thisfacilitycanbeusedtoreducesynthesiserswitching
time by preparing the non-active buffer prior to the instant of
switching and can also be used to modify the open loop gain.
To ensure reliable data is loaded into the dividers the
internal control circuits ensure that the buffer data can only be
updated if the remaining M count is greater than 3. Given this
restriction, the maximum time taken to update the buffer after
the negative going ENABLE transition (or after F1/F2 has
been toggled) is:
APPLICATIONS
Introduction
This section provides the basic information required to
implement a complete digital PLL synthesiser based on the
SP8858. A typical circuit is shown in Fig. 12 and is available
on a demonstration PCB, including a serial programmer. The
demonstrationboardcanbeusedtoevaluatetheSP8858and
can be readily adapted by the system/RF designer for a
specific application to aid in rapid prototype development.
Users of the SP8853 should consult Appendix A for details
ofthedesignchangesthatarerequiredtoreplacetheSP8853
with the SP8858.
[(31M) N1A]/RF input150ns
where update time is in seconds and RF input is in Hz.
The time taken to re-program the shift register (F1or F2)
isdeterminedbytheclockrateandthenumberofbitsrequired
and is equal to:
PLL Basics
A system level specification for a stable radio signal will
include measures of signal stability such as a single sideband
phase noise specification and a spurious output specification.
The power spectrum of the composite RF output signal is
influenced by a number of factors:
243tREP1tS1tE (see Fig. 4)
If the reference buffer is selected (C2 = 1, C1 = 1), only the 16
LSBsoftheshiftregisterareused.13bitsprovidethedataforthe
Reference divider. Two bits, PD1 and PD2, control the charge
pump and the divider output buffer as shown in Table 4.
● Residual phase noise of the dividers
● Active loop filter residual noise
● Feedback divider ratio
● Phase detector gain
PD2
PD1
Result
● VCO signal phase noise and gain
● Reference signal phase noise
● The closed loop root locations (an under damped loop will
cause a noise peak)
0
0
FREF and FPD outputs off, charge pump on
0
1
1
1
0
1
FREF and FPD outputs on, charge pump on
FREF and FPD outputs on, charge pump off
● Environmental influences such as EMI and power supply
noise
FREF and FPD outputs on, charge pump
disabled by lock detect
A single-loop synthesiser based around the SP8858 is
suitable for the synthesis of highly stable, low phase noise
signals provided each of the points above are carefully
considered.
Table 4
The remaining bit of the Reference word is used to select
the prescaler modulus. A ‘1’ in this position selects the 8/9
mode. Note that when the 8/9 mode is selected the A divider
only requires 3 bits; the 4th bit must be set to ‘0’.
The block diagram of a simple PLL is shown in Fig. 7.
To ensure reliable data is loaded into the dividers the
internal control circuits ensure that the buffer data can only be
updated if the remaining R count is greater than 1. Given this
restriction, the maximum time taken to update the buffer after
the negative going ENABLE transition (or after F1/F2 has
been toggled) is:
PHASE
DETECTOR
(mA/RAD) K
LOOP FILTER
(V/mA)
VCO
(RAD/SEC/V)
PD
F
O
(Hz) fo(s)
F
REF
(Hz)
K
VCO
s
(s)
fi
F(s)
1/FREF150ns
F
(Hz)
PD
DIVIDER
Only 16 bits are required to program the reference buffer,
therefore reference programming time tREF is:
4N
tREF =163tREP1tS1tE (see fig. 4)
If the Active A mode is programmed (C2=0. C1=1) only the
four A divider bits are updated at the end of the M count. The
M divider data, multiplication factor and phase detector sense
remain unchanged. This can be used to frequency hop to an
adjacent channel with the programming time reduced to:
(s)
f
o
F(s)3K
3K
PD
VCO
=
CLOSED LOOP RESPONSE =
OPEN LOOP DC GAIN =
fi(s)
s1F(s)3K
3K /N
VCO
PD
K
3K /N
VCO PD
Fig. 7
Programming time (Active A) = 63tREP1tS1tE
The basic aim is to phase-lock the VCO signal to a stable
reference signal, fi(s) and, ideally, set a relatively wide
closed loop bandwidth and a high DC loop gain
(KPD3KVCO/N). This combination will ensure that the free-
running VCO phase noise is attenuated and that both the
long-term and the short-term stability of the output signal is
determined by the properties of the reference signal. A wide
loop bandwidth would also be consistent with the requirement
of many synthesiser specifications to change frequency and
regain phase lock within a specified time limit. In practice, the
following considerations limit the closed loop bandwidth and
the DC gain and, consequently, limit the extent to which the
ideal system is achieved:
Theprogrammingdetailsdiscussedabovearesummarised
in Fig. 6.
Lock Detect
A simple Exclusive-OR phase detector together with an
integrator and comparator are used to indicate phase lock.
Capacitor CD on pin 28 sets the integrator time constant
and hence the sensitivity of the lock detect function. The
comparator controls a current sink connected to pin 27 which
can be used together with an external LED or resistor to
indicate phase lock.
The lock detect can also be used to disable the charge pump
by programming PD1 and PD2 of the reference word (Table 4).
8
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