NJ88C50
Dual Low Power Frequency Synthesiser
DS3805 - 1.8 July 1995
The NJ88C50 is a low power integrated circuit, designed
as the heart of a fast locking PLL subsystem in a mobile radio
application. It is manufactured on Mitel Semiconductor 1.4
micron double polysilicon CMOS process, which ensures that
low power and low noise performance is achieved. The device
contains two synthesisers, one for the generation of VHF
signals up to 125MHz and a second for UHF (when used with
a mulitmodulus prescaler such as the SP8713/14/15). The
main synthesiser has the capability of driving a dual speed
loop filter and also can perform Fractional-N interpolation.
Both synthesisers use current source outputs from their
phase detectors to minimise external components. Various
sections may be powered down for battery economy.
FEATURES
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30MHz main synthesiser
125MHz auxiliary synthesiser
Programmable output current
from phase detector - up to 10mA
High input sensitivity
Fractional-N interpolator
Supports up to 4 modulus prescalers
SSOP package
NP20
Fig.1 Pin assignment
ABSOLUTE MAXIMUM RATINGS
Storage temperature
Operating temperature
Supply voltage
Voltage on any pin
-55°C to +150°C
-40°C to +85°C
-0.5 to 7.0V
-0.3V to (V
DD
+ 0.3V)
APPLICATIONS
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s
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NMT, AMPS, ETACS cellular
GSM, IS-54, RCR-27 cellular
DCS1800 microcellular
DLMR, DSRR, TETRA
DECT, PHP cordless telephones
ORDERING INFORMATION
NJ88C50\IG\NPAS - (Industrial temp range in SSOP
package)
Fig.2 Simplified block diagram
NJ88C50
ARCHITECTURE
Fig.2 shows a simplified block diagram of the NJ88C50, a
more detailed description of each block and its function is
given later in this datasheet.
The synthesiser consists of the following blocks
- 35MHz reference frequency input buffer
- 35MHz programmable reference divider
- 125MHz Auxiliary synthesiser input buffer
- 125MHz Auxiliary synthesiser programmable divider
- Auxiliary synthesiser phase detector with current source
outputs
- 30MHz main synthesiser input buffer (differential inputs)
- 30MHz main synthesiser programmable divider and control
logic
- Main synthesiser Fractional-N interpolation system
- Main synthesiser phase detector with dual current source
outputs
PIN DESCRIPTION
Pin
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Name
AVDD
FIM
FIMB
DATA
CKIN
STROBE
RI
FIA
RSA
PDA
PDI
GND
PDP
VDD
RSM
RSC
SCREEN
MOD1
MOD2
AGND
Function
Analog supply pin (nominally 5V).
Main synthesiser balanced input buffer, may be used with single ended prescaler output if Fimb
is biased.
Main synthesiser balanced input buffer, may be used with balanced prescaler output, or biased
for single ended operation.
Serial input for programming data.
Serial clock input for programming bus.
Program enable pin, active low.
Master reference frequency input, should be a.c coupled from an accurate source.
Auxiliary synthesiser frequency input, should be a.c coupled.
Current setting resistor connection defining auxiliary phase detector output current.
Tristate current output from auxiliary phase detector.
Tristate current output from the main synthesiser's phase detector giving integral control.
Digital ground supply pin.
Tristate current output from the main synthesiser's phase detector giving proportional control.
Digital supply pin (nominally 5V).
Current setting resistor connection defining main synthesiser's phase detector output currents.
Current setting resistor connection defining the compensation current for fractional-N ripple
elimination in the main synthesiser's current source outputs.
To be connected to ground to provide isolation of the modulus control pins from RF interference.
Modulus control pin (see truth table).
Modulus control pin (see truth table).
Analog ground supply pin.
It is recommended that power supply pins are well decoupled to minimise power rail born interference.
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NJ88C50
FUNCTIONAL DESCRIPTION
The NJ88C50 has been designed using a modular
concept, and its operation can be best summarised as these
component blocks.
Reference divider
The reference divider is used to provide the reference
signals needed for both the main and auxiliary synthesiser
phase detectors. The divider allows for a twelve bit number to
be loaded, via the serial bus, to select the required division
ratio. Division ratios of 3 to 4095 can be used.
The reference divider input stage will accept a low level,
AC coupled, sinewave input. It is anticipated that in most
systems this will be provided by a stable reference source up
to 35MHz, and so encompasses all the common TCXO
(temperature controlled crystal oscillator) frequencies, such
as 9.6, 12.8, 13.0, 19.44 and 26MHz.
A standby mode is supported so that the reference divider
can be powered down, this is achieved using two of the serial
program control bits.
To reduce the possibility of unwanted interaction between
the main and auxiliary synthesisers, the charge pumps do not
take current at the same time. To achieve this the output of the
reference divider has a duty factor of approximately 50:50,
which then allows the Q and QBAR taps to be used for the
auxiliary and main synthesisers respectively. By doing this the
current pulses can be taken alternatively, minimising
modulation of the power supply rails as current is drawn.
The reference divider consists of a 12 bit programmable
divider followed by a 4 bit binary counter. This 4 bit counter
gives a choice of divide by M, 2M, 4M or 8M.
A pair of programmable control bits are used to determine
which of the divide by M, 2M, 4M or 8M outputs is supplied to
the auxiliary synthesiser’s phase detector and a further pair of
control bits are used to determine which are supplied to the
main synthesiser’s phase detector.
Auxiliary synthesiser
The auxiliary synthesiser operates over an input frequency
range from 1 to 125MHz, without the use of an external
prescaler. The synthesiser consists of a 12 bit N divider and
a digital phase comparator with current source outputs. The
reference frequency is supplied by the shared reference
divider. Current source outputs allow a passive loop filter to
be used.
When the auxiliary synthesiser is not in use, a standby
mode is supported so that power consumption is reduced.
This is achieved using one of the serial program control bits.
The divider is programmed with a 12 bit word allowing
division ratios of 3 to 4095 to be used.
The auxiliary phase detector consists of the 2 D-type
phase and frequency detector shown in Figure.3 below, the
high and low outputs of which drive on-chip, opposing
complementary charge pumps. This type of phase detector
design eliminates non linearity or deadband around the zero
phase error (locked) condition.
The charge pump output current level is set by an external
resistor on the RSA pin (pin 9) up to a limit of 250µA +/-10%.
A pull up current pulse will indicate that the VCO frequency
must be increased, whilst a pull down pulse indicates that the
frequency must be decreased.
Fig.3 Auxiliary phase detector
Main Synthesiser
The main synthesiser is capable of operating at
frequencies up to 30MHz. The synthesiser uses the 12 bit
reference divider, shared with the auxiliary synthesiser, a 12
bit up/down N divider and a digital phase comparator with
current source outputs.
The device also has a number of features which increase
the design flexibility and performance of the synthesiser.
These include fractional-N operation, speed up mode and
support of 2, 3 and 4 modulus prescalers. A description of the
operation and advantages of each of these features is given.
The main N divider input buffer will accept inputs from
an external prescaler, either as balanced (2 wire) ECL levels
at frequencies up to 30MHz, or DC coupled to a single ended
prescaler output. Single ended operation requires the other
buffer input (pin 3) to be externally biased to the correct slicing
voltage for the prescaler and also externally decoupled.
If the inputs are in the form of balanced ECL levels, there
must not be a skew of greater than 2ns between one input
changing and the second input changing. The relationship of
the signals is shown below in Fig.4.
Fig.4 Maximum input skew
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NJ88C50
The main N divider is programmable so that it can
determine how many cycles of each division ratio the external
prescaler will perform.
The total division ratio of the output from the system VCO
to the synthesiser's phase detector may be expressed as N
TOT
and R1, R2, R3 and R4 are the available prescaler ratios and
N1, N2, N3 and N4 are the corresponding number of cycles for
each ratio selected, within one complete division cycle.
The divider is programmed via the serial data bus and the
values needed to be programmed for each of the possible
prescaler ratios are as follows:-
In 2 modulus mode (division ratios R1, R2)
N
TOT
= N1.R1 + N2.R2
Programmed values needed:
N1 - a 12 bit value giving the number of times R1 is to be used
N2 - a 8 bit value giving the number of times R2 is to be used
In 3 modulus mode (division ratios R1, R2, R3)
N
TOT
= N1.R1 + N2.R2 + N3.R3
Programmed values needed:
N1 - a 12 bit value giving the number of times R1 is to be used
N2 - a 4 bit value giving the number of times R2 is to be used
N2+N3 - a 4 bit value where N3 is the number of times R3 is
to be used and (N2+N3) is modulo-16 addition
In 4 modulus mode (division ratios R1, R2, R3, R4)
N
TOT
= N1.R1 + N2.R2 + N3.R3 + N4.R4
Programmed values needed:
N1 - a 12 bit value giving the number of times R1 is to be used
N2 - a 4 bit value giving the number of times R2 is to be used
N2+N3 - a 4 bit value where N3 is the number of times R3 is
to be used.
N2+N3+N4 - a 4 bit value where N4 is the number of times R4
is to be used. (N2+N3) and (N2+N3+N4) are modulo-16
addition.
To facilitate the use of multimodulus prescalers the N
divider is based upon a twelve bit up/down counter which
functions as follows
The first value, N1, is loaded into the counter which then
counts down from N1 to zero. During this time, the modulus
ratio R1 is selected.
When the counter reaches zero modulus R2 is selected
and the counter then counts up to the N2 value. If 2 modulus
operation is chosen, the counter is then reloaded with N1 and
the count is repeated.
For operation with 3 or 4 modulus devices, the counter
continues to count up once it has reached the N2 value. The
count continues to the N2+N3 value and during this time the
R3 ratio is selected. In the 3 modulus case, when the N2+N3
value is reached the counter is then reloaded with the N1
value and the modulus ratio R1 is selected.
For 4 modulus operation the counter will continue its count
up to the N2+N3+N4 value before reloading the N1 value.
During this time the R4 modulus is selected.
If N2, N3, or N4 are set to zero this will give a full count of 16
for the corresponding modulus.
The N divider block also has a special control line from the
Fractional-N logic. When required this control will cause the
total division ratio to be increased from N to N+1. This is
achieved by forcing a cycle which would have normally used
a prescaler ratio R1 to use ratio R2 instead. R1 and R2 are
chosen so that R2 equals R1+1.
Further explanation of the operation of the synthesiser
when using 2, 3 or 4 modulus prescaler is given in the section
on multimodulus division (page 8).
The phase detector used on the main synthesiser is
similar to the type used on the auxiliary synthesiser (Figure.3).
In this case, however, the detector will drive two pairs of
complimentary charge pumps, one of which is intended to
drive the loop integrator capacitor to provide integral control,
whilst the other provides proportional control for the VCO.
This system is shown in Fig 5, and has applications where fast
locking of the loop is required.
Fig.5 Loop filter using both charge pumps
MODES OF OPERATION
Normal Mode
The synthesiser will operate in normal mode while the
strobe line of the serial data bus is low. In this mode the
following current levels are produced. The charge pump
providing the proportional feedback term will have a normal
current level designated by Iprop(0), that is set by an external
bias resistor, RSM. Iprop(0) will vary when different N-divider
ratios are programmed, so that it is proportional to the total
division ratio. To avoid the necessity of computing the total
division ratio on chip, an eight bit number representing the
most significant bits of Ntot will be loaded via the serial data
bus. Iprop(0) is therefore given by
Iprop(0) = CN.Ibo
where CN is the loaded eight bit number and the value Ibo is
scaled from the external current setting resistor RSM where
Ibo = Irsm/32. Typically Ibo = 1µA ,and therefore Iprop(0) will
have a maximum value equal to 255µA.
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NJ88C50
The normal value of Iprop, Iprop(0), is obtained while the
strobe line of the serial programming bus is held low. In this
condition, the second charge pump providing the integral
feedback term is inactive.
Speed up Mode
In speed up mode the loop bandwith during switching
is increased to allow faster initial frequency acquisition. This
is done by using the dual phase detector outputs (PDP and
PDI) connected to a standard passive loop filter as shown in
fig.5. The effect of this is to increase the loop gain and hence
the bandwidth while maintaining a constant phase margin
when switching between speed up mode and normal mode.
The synthesiser operates in speed up mode when the
strobe line goes high loading either word A or word A2 (see
programming section Page 8-Page 9) and it will stay in this
mode until the strobe line goes low. In this mode the following
current levels are produced. The charge pump providing the
proportional feedback will increase its current from Iprop(0) to
a value Iprop(1), where
Iprop(1) = 2
L+1
.Iprop(0)
where L is a two bit number loaded as part of the serial
programming data. Iprop(1) will therefore be 2, 4, 8 or 16
times Iprop(0). The charge pump supplying Iprop is specified
up to a value of 1mA.
Also when the strobe line goes high loading word A or
word A2, the charge pump providing the integral feedback
term becomes active at a current level Iint given by
Iint = K.Iprop(1)
where K is a four bit number loaded as part of the serial
programming data. Although Iint can be programmed to be
240 times greater than Iprop(0), the charge pump supplying
Iint is only specified up to a value of 10mA.
For all charge pumps, a pull-up current indicates the VCO
frequency should be increased while a pull-down current
indicates the VCO frequency should be decreased.
For the proportional and integral charge pumps, the
selected pulse current levels will remain substantially
constant over the charge pumps output voltage ranges
tabulated in the electric characteristics. “Substantially
constant” means that the current will not have changed by
more than 10% of the value measured at 2.5 volts on the
output .
ratio. Using fractional-N the value of N is alternated between
N and N+1 in order to simulate a fractional part. For example
9000.375 would be simulated by alternating between 9000
and 9001 in the pattern
9000, 9000, 9001, 9000, 9000, 9001, 9000, 9001 (mean value of 9000.375).
On the NJ88C50 the fractional-N circuit consists of an
accumulator which can be set to overflow at a value of 5 or 8
(FMOD in programming word D, see page 9). The value in the
accumulator, A, is incremented once every comparison cycle
of the main phase detector and every time the accumulator
overflows the total division ratio of the synthesiser and
prescaler is increased from N to N+1. To obtain the pattern
described above N=9000 and FMOD would be set to mod8
and the incremental value, NF(programmed in word A) would
be set to 3. The accumulator would then behave as shown
below.
Increment
Value
3
3
3
3
3
3
3
3
Accumulator
Value
3
6
1
4
7
2
5
0
Total Division
Ratio
9000
9000
9001
9000
9000
9001
9000
9001
Varying NF allows different fractions to be obtained. If NF=1
and FMOD=8 the accumulator would overflow once in every
8 cycles giving a value of 9000.125. Similarly if NF=4 the
accumulator overflows every other cycle giving 9000.5.
For a given step size this increase in resolution means a higher
comparison frequency at the phase detector, and therefore a
lower overall division ratio. For example,
with a
step size = 200kHz
and carrier frequency = 900MHz
Non fractional-N synthesiser
Comparison frequency=200kHz
Division ratio=900MHz=4500
200kHz
Fractional-N synthesiser (using 5ths)
Comparison frequency=1MHz
Division ratio=900MHz=900
1MHz
In most applications the phase noise is proportional to the
overall division ratio. Therefore fractional-N gives lower phase
noise. This higher comparison frequency and lower phase
noise allows circuits to be built with wider loop bandwidths
while keeping the same stability. This means that phase
locked loops (PLLs) can be made to either switch faster for a
given phase noise or be quieter for a given switching speed,
compared to conventional designs.
FRACTIONAL-N OPERATION
Conventional, non fractional-N synthesisers have a
frequency resolution or step size equal to the phase detector
comparison frequency. Fractional-N refers to a technique
which allows finer frequency steps to be obtained.
The synthesised frequency with a conventional
synthesiser is equal to N times the phase detector comparison
frequency, where N is the programmable integer loop divide
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