DS2438 - 2.3
NJ88C24
Frequency Synthesiser with non-resettable counters
The NJ88C24 is a synthesiser circuit fabricated on the GPS
CMOS process and is capable of achieving high sideband
attenuation and low noise performance. It contains a reference
oscillator, 11-bit programmable reference divider, digital and
sample-and-hold comparators, 10-bit programmable `M' counter,
7-bit programmable `A' counter and the necessary control and
latch circuitry for accepting and latching the input data.
Data is presented serially under external control from a
suitable microprocessor. Although 28 bits of data are initially
required to program all counters, subsequent updating can be
abbreviated to 17 bits, when only the `A' and`M' counters require
changing.
The NJ88C24 is intended to be used in conjunction with a
two-modulus prescaler such as the SP8710 or SP8705 series
to produce a universal binary coded synthesiser for up to
1100MHz operation.
Fig.1 Pin connections - top view (not to scale)
FEATURES
s
Low Power Consumption
s
High Performance Sample and Hold Phase Detector
s
Serial Input with Fast Update Feature
s
>20MHz Input Frequency
s
Fast Lock-up Time
DG16, DP16
MP18
Fig.2 Block diagram
7 (9)
8 (10)
10 (12)
12 (14)
11 (13)
4 (5)
6 (7)
5 (6)
OSC IN
OSC OUT
DATA
ENABLE
CLOCK
F
IN
V
DD
V
SS
REFERENCE COUNTER
(11BITS)
LATCH 6
LATCH 7
LATCH 8
LATCH 1
LATCH 2
LATCH 3
`M' COUNTER
(10 BITS)
`M' REGISTER
`R' REGISTER
CONTROL LOGIC
LATCH 4
LATCH 5
`A' COUNTER
(7 BITS)
`A' REGISTER
4
2
SAMPLE/HOLD
PHASE
DETECTOR
FREQUENCY/
PHASE
DETECTOR
V
SS
PDA
PDB
LOCK DETECT (LD)
MODULUS
CONTROL
OUTPUT (MC)
1 (1)
2 (2)
3 (4)
14 (16)
RB CAP CH
15 17 16
f
r
f
V
(17) (15) (18)
ORDERING INFORMATION
NJ88C24 MA DG Ceramic DIL Package
NJ88C24 MA DP Plastic DIL Package
NJ88C24 MA MP Miniature Plastic DIL Package
205V to 7V
7V
V
SS
203V to V
DD
103V
255
C to
1125
C
(DP and MP packages)
265
C to
1150
C
(DG package)
ABSOLUTE MAXIMUM RATINGS
Supply voltage, V
DD
2V
SS
:
Input voltage
Open drain output, LD pin:
All other pins:
Storage temperature:
1
2
3
4
5
6
7
8
9
18
17
16
15
14
13
12
11
10
NJ88C24
PDA
PDB
NC
LD
F
IN
V
SS
V
DD
NC
OSC IN
CH
RB
MC
CAP
ENABLE
CLOCK
DATA
NC
OSC OUT
CH
RB
MC
CAP
ENABLE
CLOCK
DATA
NC
PDA
PDB
LD
FIN
V
SS
V
DD
OSC IN
OSC OUT
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
NJ88C24
NJ88C24
Characteristic
Units
Conditions
Min.
Typ.
Max.
F
IN
and OSC IN input level
200
mV RMS 10MHz AC-coupled sinewave
Max. operating frequency, f
F
IN
and f
osc
20
MHz
Input squarewave V
DD
to V
SS
,
25
C.
Propagation delay, clock to modulus control MC
30
50
ns
See note 2
Programming Inputs
Clock high time, t
CH
05
s
Clock low time, t
CL
05
s
Enable set-up time, t
ES
02
t
CH
s
Enable hold time, t
EH
02
s
Data set-up time, t
DS
02
s
Data hold time, t
DH
02
s
Clock rise and fall times
02
s
High level threshold
V
DD
208
V
See note 1
Low level threshold
08
V
See note 1
Hysteresis
10
V
See note 1
Phase Detector
Digital phase detector propagation delay
500
ns
Gain programming resistor, RB
5
k
Hold capacitor, CH
1
nF
See note 3
Programming capacitor, CAP
1
nF
Output resistance, PDA
5
k
Characteristic
Units
Conditions
Min.
Typ.
Max.
Supply current
55
mA
f
osc
, f
F
IN
= 10MHz
1.5
mA
f
osc
, f
F
IN
= 1MHz
Modulus Control Output (MC)
High level
46
V
I
SOURCE
= 1mA
Low level
04
V
I
SINK
= 1mA
Lock Detect Output (LD)
Low level
04
V
I
SINK
= 4mA
Open drain pull-up voltage
70
V
PDB Output
High level
46
V
I
SOURCE
= 5mA
Low level
04
V
I
SINK
= 5mA
3-state leakage current
01
A
2
ELECTRICAL CHARACTERISTICS AT V
DD
= 5V
Test conditions unless otherwise stated:
V
DD
V
SS
=5V
05V. Temperature range = 40
C to +85
C
DC Characteristics
Value
AC
Characteristics
All timing periods
are referenced to
the negative
transition of the
clock waveform
Value
0 to 5V
square
wave
NOTES
1. Data, Clock and Enable inputs are high impedance Schmitt buffers without pull-up resistors; they are therefore not TTL compatible.
2. All counters have outputs directly synchronous with their respective clock rising edges.
3. The finite output resistance of the internal voltage follower and `on' resistance of the sample switch driving this pin will add a finite time constant
to the loop. An external 1nF hold capacitor will give a maximum time constant of 5
s.
4. The inputs to the device should be at logic `0' when power is applied if latch-up conditions are to be avoided. This includes the signal/osc.
frequency inputs.
NJ88C24
PIN DESCRIPTIONS
Name
Description
Analog output from the sample and hold phase comparator for use as a `fine' error signal. Voltage
increases as f
v
(the output from the `M' counter) phase lead increases; voltage decreases as f
r
(the
output from the reference counter) phase lead increases. Output is linear over only a narrow phase
window, determined by gain (programmed by RB). In a type 2 loop, this pin is at (V
DD
2V
SS
)/2 when the
system is in lock.
Three-state output from the phase/frequency detector for use as a `coarse' error signal.
f
v
. f
r
or f
v
leading: positive pulses with respect to the bias point V
BIAS
f
v
, f
r
or f
r
leading: negative pulses with respect to the bias point V
BIAS
f
v
= f
r
and phase error within PDA window: high impedance.
Not connected.
An open-drain lock detect output at low level when phase error is within PDA window (in lock); high
impedance at all other times.
The input to the main counters. It is normally driven from a prescaler, which may be AC-coupled or,
when a full logic swing is available, may be DC-coupled.
Negative supply (ground).
Positive supply (normally 5V)
Not connected.
These pins form an on-chip reference oscillator when a series resonant crystal is connected across
them. Capacitors of appropriate value are also required between each end of the crystal and ground
to provide the necessary additional phase shift. The addition of a 220
resistor between OSC OUT and
the crystal will improve stability. An external reference signal may, alternatively, be applied to OSC IN.
This may be a low-level signal, AC-coupled, or if a full logic swing is available it may be DC-coupled.
The program range of the reference counter is 3 to 2047 , with the total division ratio being twice the
programmed number.
Not connected.
Information on this input is transferred to the internal data latches during the appropriate data read time
slot. DATA is high for a `1' and low for a `0'. There are three data words which control the NJ88C24;
MSB is first in the order: `A' (7 bits), `M' (10 bits), `R' (11 bits).
Data is clocked on the negative transition of the CLOCK waveform. If less than 28 negative clock
transitions have been received when the ENABLE line goes low (i.e., only `M' and `A' will have been
clocked in), then the `R' counter latch will remain unchanged and only `M' and `A' will be transferred from
the input shift register to the counter latches. This will protect the `R' counter from being corrupted by
any glitches on the clock line after only `M' and `A' have been loaded If 28 negative transitions have
been counted, then the `R' counter will be loaded with the new data.
When ENABLE is low, the DATA and CLOCK inputs are disabled internally. As soon as ENABLE is
high, the DATA and CLOCK inputs are enabled and data may be clocked into the device. The data is
transferred from the input shift register to the counter latches on the negative transition of the ENABLE
input and both inputs to the phase detector are synchronised to each other.
This pin allows an external capacitor to be connected in parallel with the internal ramp capacitor and
allows further programming of the device. (This capacitor is connected from CAP to V
SS
).
Modulus control output for controlling an external dual-modulus prescaler. MC will be low at the beginning
of a count cycle and will remain low until the `A' counter completes its cycle. MC then goes high and
remains high until the `M' counter completes its cycle, at which point both `A' and `M' counters are reset.
This gives a total division ratio of
MP1A, where P and P11 represent the dual-modulus prescaler
values. The program range of the `A' counter is 0-127 and therefore can control prescalers with a
division ratio up to and including 4128/129. The programming range of the `M' counter is 8-1023
and, for correct operation,
M>A. Where every possible channel is required, the minimum total division
ratio
N should be: N>P
2
2
P, where N = MP1A.
An external sample and hold phase comparator gain programming resistor should be connected
between this pin and V
SS
.
An external hold capacitor should be connected between this pin and V
SS
.
PDA
PDB
NC
LD
F
IN
V
SS
V
DD
NC
OSC IN/
OSC OUT
NC
DATA
CLOCK
ENABLE
CAP
MC
RB
CH
1
2
3
4
5
6
7
8
9,10
12
13
14
15
16
17
18
DG,DP MP
Pin no.
1
2
3
4
5
6
7, 8
9
10
11
12
13
14
15
16
3
NJ88C24
Note that
M>A and
f
VCO
fcomp
For example, if the desired VCO frequency = 275MHz, the
comparison frequency is 125kHz and a two-modulus prescaler
of 464/65 is being used, then
275310
6
125310
3
Now,
N = MP1A, which can be rearranged as N/P = M1A/P.
In our example we have
P = 64, therefore
such that
M = 343 and A /64 = 075.
Now,
M is programmed to the integer part = 343 and A is
programmed to the fractional part364 i.e.,
A = 075364 = 48.
NB The minimum ratio
N that can be used is P
2
2
P (=4032 in
our example) for all contiguous channels to be available.
To check:
N = 343364148 = 22000, which is the required
division ratio and is greater than 4032 ( =
P
2
2
P ).
When re-programming, the counters are changed only at
the zero state. There is no reset to zero , which means that the
synthesiser loop lock-up time will be variable. When only small
changes in frequency are required, the non-resettable
synthesiser should achieve the shortest loop lock-up times.
Fig. 3 Typical supply current v. input frequency
Fig. 4 Typical supply current v. input level, OSC IN
f
osc
23
fcomp
PROGRAMMING
Reference Divider Chain
The comparison frequency depends upon the crystal
oscillator frequency and the division ratio of th `R' counter,
which can be programmed in the range 3 to 2047, and a fixed
divide by two stage.
where
fosc
=
oscillator frequency,
fcomp
= comparison frequency,
R = `R' counter ratio
For example, where the crystal frequency = 10MHz and a
channel spacing comparison frequency of 125kHz is required,
10
7
23125310
3
Thus, the `R' register would be programmed to 400 expressed
in binary. The total division ratio would then be 23400 = 800
since the total division ratio of the `R' counter plus the 42 stage
is from 6 to 4094 in steps of 2.
VCO Divider Chain
The synthesised frequency of the voltage controlled oscillator
(VCO) will depend on the division ratios of the `M' and `A'
counters, the ratio of the external two-modulus prescaler
(
P/P11)and the comparison frequency .
The division ratio
N = MP1A,
where
M is the ratio of the `M' counter in the range 8 to 1023
and
A is the ratio of the `A' counter in the range 0 to 127.
R =
R =
= 400
N =
= 22310
3
N =
Fig. 5 Timing diagram showing timing periods required for correct operation
t
EH
t
EH
t
ES
t
DH
t
ES
t
DS
t
CH
t
CL
CLOCK
ENABLE
DATA
4
V
DD
= 5V
OSC IN, F
IN
= 0V TO 5V SQUARE WAVE
F
IN
OSC IN
INPUT FREQUENCY (MHz)
1 2 3 4 5 6 7 8 9 10
SUPPLY CURRENT (mA)
20
15
10
05
TOTAL SUPPLY CURRENT IS
THE SUM OF THAT DUE TO
F
IN
AND OSC IN
INPUT LEVEL (V RMS)
02 04 06 08 10 12 14 16
SUPPLY CURRENT (mA)
8
7
6
5
4
3
2
1
V
DD
= 5V
F
IN
= LOW FREQUENCY
0V TO 5V SQUARE WAVE
1MHz
10MHz
22
3
10
3
64
=
M1
A
64
NJ88C24
5
1
2
3
4
5
(15)26
(16)27
(17)28
A
6
A
5
A
4
A
3
A
2
(M
2
)R
2
(M
1
)R
1
(M
0
)R
0
CLOCK
ENABLE
DATA
Fig.6 Timing diagram showing programming details
is sampled at the reference frequency to give the `fine' error
signal, PDA. When in phase lock, this output would be typically
at (V
DD
2V
SS
)/2 and any offset from this would be proportional
to phase error.
The relationship between this offset and the phase error is
the phase comparator gain,
K
PDA
, which is programmable with
an external resistor, RB, and a capacitor, CAP. An internal
50pF capacitor is used in the sample and hold comparator.
CRYSTAL OSCILLATOR
When using the internal oscillator, the stability may be
enhanced at high frequencies by the inclusion of a resistor
between the OSC OUT pin and the other components. A value
of between 150
and 270
is advised, depending on the
crystal series resistance.
PROGRAMMING/POWER UP
Data and signal input pins should not have input applied to
them prior to the application of V
DD
, as otherwise latch-up may
occur.
PHASE COMPARATORS
Noise output from a synthesiser loop is related to loop gain:
K
PD
K
VCO
N
where
K
PD
is the phase detector constant (volts/rad),
K
VCO
is
the VCO constant (rad/sec/volt) and
N is the overall loop division
ratio. When
N is large and the loop gain is low, noise may be
reduced by employing a phase comparator with a high gain.
The sample and hold phase comparator in the NJ88C24 has
a high gain and uses a double sampling technique to reduce
spurious outputs to a low level.
A standard digital phase/frequency detector driving a three-
state output,PDB, provides a `coarse' error signal to enable
fast switching between channels.
The PDB output is active until the phase error is within the
sample and hold phase detector window, when PDB becomes
high impedance. Phase-lock is indicated at this point by a low
level on LD. The sample and hold phase detector provides a
`fine' error signal to give further phase adjustment and to hold
the loop in lock. An internally generated ramp, controlled by the
digital output from both the reference and main divider chains,
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