SP8852D データシートの表示（PDF） - Zarlink Semiconductor Inc
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SP8852D Datasheet PDF : 14 Pages
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SP8852D
ADDRESS
+5V
STROBE
CONTROL
MICRO
1k
VCO
APPLICATION USING
CRYSTAL REFERENCE
SP8852
27
28
100p
33p
10MHz
CRYSTAL
1n
1n
7
8
9
10
11
12
13
14
15
16
17
2k2
10n
SP8852D
Fpd Fref
* 100p
1m 10n
39
38
37
36
* VALUES DEPEND
ON APPLICATION
35
34
33
32
31
LOOP
30 * FILTER *
29
*
+30V
–
+ OP27
ETC
100n
1n
Fig. 5 Typical application diagram
Ref in
DESCRIPTION
Prescaler and AM counter
The programmable divider chain is of A M counter
construction and therefore contains a dual modulus front end
prescaler, an A counter which controls the dual modulus ratio
and an M counter which performs the bulk multi–modulus
division. A programmable divider of this construction has a
division ratio of MN+A and a minimum integer steppable
division ratio of N(N–1), where N is the prescaler value.
Data entry and storage
Data is loaded from the 16 bit bus into one of the internal
buffers by applying a positive pulse to the strobe input. The
input bus can be driven from TTL or CMOS logic levels. When
the strobe input is low, the inputs are isolated and the data can
be changed without affecting the programmed state. The data
is loaded into the RF buffer when the address input is high and
into the reference buffer when low. When the strobe input is
taken high, the A and M and reference counters are reset and
the input data is applied to the internal storage register. When
the strobe input is again taken low, the data on the input bus
is stored in the selected register and the counters released.
The strobe input is level triggered so that if the data is changed
whilst the input is high, the final value before the strobe goes
low will be stored.
In order to prevent disturbances on the VCO control voltage
when frequency changes are made, the strobe input disables
the charge pump outputs when high. During this period the
VCO control voltage will be maintained by the loop filter
components around the loop amplifier, but due to the
combined effects of the amplifier input current and charge
pump leakage a gradual change will occur. In order to reduce
the change, the duration of the strobe pulse should be
minimised. Selection of a loop amplifer with low input current
will reduce the VCO voltage droop during the strobe pulse and
result in minimum reference sidebands from the synthesiser.
Reference input
The reference source can be either driven from an external
sine or square wave source of up to 100MHz or a crystal can
be connected as shown in Fig. 5.
Phase Comparator and Charge pump
The SP8852D has a digital phase/frequency comparator
driving a charge pump with programmable current output. The
charge pump current level at the minimum gain setting is
approximately equal to the current fed into the Rset input pin
19 and can be increased by programming the bus according
to Table 1 by up to 4 times.
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