HEF4750V datasheet(7/17 Pages) PHILIPS

7 / 17 page

January 1995

Philips Semiconductors

Product speciﬁcation

Frequency synthesizer

HEF4750V

LSI

Fig.6 Phase characteristic of PC2.

Strobe function

The strobe function is intended for applications requiring

extremely fast lock times. In normal operation the

additional strobe input (STB) can be connected to the

V-input and the circuit will function as described in the

previous sections.

In single, phase-locked-loop type frequency synthesizers,

the comparison frequency generally used is either the

nominal channel spacing or a sub-multiple. PC2 runs at

the higher frequency (a higher reference frequency must

also be used), whilst strobing takes place on the lower

frequency, thereby obtaining a decrease in lock time. In a

system using the Universal Divider HEF4751V, the output

OFS cycles on the lower frequency, the output OFF cycles

on the higher frequency.

Out-of-lock function

There are a number of situations in which the system goes

from the locked to the out-of-lock state (OL goes HIGH):

1. When V’ leads R, however out of the range of PC1.

2. When R leads V’.

3. When an R-pulse is missing.

4. When a V-pulse is missing.

5. When two successive STB-commands occur, the first

without corresponding V-signal.

Phase modulator

The phase modulator only uses one external capacitor,

CB at pin TCB. A negative-going transition at the V-input

causes CB to produce a positive-going linear ramp. When

the ramp has reached a value almost equal to the

modulation input voltage (at MOD), the ramp terminates,

CB discharges and a start signal to the CA-ramp at TCA is

produced. A linear phase modulation is reached in this

way. If no modulation is required, the MOD-input must be

connected to a fixed voltage of a certain positive value up

to VDD. Care must be taken that the V’ pulse is never

smaller than the minimum value to ensure that the external

capacitor of PC1 (CA) can be discharged during that time.

Since the V’ pulse width is directly related to the TCB ramp

duration, there is a requirement for the minimum value of

this ramp duration.

Reference oscillator

The reference oscillator normally operates with an external

crystal as shown in Fig.2. The internal circuitry can be

used as a buffer amplifier in case an external reference

should be required.

Reference divider

The reference divider consists of a binary divider with a

programmable division ratio of 1 to 1024 and a prescaler

with selectable division ratios of 1, 2, 10 and 100,

according to the following tables:

Binary divider

Prescaler

N (A0 TO A9)

DIVISION RATIO

1024

≤ N ≤ 1023

PROGRAMMING WORD

(NS0, NS1)

DIVISION RATIO

210

100

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