M491B datasheet(6/16 Pages) STMICROELECTRONICS

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PIN 3 : MEMORY TIMING OUTPUT

This output gives the timing for the pulses to be

applied at Pin 2 during the modify and read cycles.

The output consists of an open drain transistor.

PIN 4 : FINE TUNING D/A (see Figure 5)

A D/A converter with 16-step resolution and a fre-

quency of 15kHz can be used to generate a voltage

which, if fed to a varicap diode in parallel to the AFC

discriminator, will detune the receiver by a small

∆f

while maintaining the action of the Digital AFT. This

output can be used in conjunction with both Linear

and Digital AFT modes of operations.

The Fine tuning function operates as follows :

- At the start of any automatic or manual search,

the output is set at the mid range.

- When the search has been completed it is possi-

ble to operate on FT

± commands.

The store command memorizes this information

together with the 13 tuning voltage bits and 2

information bits.

- Modification time of FT D/A is of 1 step every

200ms if issued locally or every 2 received signals

from Remote control transmitter.

PIN 5 : TUNING D/A (see Figure 6)

A 2

13 = 8192 step pulse modulated signal for the

tuning voltage is available on this pin.

Pulse modulation is implemented by combination

of a rate multiplier and pulse width principle.

With a tuning voltage increasing from zero, the

number of pulses increases continuously up to

8 = 256 ; starting from this point the number of

pulses remains the same but the pulses get larger

until they reach the maximum content of the inter-

nal counter. The output consists of an open drain

transistor which offers a low impedance to ground

when in the ON state.

OUT

µs

Mid

Range

Fine Tuning Output

Figure 5

M491B

6/16

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