TDA7540N datasheet(36/76 Pages) STMICROELECTRONICS

36 / 76 page

Functional description

TDA7540N

36/76

Sampling timer

A sampling timer generates the gate signal for the main counter. The basically sampling

time are in FM mode 6.25kHz (tTIM=160μs) and in AM mode 1kHz (tTIM=1ms). This is

followed by an asynchronous divider to generate several sampling times.

Intermediate frequency main counter

This counter is a 11 - 21-bit synchronous autoreload down counter. Five bits (CF) are

programmable to have the possibility for an adjust to the centre frequency of the IF-filter.

The counter length is automatic adjusted to the chosen sampling time and the counter mode

(FM, AM-UPC, AM).

At the start the counter will be loaded with a defined value which is an equivalent to the

divider value (tSample xfIF).

If a correct frequency is applied to the IF counter frequency input at the end of the sampling

time the main counter is changing its state from 0h to 1FFFFFh.

This is detected by a control logic and an external search stop output is changing from LOW

to HIGH. The frequency range inside which a successful count result is adjustable by the

EW bits.

tCNT = (CF + 1696+1) / fIF

FM mode

tCNT = (CF + 10688+1) / fIF

AM up conversion mode

tCNT = (CF + 488+1) / fIF

AM mode

Counter result succeeded:

tTIM ≥ tCNT - tERR

tTIM ≤ tCNT + tERR

Counter result failed:

tTIM > tCNT + tERR

tTIM < tCNT - tERR

tTIM = IF timer cycle time (sampling time)

tCNT = IF counter cycle time

tERR = discrimination window (controlled by the EW registers)

The IF counter is only started by inlock information from the PLL part. It is enabled by

software (IFENA).

Adjustment of the measurement sequence time

The precision of the measurements is adjustable by controlling the discrimination window.

This is adjustable by programming the control registers EW.

The measurement time per cycle is adjustable by setting the registers IFS.

Adjust of the frequency value

The center frequency of the discrimination window is adjustable by the control registers CF.

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