TDA7540N Datasheet, PDF (36/76 Pages) STMicroelectronics – AM/FM car radio tuner IC with stereo decoder and intelligent selectivity system (ISS)

Functional description
TDA7540N
Sampling timer
A sampling timer generates the gate signal for the main counter. The basically sampling
time are in FM mode 6.25kHz (t_TIM=160μs) and in AM mode 1kHz (t_TIM=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 (t_Sample x f_IF).
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.
t_CNT = (CF + 1696+1) / f_IF
t_CNT = (CF + 10688+1) / f_IF
t_CNT = (CF + 488+1) / f_IF
FM mode
AM up conversion mode
AM mode
Counter result succeeded:
t_TIM ≥ t_CNT - t_ERR
t_TIM ≤ t_CNT + t_ERR
Counter result failed:
t_TIM > t_CNT + t_ERR
t_TIM < t_CNT - t_ERR
t_TIM = IF timer cycle time (sampling time)
t_CNT = IF counter cycle time
t_ERR = 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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