TDA5235
Functional Description
Attack and Decay coefficients PMF-UP & PMF-DOWN:
The settling time of the loop is determined by means of the integrator gain coefficients
PMFUP and PMFDN, which need to be calculated from the wanted attack and decay
times.
The ADC is running at a fixed sampling frequency of 274kHz. Therefore the integrator is
integrating with PMFUP*274k per second, i.e. time constant is 1/(PMFUP*274k). The
attack times are typically 16 times faster than the decay times.
Typical calculation of the coefficients by means of an example:
• PMFUP = 2^-round( ln(AttTime / BitRate * 274kHz) / ln(2) )
• PMFDN = 2^-round( ln(DecTime / BitRate * 274kHz) / ln(2) ) / PMFUP
where AttTime, DecTime = attack, decay time in number of bits
Note: PMFDN = overall_PMFDN / PMFUP
Example:
BitRate = 2kbps
AttTime = 0.1 bits
=> PMFUP = 2^-round(ln(0.1bit/2kbps*274kHz)/ln(2)) = 2^-round(3.8) = 2^-4
DecTime = 2 bits
=> PMFDN = 2^-round(ln(2bit/2kbps*274kHz)/ln(2))/PMFUP = 2^-round(8.1)/2^-4 = 2^-4
Note: In case of ASK with large modulation index the attack time (PMFUP) can be up to
a factor 2 slower due to the fact that the ASK signal has a duty cycle of 50% - during the
ASK low duration the integrator is actually slightly discharged due to the decay set by
PMFDN.
The AGC start and freeze times are programmable. The same conditions can be used
as in the corresponding AFC section above. They will however, be programmed in
separate SFR registers.
Data Sheet
33
V1.0, 2010-02-19
INFINEON [ Infineon ]
