AN332
Rev. 0.8
167
2. Determine sensitivity RF input and SINAD requirements.
3. Set frequency to 1000kHz.
4. With source impedance in #1 and RF input in #2, adjust MIN_GAIN_INDEX until SINAD requirements are
achieved with minimum necessary margin.
5. Program this value into Si474x MIN_GAIN_INDEX as part of initialization after POWERUP command.
ATTN_BACKUP insures the AGC gain indexes are monotonic and is used when the external attenuator is engaged
via GPO1/AGC2. The actual attenuation achieved depends on the source impedance or AM antenna dummy.
Since AGC gain implementation is subject to change, the optimum value is best determined with specific antenna
and board design.
The recommend ATTN_BACKUP optimization procedure is:
1. Determine source impedance and AM antenna dummy.
2. Determine maximum RF input and associated SINAD requirements.
3. Set frequency to 1710kHz.
4. With ATTN_BACKUP set to 12 (default), disable the AGC at AMAGCNDX=47 using AM_AGC_OVERRIDE
command.
5. With source impedance in #1 and RF input in #2, adjust attenuator impedance until SINAD requirements are
achieved with minimum necessary margin. For Si4743EVB Rev 1.3, C7 (1200pF) attenuates against passive
antenna sources and R8 (1 ohm) attenuates against active (50 ohm) sources.
6. Enable the AGC using AM_AGC_OVERRIDE.
7. Sweep the RF input from 0 to 126 dBuV and then from 126 to 0 dBuV in 1 dB steps and observe the
AMAGCNDX at each RF level using AM_AGC_STATUS command.
8. If AMAGCNDX is observed to oscillate at any RF level, increase ATTN_BACKUP by 1 and repeat from step 7.
9. If AMAGCNDX is observed not to oscillate at any RF level, decrease ATTN_BACKUP by one and repeat from
step 7.
10. Add one to smallest ATTN_BACKUP for which no oscillations are observed and program this value into Si474x
ATTN_BACKUP as part of initialization after POWERUP command.
Table 16. Recommended Values for MIN_GAIN_INDEX and ATTN_BACKUP with FW2.E and later,
Si4743EVB Rev 1.3 and Various AM Antenna Dummies
AM Antenna Dummy
MIN_GAIN_INDEX
ATTN_BACKUP
50
/15 pF/62 pF (Silabs)
19
12
50
/40 pF/40 pF
19
12
50MN Series
19
12
Active (50
)
19
20