AN851 Datasheet, PDF (17/25 Pages) Silicon Laboratories – Si468X SCHEMATICS AND LAYOUT GUIDE

AN851
APPENDIX A—PROCEDURE TO OPTIMIZE CONDUCTED
SENSITIVITY THROUGH VARACTOR TUNING
There are 3 properties related with varactor-tuning procedure in each image:
Table 8. Varactor Tuning Properties
ID
FM/DAB
Note*
0x1710
FM/DAB_TUNE_FE_- Slope of Varactor vs Frequency Curve
VARM
0x1711
FM/DAB_TUNE_FE_- Intercept of Varactor vs Frequency Curve
VARB
0x1712
FM/
Configure VHFSW switch from open to close
DAB_TUNE_FE_CFG
*Note: See AN649 for detailed descriptions of these properties.
The following procedure is intended to show how to achieve the best conducted sensitivity result for Si468x for FM/
DAB band by tuning the on-chip varactor. The end goal is for the customer to identify the desired values of the
above 3 properties in each functional FW image. After the part is booted, the 3 properties are set to 0 by default.
For each PCB design, the customer needs to set identified values so that the customer can use the automatic
tuning function to achieve the target of conducted sensitivity specification.
In the tuning command of FM/DAB image (FM/DAB_TUNE_FREQ), there is an ANTCAP parameter. Once the
ANTCAP is issued with the tuning command as any value other than 0, the automatic tuning function (determined
by VARM/VARB settings) is bypassed and the on-chip varactor is set as the ANTCAP value as specified by users.
This feature is utilized in the procedure described below for DAB.
Set DAB_TUNE_FE_CFG property as 1, which closes the VHFSW.
How to optimize varactor value for each individual frequency X:
1. Connect the signal-generator to DUT with an SMA barrel.
2. Set the signal-generator to frequency X with 40 dBuV rf level and FM/DAB modulation.
3. Tune Si468x to frequency X with ANTCAP set to 1.
4. Call Test_Get_RSSI command 5 times and get the average of the 5 RSSi measurements.
5. Increment the ANTCAP by 1 and re-issue the tune command to Si468x.
6. Repeat Step 4 and Step 5 until reaching the max tunable varactor range of 128.
7. Identify the varactor value with the max RSSI reading.
Repeat the test for frequencies across the DAB band.
If describing the varactor’s relationship with frequency by varactor_value=m/1000*frequency (in MHz) +b,
m represents the slope (DAB_TUNE_FE_VARM), b represents the intercept
(DAB_TUNE_FE_VARB). Given the data collected, identify the low frequency range where the optimal
varactor is not lower than 4. This is illustrated in the graph* below.
Rev. 0.2
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