How do I know the exact RX filter bandwidth on Si446x?
There are two methods the Rx bandwidth (also referred to as IF bandwidth) gets selected on Si446x devices.
1. Based on the reference XO/TCXO accuracies entered on tab “Frequency and power” on any Rx related example project in the RCA (Radio Configuration Application) WDS calculates the necessary Rx bandwidth to accommodate the Tx signal even at worst case frequency misalignments.
2. On the “RF parameters” tab users can bypass the automatic calculation mentioned in pont#1 by ticking the check-box next to the “RX bandwidth” entry. By doing so the entry becomes active and an arbitrarily RX bandwidth selected by the user can be entered (and the “Crystal tolerance Rx/Tx” entry boxes disappear on the “Frequency and power” tab).
On the chip, however only a limited number of filter bandwidths can be set. So no matter which method the Rx bandwidth is set by it may happen that the exact required bandwidth cannot be set and the one wider bandwidth will be selected. It may therefore be desired to know what the exact bandwidth is on the part.
If you press the “Create batch” button located in the lower left corner of the RCA projects and select “Preview batch” from the pop up menu the chip configuration will be listed as shown below. Now, right above the RESET command you will find the exact receive bandwidth: # WB filter 1 (BW = 99.20 kHz); NB-filter 1 (BW = 99.20 kHz) #
Why do we have two filter bandwidths listed? : # WB filter 1 (BW = 99.20 kHz) is the bandwidth of the filter bandwidth in acquisition (aka: preamble search) mode while NB-filter 1 (BW = 99.20 kHz) is the filter bandwidth in tracking mode. In this particular example the two bandwidths are equal. If filter bandwidth gearing is enabled, however (by ticking the “Enable Adaptive Ch. Fil. Bw” on the “RF parameters tab” – only visible if “Enable PLL AFC ” is ticked) the two bandwidths may be different. Typically the tracking filter is narrower bandwidth than the acquisition filter. (Hence the WB- Wide Band and NB – Narrow Band abbreviations.) You will find more on filter bandwidth gearing in chapter 3.4 in AN734.