I would like to use Antenna diversity with Si4461 and i am trying to design a possible layout, but i have a problem.
In the datasheet of the component, it is write that I have to use a SPDT switch, but on the layout of the 4463CPCE20A915, a DPDT switch it used.
So my question is, which type of switch I have to use?
If you want to connect both the TX and RX paths to the antennas, then you need a DPDT switch. If only TX or RX is to be used, an SPDT switch is enough.
i want to connect both Tx and Rx paths to the switch.
My idea, is to connect RX and TX paths before the switch, and then, connect this point to the input of the switch. Can I do this?
Yes, you can connect the TX and RX paths directly: this is called a Direct-tie matching network. In this case an SPDT switch can be used for Antenna Diversity.
You can find Direct-tie matching network topologies and recommended component values for Si4461 in AN627 (Figure 4, Table 9, Table 11):
Note that even in this case a 3-element low-pass filter similarly as in the 4463CPCE20A915 design should be added after both antenna ports of the RF switch to suppress possible harmonic re-generation by the RF switch.
So, i can use the following schematic?
The low-pass filter should be separated into two 3-element low-pass filters: one should be added before the RF switch, the other one after the RF switch. For 915MHz the recommended matching network component values for Si4461 Direct-tie configuration are shown in Table 11 in AN627. Before the RF switch the values from the table should be copied: 22pF-6.8nH-3.9pF-6.8nH-3.9pF. After the RF switch the same 3-element low-pass filter can be used: 3.9pF-6.8nH-3.9pF.
Note that we typically recommend DPDT switch for Antenna Diversity to minimize the loss caused by the matching network. In a combined Direct-tie + RF switch solution the Direct-tie connection causes approximately 2dB RX sensitivity hit, while the RF switch also has some additional insertion loss. Overall with a Direct-tie + RF switch combined matching you can expect 3dB RX sensitivity loss compared to a split TX / RX case. With a DPDT switch the TX and RX paths would be separated and thus the only sensitivity loss would be caused the insertion loss of the RF switch which is typically 0.5dB.
I try to used DPDT switch to obtain the max sensitivity possible.
Excuse me, last question. Which frequency range of work this DPDT switch must have?
The DPDT switch should work optimally in the frequency of operation (915MHz in this case), but typically these switches work in a wide range of frequencies. You can find a list of recommended DPDT switch types in the following KBA:
It is what i was searching.
Why Silicon labs use switchs manufactur by CEL for their demo boards?