Using RF switch as DC switch

[tungchang]

I used RF switch (SP3T) as DC switch. When I increase the input voltage from 0V to 2.4V, the out level will follow that. But the input increase higher than 2.4V to 5V,
the output level will be hold in 2.4V. Someone can help me explan that? thx.

 

[biff44]

I would not do it. Microwave switches have both shunt and series FETs. They are not designed to switch DC. Analog switches have only series FETs. I would assume that your shunt fets are biasing ON under certain conditions.

 

[tungchang]

Yes,I know what you said. The shut Fets which are in the RF Switch will increase the device's isolation. But my point is why the output voltage leavel does not change by increasing the input's. It's always keep on 2.4V when the input voltage is higher than 2.4V. Thanks.



ps. I know it's a strange design and I just want to prof if the RF switch can use in DC state. The datesheet always say they can be used from "DC to 20GHz and so on.

 

[vfone]

The RF switch cannot transfer DC. What voltage you see there is the DC bias of RX and TX nodes.

 

[sapphire]

When you increase your input voltage, at certain point your fet will get clipped on.

 

[tungchang]

Yes, When I increase the input voltage, the output will increase until 2.4V.
ex. input ouput
0.5V 0.5V
1V 1V
1.5V 1.5V
2.0V 2.0V
2.4V 2.4V
3V 2.4V
4V 2.4V
5V 2.4V