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SN74CB3Q3125: RF signal usage

Part Number: SN74CB3Q3125

Hi Team,

Would you advise below questions of my customer's inquiry that she want to ON/OFF the RF signal

of about several tens of MHz for the purpose of eliminating leakage from the antenna when using this product as an RF switch.

1. Can it be used at several tens of MHz?

2. Would you please give me a circuit diagram that can be used as a reference for the above applications?

3. Can you tell me the necessary circuits such as DC blocking capacitors when connecting to the RF signal line?

4. When VDD is below the operating voltage, is it safe to assume that the xA and xB terminals are insulated?

5. If you have any other points or precautions on how to use it, could you please tell me?

Thanks

Best regards,

Shidara

  • Hi Shidara,

    For your questions please see below:

    1. The nominal bandwidth for this part is about 500MHz - however higher output capacitances can degrade this value,  so a few tens of MHz should be fine as long as there isn't a ton of extra output capacitance from the mux .

    2. Please refer the figure below. The only external components needed to run this device are the supply decoupling capacitor. The active low enables /xOE can be shorted together or controlled individually. At least 1 of the supply caps are recommended but adding a smaller capacitor to help dampen high frequency transients on the power line.

     

    3. For the addition of a DC blocking capacitor I'd try to put this at the output of the switch. This part cannot pass negative voltages - so having a DC bias is necessary that all of the AC component of the signal is transferred through the switch. You'd want to remove a DC bias/level after the switch or else the switch can take damage if the voltage dips below -0.5V and it won't pass a signal that is < 0V w.r.t. ground. 

    4. Yes this part supports partial power down protection which causes the switches to go high impedance when VDD = 0V. There is still some leakage associated with the high impedance state (max of 1uA of leakage).

    5. The largest concern I see is negative voltages. If the original signal swings below 0V w.r.t ground a DC bias is needed to ensure that the AC data of the signal is transferred through the switch. A DC blocking capacitor can appear at the output so that the antenna has the RF signal only.

    If you have other questions please let me know!

    Best,

    Parker Dodson

  • Hi Parker-san 

    Thanks for your advice.

    I informed this to the customer.

    I'll re-post if I get additional question then please close this thread.

    Thanks

    Best regards,

    Shidara