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HMC494 how to prevent self oscillation in differential mode

Sam_raman
on Aug 11, 2017

Hi, I am using HMC494 in differential mode. A PLL differential output is driving the HMC494. It is AC coupled.

1)How can i prevent self oscillation? Can i connect a 8.25K on nIN to GND?

2) Datasheet suggests NC pin to leave it open. So we left it no connect. In other dividers NC pin was suggested to GND.

   Can we leave  NC pin  to float?

3) what is the abs max power that can be applied on IN and nIN pins when operation in differential mode?

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  • 0 on Aug 15, 2017 3:47 AM

    Hi Marty, thanks for your reply.

    Yes all the DC blocking caps are broadband caps.

    1) Regarding oscillation. Yes i am driving differential. There is an RF switch is between the PLL output and the HMC494 divider inputs. The differential signal is not always present at the input of the HMC494. During certain instance the HMC494 inputs are connected to 50 ohms termination inside the RF switch ( HMC547 per input of HMC494 ). So we like to prevent the self oscillation when PLL is not driving the inputs of HMC494. 

    3) regarding power level okay we will optimize it at around -2dBm for each input.

    During power up sequences or while configuring the PLL we may have up to +10dbm at each input of HMC494.

    The abs max in data sheet is +13dBm (for HMC494). So we are assuming this per input. Is this assumption correct?

    Will 10dBm power at each input during couple of seconds cause any problem to the HMC494?

     2) Regarding my 2nd question can we leave the NC pins (1,4-9,12,13,16) floating ? Datasheet says No connection.

    I remember in few other product data sheets it was recommended to ground NC pins.

Reply
  • 0 on Aug 15, 2017 3:47 AM

    Hi Marty, thanks for your reply.

    Yes all the DC blocking caps are broadband caps.

    1) Regarding oscillation. Yes i am driving differential. There is an RF switch is between the PLL output and the HMC494 divider inputs. The differential signal is not always present at the input of the HMC494. During certain instance the HMC494 inputs are connected to 50 ohms termination inside the RF switch ( HMC547 per input of HMC494 ). So we like to prevent the self oscillation when PLL is not driving the inputs of HMC494. 

    3) regarding power level okay we will optimize it at around -2dBm for each input.

    During power up sequences or while configuring the PLL we may have up to +10dbm at each input of HMC494.

    The abs max in data sheet is +13dBm (for HMC494). So we are assuming this per input. Is this assumption correct?

    Will 10dBm power at each input during couple of seconds cause any problem to the HMC494?

     2) Regarding my 2nd question can we leave the NC pins (1,4-9,12,13,16) floating ? Datasheet says No connection.

    I remember in few other product data sheets it was recommended to ground NC pins.

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