How to bias HMC8410 with HMC981?

Hi,

I need to bias HMC8410 with HMC981 bias controller. However, I didn't understand how I can get -2V? (Vneg=-2,5V).

Thank you

Parents
  • +1
    •  Analog Employees 
    on Dec 5, 2019 4:49 PM 10 months ago

    Hi tlhnt,

    I'm not sure I follow your question 100%. Are you trying to set the output of the HMC981 to -2V and leave it there? That would result in the amplifier being essentially disabled with very little to no current flow.

    For the HMC8410, you should be safe to use the HMC981 as intended, but it is possible to limit the VNEG and VGATE voltages using external resistors. An example of this is shown in Figure 26 of AN1363. Using the designators from that figure, R4 should be 1620kΩ and R6 should be 866kΩ. This will limit VNEG to -2V and VGATE to -1.7V.

    Thanks,
    Weston

  • Hi wsapia,

    I see my question was not clear enough. My problem is that to startup HMC8410, I first need to set Vgg to -2V than increase the voltage up to the point where Id is 65mA. However, the starting/reference point of Vgg is equal Vneg of HMC981. In datasheet of HMC 981, they say that Vneg is -2.5V but I need -2V for start the startup. To sum up, How do i set Vneg of HMC981 to -2V. If i use R4 as 1620kohm, does it set Vneg as -2V? Is there any equation for it? I attached a schematic, is that ok?

Reply
  • Hi wsapia,

    I see my question was not clear enough. My problem is that to startup HMC8410, I first need to set Vgg to -2V than increase the voltage up to the point where Id is 65mA. However, the starting/reference point of Vgg is equal Vneg of HMC981. In datasheet of HMC 981, they say that Vneg is -2.5V but I need -2V for start the startup. To sum up, How do i set Vneg of HMC981 to -2V. If i use R4 as 1620kohm, does it set Vneg as -2V? Is there any equation for it? I attached a schematic, is that ok?

Children
  • +2
    •  Analog Employees 
    on Dec 9, 2019 2:15 PM 10 months ago in reply to tlhnt

    Hi tlhnt,

    Using the 1.6MΩ resistor between VNEG and VNEGFB will set VNEG to -2V. VGATE should be set to -1.7V using the other resistor, so your value is wrong in the schematic. This resistor should be 866kΩ.

    A few more comments:
    - The charge pump circuitry should connect to VNEG, not VGATE.
    - The VREF pin should have a capacitor to GND.
    - The VDD input to the HMC8410 will need to have an inductor to block RF.
    - It's much better practice to drive the EN pin rather than let it float up.
    - Be sure to sequence the HMC981 supplies as below on startup and reverse the order on shutdown.
        1. VDIG
        2. VDD
        3. EN

    Thanks,
    Weston

  • Hi wsapia,

    Thanks for your reply. You are right, I have mistakes on the schematic. I fixed them. However I could not understand why I should set the Vgate as -1.7V.