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LMX2492: control high voltage Vtune VCO

Part Number: LMX2492

Hi Texas Instruments,

I want to make 11GHz output on LMX2492 and external VCO, MAOC-009265-TR0500(VCO on LMX2492evm) or most of VCO on market have 12V Vtune, but CPout of LMX2492 is 5V. Therefore, I need an active loop filter.

I use Pllatinum Sim, there are 3 active filters are: type A, B, C.

1. Type A and B: have not AmpGain option, can I use type A or type B to control 12V Vtune VCO? 

I try this Schematic but it not working, Cpout connect CPout on LMX2492, Vtune connect Vtune of VCO

2. Also, I try type C with ampgain is 3(5V CPout x 3 = 15V Vtune), its not working again, can you help me?

here is my Schematic:

Where is my wrong?

Thank you!

Best Regards,


  • Hi Ngandv,

    Type B is an inverting amplifier, did you change the register CPPOL = 0x1? 

  • Hi Noel Fung,

    1. I have changed CPPOL, it is still not working. Can I use type B for 12V Vtune VCO?

    2. If type B is OK, How to calculate DC gain of op-amp?

    3. How about another question?

    Thank you so much.

  • Hi Ngandv,

    12V, 15V or 30V is the supply voltage to the op-amp, this will affect Vtune voltage but not loop filter type dependent. 

    You said not working, then what was the Vtune voltage?

    Your type B schematic is fine, this is my favor active filter architecture, we use the same active filter type in the EVM.  However, I think we should put a shunt capacitor at pin3 in order to eliminate any AC signal. The gain in type B is -1. 

  • Hi Noel Fung,

    Passive filter is working, max CPout is 5V, you can't make 12V on Vtune of VCO. 

    Example: MAOC-009265 is VCO on LMX2492evm, with 5V maximum on CPout, you can't get 11GHz output, then you need a active filter.

    type B active filter have gain is -1, you can't make 12V on Vtune. Then you can't get 11GHz output, that is right ?

    Only type C active filter can make 12V on Vtune, because type C have |gain DC| > 1 ?

    Thank you so much.

  • Hi Ngandv,

    CPout cannot generate Vtune higher than 5V but the active filter can, even if its gain is unity. The problem with type C is, in addition to Vtune amplification, it will also amplify the op-amp noise. that's why type B is more popular. 

    Below is a simple sim, the output will swing around the supply voltage of the op-amp.

  • Hi Noel Fung,

    Type B maybe good.

    Your sim: output from 11.968V to 11.978V, how about input(Vs), can you show to me?

    In reality, if Vs = CPout(0V-5V), TPv1 can reach 12V?

    Thank Noel Fung.

  • Hi Ngandv,

    below has a better visual result with a 100kHz square wave input (blue trace).

  • Hi Noel Fung,

    Thanks for your reply.

    I simulate your schematic on Proteus. When input from 2.3V to 2.7V, output is 12V square wave(0V-12V), that's the same your result.

    Uhhhh. That is not my goal, but I try a real circuit board.

    It is working, that's great.

    Maybe, my wrong is input of sim, input should be current source.

    Thank you very much.

    Best Regards