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LMX2820: MUTE pin

David Cipolle
Prodigy 120 points
Part Number: LMX2820
Other Parts Discussed in Thread: LMX2592

Has anyone used the MUTE pin to switch between synthesizers?  I want to ping pong two LMX2820's and think I may be able to save a switch by tying two outputs together and only enabling one synth at a time with the MUTE pin.  I am using it around 6 GHz so of course impedance is an issue.  What do the outputs look like?  The datasheet says there is a 50 ohm pullup- is that still there when the output is muted?  Any experience with the best way to do this?  I could use a splitter to combine them, but has anyone used something simpler like a resistive split?

BTW- Lock times look very nice with the instant cal- very good idea, wish your lower freq PLL parts (e.g. LMX2592) had this feature.

Thanks

  • 0
    TI__Guru* 81900 points

    Hi David,

    The 50Ω is still there when it is muted. The isolation from mute is around 50dB at 6GHz, so a switch is probably needed when you do ping-pong. The switch can provide another 30dB isolation. 

  • 0
    TI__Mastermind 29100 points

    Just to clarify:

    The output structure is open-collector with integrated 50Ω pull-up. If you try to combine the outputs with one device muted, you'll see one of two things:

    • DC-coupled: 25Ω pull-up to VCC, half of which is on another device. Not what you want.
    • AC-coupled: a 50Ω load to AC-GND, embedded in another device. Probably also not what you want.

    You could use a combiner, but the mute isolation will not be perfect. As Noel suggested, we recommend using a switch.

    Regards,

    Derek Payne