Adalm Pluto & GPSDO

  • I'm still struggling with this…


    I'm a bit confused about the output voltage of the GPSDO because the homepage states the following:

    Both output signals square wave at 3.3V CMOS levels with 50 Ohms characteristic impedance.

    Output power level (measured at 10MHz, fundamental power channel):

    +13.3dBm, drive setting 32mA

    +12.7dBm, drive setting 24mA

    +11.4dBm, drive setting 16mA

    +7.7dBm, drive setting 8mA


    From my point of view these values are contradictory because CMOS voltage != dBm related voltage:

    7.7dBm at 50 Ohms = 1.535V p-p

    13.3dBm at 50 Ohms = 2.924V p-p


    I can also measure with a oscilloscope an output voltage of ~3.3V p-p (no matter which drive setting).


    Where is my mistake?

  • The drive setting will only have an effect if the clock is terminated with 50 ohms.
    Your dBm to Vpp conversion assumes sinewaves with a rms to peak (crest-)factor of 2xsqrt(2)=2.818. A squarewave has a crest factor of 1, so 1Vrms=2Vpp (p-p is 2x amplitude).

  • It 'sme again.

    I did the math but I'm still confused.

    Because as I said in one of my last post I can not understand why there are two values (dBm / CMOS voltage).

    If I take the dBm value and calculate the corresponding voltage I could not get close to the 3.3V CMOS.


    square wave +13.3dBm @ 50 Ohms =

    21.38 mW, 1.034 Vrms (2.068V p-p), 20.68mA


    Help - where's the error in my logic?


    Thanks in advance!

  • Probably none.. just unprecise specs given by the manufacturer. As an example, if you have a 50 ohm output but don't load it at all with a terminator, you may have 3.3Vpp. But if you attach a 50 Ohms termination, the signal will be reduced to 50% just because of the resistive divider (50 Ohms in the source, 50 Ohms in the termination). So you end up with 1.65Vpp.

    Next: what does the 32mA setting actually mean? We don't know, it could be just an internal current setting. 32mA vs. 16mA should be 6dB in power level.. it is not.

    Next:

    Output power level (measured at 10MHz, fundamental power channel):

    Fundamental power level means although you have a square wave they only quote the power in the 10 MHz component, not in all the harmonics. So again something to be corrected for. Why do they quote it that way? Unknown..

    Bottom line: if you can measure it, do it. Adjust for the required PLUTO p-p level with a proper AC block and 50 ohms termination before you connect and enjoy.

  • Very helpful people here! Again, thank you very much for the detailed explanations.

    I have less experience in electronics, so I trusted the information from manufacturer (too much) and didn't question it.