VCTCXO

    Hello OMs,

    for a flexible solution with a better TCXO on the Adalm Pluto I'm working on external oscillator.

    40MHz for Adalm Pluto + 25MHz 0,28ppm.

    The 40MHz is a VCTCO. For <0.5ppm I didn't find any TXCO, only this VCTCXO, 15Euro = OK.

    I tested the temperatur drift and pulling range by control voltage.

    The test setup is on a stripline PCB, a blocking capacitor 100n cer. diagonally over the chip, powered by 3,3V-LDO .

    The frequency counter is a VNWA3 with the GPSDO Lea Bodnar as a 10MHz reference.

    Sweep: 10ms/per point * 1000points



    From left: Vc~ 0,5 - 1V / from center: back to 0,5V

    Scale: 5Hz / div.

    I was expecting a constant frequency change of around 200Hz (5ppm).

    What is this sawtooth per ~0.1V each?

    Does anyone have an explanation ?


    73

    Hans-Jürgen

    Hello Hans-Jürgen,


    The observed sawtooth looks like the phase wraparound of the VNA that you use for your frequency measurement. Each time a step from +10Hz to -10Hz (or vice-versa) occurs, the real frequency curve just continues to go up or down. There should be an option in your software to show the unwrapped frequency.


    I used the same VCTCXO in my 40MHz PLL, it didn't show any such non-linear effect.


    73 Jean

    Hello Jean,

    thank you for your hint.

    This chaotic behavior is not only dynamic. It is also static.

    I don't need dias tuning with the voltage. But if the constant voltage is at a jump, the frequency would be unstable.

    Now I'm on the phone with an OM: He lends me a suitable frequency counter.
    These measurements are for my knowledge only. In any case I can install the oscillator. I am flexible with the external variant via an SMA connector.

    More information in next days

    73

    Hans-Jürgen