Adalm Pluto & GPSDO

  • Hi all,

    I'm using a Adalm Pluto with an external GPSDO from Leo Bodnar (40 MHz / 8mA drive) . Everything works quite good.

    Today I had a QSO with an OM via QO-100 who told me that I should not connect the GPSDO directlyto the Pluto because the SDR chip might get fried. After the QSO I was a bit scarred and therefore I searched in this forum for more information. But I could not find any recommendation to add an attenuator between the GSPDO and the Pluto.

    Sorry - maybe a strupid question:

    Is it really true that my setup might damage my Pluto?


    73 Mic

  • I have it also directly connected to one of my Pluto's and no problems at all since a few months.

    I remember that I did some measurements right after the modification with a RF Probe and Oscilloscope and the amplitude at the lowest output setting was not really reaching critical levels (but I forgot how much it was exactly - maybe someone else could measure and post here?)

    73, Oscar

  • Uh - 1.3 V p-p

    I wasn't aware about the input limits of Pluto's SDR chip. Thank you for highlighting this!

    According to the GPSDO specifications the output signal is a square wave at 3.3 V CMOS level. Due to the lack of a proper oscilloscope I can only measure up to 20 MHz. Up to 15 MHz I can measure an output level of 3.3 V p-p (without load).

    So 3.3 V p-p is obviously way too much.

    Maybe a simple voltage divider is a good solution to protect the Pluto from getting fried?

    DJ0MY: I really appreciate your feedback. I'm wondering why my Pluto is still alive because of the high clock input. I have also set the GPSDO drive to the lowest level (8 mA).

    Hopefully someone has still the measured results and can post them here.

  • You should measure it under load at the Pluto...I recall my readings to be much lower, but have no access to a suitable scope right now...maybe someone here could measure some proper measurements at the Pluto TRX chip input.


  • An interesting problem came up yesterday, when I was using two Plutos in parallel from the same LO. One pluto for NB and the other with Evaristes firmware for DATV. Both connected via LAN to PC and LO connected by -3dB splitter (in this case LO from the 40 MHz output of the AMSAT-DL-Converter V3). When I switch on the second Pluto the first one is stopping. It seems that the LO signal is dropping because the decoupling by my splitter is not sufficent. Any ideas? Will have to use a distribution amplifier for frequency standards?

  • OE3GBB Are they powered from separate power supplies ? If not try to power them from separate power supplies as it can be a drop in the power supply due to initial current of the second. I have a similar experience when I switch on some devices in the Pluto power supply ..

  • The original circuit of the unmodified PLUTO used just a 18pF capacitor in series to use together with the internal capacitance of the REFin pin to knock it down to 1.3Vpp. A capacitive voltage divider so to speak. Exchanging the 18pF series cap to a 4.7pF cap should do the trick.

  • Here's another possible solution:

    According to the schematic you could also install C124 (which is in the unmodified Pluto not installed) with a value of 18 to 27pF and keep the existing 18pF series capacitor (C123).

    In combination with the capacitance of the XTALN input (approx 10pF) they would form a capacitive voltage divider like this:

    approx. 1.3 to 1.1Vpp @ XTALN

    In case if you would like to reinstall the org. TCXO you only have to remove C124.

  • No, it is all different supplies. I was thinking therefore it could be a ground loop. Will try to feed the Plutos with HF-transformers next and avoid a DC-connection.

    I have found a solution. It was quite a simple error of my setup. I did have both Plutos at the same IP adress! Grrrrrr!!!

  • Back to the question: Can a Leo Bodnar GPSDO (with 8mA set-up) directly substitute the original Pluto TXO?
    I am in front of the same question. I guess DJ0MY is right, it should be ok and not too strong.

    Please correct me if I am wrong:
    - 8mA on 50Ohm are 0,4V ==> 0,4*1,414 = is 0,566Vp ===> 2*0,566V= 1,13Vpp so close to 1,3Vpp which the Pluto require.


  • be careful: your calculation assumes 50 Ohms load. There is no such termination in the Pluto so you have to provide the 50 Ohm resistor yourself. Otherwise you might end up with a significantly higher Vpp value.

  • @DO1OHB:

    Details about the input resistance of the SDR chip can be found in the reference manual (-> page 15).

    In short: you can't assume a 50 Ohms load.

    As mentioned in one of my last articles above, you could build a voltage divider (C123/C124) or use an external coax attenuator, as others have suggested.

  • DH2VA and DB8DT, thank you for the feedback.

    For me the easiest way would be a coax attenuator from the GPSDO to the Pluto prior TXO-Chip pins.

    Do you know which level would be the right one (Leo Bodnar GPSDO xxmA level and the related attenuator in dB)?

    73, Oliver