Posts by DM5RM


    Hallo Hans,

    die "Linse", eigentlich ist es ja eine dielektrische Stabantenne, wirkt in erster Näherung nur für den Empfang mit einem LNB.

    Bei deiner Messung mit dem WebSDR ist es aber trotzdem bemerkenswert, dass sich das Sendesignal um mehr als 1dB ändert.

    Es wäre interessant zu wissen, inwieweit das TX-Richtdiagramm (und evtl. das TX-Phasenzentrum) von der bei 2.4GHz elektrisch eher kleinen "Linse" abhängt.

    Hat das mal jemand simuliert?

    73, Roland


    Looks very good. :)

    By the way, the directional coupler at the output can be omitted and thus may bring a few tenths of dB more output power to the antenna. The amplifier already has a coupler built in and you can simply connect the power meter to the second MMBX socket. The corresponding calibration value has to be determined of course, but this would be necessary for the external coupler as well.

    But one could also realize a deluxe version and use the external coupler to measure additionally the reflected power.


    Hi Piero,

    the main reason for the input transformer is to avoid a ground loop, which can cause problems such as increased phase noise and spurious signals. Besides, one gains a degree of freedom for the adjustment to the output level of the used external OCXO or GPSDO.

    If the switching threshold of SN74LVC1G125 is assumed to be 0.9V and its input protection diode conducts from 0.5V, you get about 2*(0.5+0.9)=2.8Vpp as optimal input voltage for the chip. This applies at least for sinusoidal driving to achieve about 50% duty cycle at the output. A different transformation ratio obviously requires a correspondingly adjusted value for R1. Placing R1 on the secondary winding of the (non-ideal) transformer reduces overshoot when the input signal is square wave.

    The transformer is certainly not the cause of the 10 MHZ problem. In the ADI forums it is reported that at this frequency a sine wave with 1Vpp at the AD936x clock input is mandatory. The SN74LVC1G125 simply cannot do this.

    But for us 25MHz are much more interesting anyway, because they are also needed for the LNB.

    vy 73,


    Now the practical part.

    For the test the circuit was simply soldered together, quick and dirty, without a printed circuit board.

    U1 and the Pluto-XO are almost pin-compatible. However, the metal housing of Y3 prevents a simple piggyback mounting.

    The transistor Q1 is placed upside down, with the emitter at C121. R1 consists of five parallel-connected 1k resistors, because they were just available.

    A level of about 6.5dBm is just enough to switch to external clock at the given impedance ratio of the input transformer. At more than 12 dBm, however, U1 is increasingly overdriven.

    With this circuit the Pluto was able to operate with external clock frequencies between 20 and 50 MHz. 10 MHz unfortunately did not work.

    The currently applied clock frequency (in the example 25 MHz) must be submitted to the Pluto with the following well known commands:

    fw_setenv ad936x_ext_refclk "<25000000>"

    fw_setenv xo_correction 25000000

    pluto_reboot reset

    IK1IYU recently mentioned, that it


    "Looks like the easiest pluto sdr tcxo removal….is NOT removing it at all. According to Analog Device Pluto schematics, pin 1 of the tcxo is an Enable/Disable pin and simply grounding it should disable it."

    TCXO for Pluto

    But if pin 1 is not connected statically to GND, an automatic clock selection can be realized with little effort.

    This is made possible by TI's SN74LVC1G125 chip, which has a low-active enable input and accepts input signals up to 5V with 1.8V VCC. So a single Si-npn transistor provides automatic source switching, a pull-up resistor is not necessary.

    The tested circuit diagram is attached.

    The advantages are:

    The original XO does not have to be removed.

    The Pluto still works even if no external clock is fed in.

    And last but not least, accidentally too high Clk levels or ESD no longer directly endanger the heart of Pluto.

    Meanwhile I almost believe that it won't work at all.

    On the beacon status website

    is written at the end of the page:

    "QRV: always when moon is visible with more than 10 degrees elevation at DL0SHF but only when moon declination is above 20 degrees North"

    However, Daniel, EA4GPZ, has reported that he received the beacon on May 16, 2019.…qo-100-station/#more-6594

    73 Roland

    Bitte, gern geschehen.

    Jetzt könnte man sicherheitshalber noch eine Z-Diode (etwa 12V, ist unkritisch) zwischen Gate und Source ergänzen, damit auch bei Speisung mit +28V der im Datenblatt angegebene Grenzwert für VGS (maximal ±20V ) eingehalten wird.

    Gute Idee, aber Source und Drain sollten vertauscht werden. Sonst leitet die Substratdiode die ganze Zeit.

    Good idea, but source and drain should be swapped. Otherwise the substrate diode always conducts.


    Der Magnet stammt von einer alten Magnettafel.

    Mein (ausgeschalteter) Verstärker hat ein S22 von besser als -20dB im UMTS-Bereich und nur noch -14dB bei 2.4GHz. Bringt man nun einen Magneten in die Nähe des Isolators, verschiebt sich das Bild am VNA je nach Polarität nach oben oder unten und verformt sich dabei. So war bei mir leider nur 1dB Verbesserung zu erreichen. Angefangen hatte ich mit einer 10mm Neodym-Scheibe, die aber letztlich viel zu stark war. Habe dann nach einem deutlich schwächeren Magneten gesucht, den man direkt mittig oben auf den Isolator kleben kann. Ein sehr kleiner Neodym könnte (evtl. mit Abstandsscheibe) auch funktionieren.

    Aber wie gesagt, die Verbesserung ist gering und S21 sollte wenigstens nicht schlechter werden...

    Bin gespannt auf deine Ergebnisse.

    73 & 55


    Mit einem eher schwachen keramischen Permanentmagneten ( Durchmesser 15.5 x 4.5mm^3 ) on top kann man die Eigenschaften geringfügig verändern. Bei meinem BLD6G22L- Verstärker wurde dadurch S22 etwa 1dB besser. Mehr Messungen konnte ich nicht machen, der Isolator ist noch auf dem Board.

    With a rather weak ceramic permanent magnet ( diameter 15.5 x 4.5mm^3 ) on top you can change the properties slightly. With my BLD6G22L amplifier S22 became about 1dB better. I couldn't do more measurements, the isolator is still on the board.