TX LO leakage / spurs on Pluto (plus) ?

  • Hi,

    danke für die Aufnahme ins Forum! Ich habe letztes Jahr mit Kurzwelle angefangen und bin ziemlich neu bei dem Thema QO 100 und SDRs.

    Downlink mit Bullseye LNB und Sdrplay RSP1A habe ich schon hinbekommen, jetzt schaue ich wie ich senden könnte.

    Freue mich über jede Anmerkung, und schreibe jetzt einfach auf Englisch:


    I just got a Pluto Plus which i want to use for QO 100 uplink. My goal is to create a clean signal, so i did some checks for unwanted emissions.

    Signal generation (single tone test 1k) was done with SDRangel, TX Freq was set to 2.400 Mhz with offset tuning +200KHz

    Test setup:

    SDRangel -> Pluto Plus -> EVAL-CN0417-EBZ -> 40 db attenuator -> nooelec downconverter 3GHz (IF 1.5 GHz) -> sdrplay RSP1A -> SDRuno

    Pluto Firmware version: 0.38 (original version from analog)

    SDRangel version: 7.17.3

    I noticed a signal exactly at the tx frequency (even visible and audible when nothing is transmitted), and assume this is LO leakage as described here? :

    and here https://wiki.analog.com/univer…ing/listening_to_yourself

    I tested at different gain (=attenuation) levels, see the pictures and data below:

    picture 1 (tx gain: -36 dB)

    signal level -46 dBm

    LO level: -98 dBm

    (delta: 52 dBm)

    picture 2 (tx gain: -26 dB)

    signal level: -46 dBm

    LO level: -89 dBm

    (delta: 43 dBm!)

    picture 3 (tx gain: -12 dB)

    signal level -33 dBm

    LO level: -76 dBm

    another big spur at 899.398 (-72 dBm)

    (delta: 39 dBm!!!)

    By the way i also checked the frequencies above and below, there are no additional spurs - so this looks good.

    I also did a test with ssb (tx offset = 0) and can hear the LO when on the proper frequency (see picture). It seems quite strong compared to my signal (compression was on)

    Are these values 'normal', compared to a normal pluto, or is my device broken? How to deal with this? (also checked this thread). Is my test setup even valid?

    Can someone confirm this or can give me suggestions?

    Thanks & 73, Christian DO1ZL

  • I attached another picture that shows that this LO signal is even there when you stop SDRangel transmit.

    In that picture you can also see what happens when you start/stop the transmitter device: a short distortion on all frequencies. Or could this be caused by the sdrplay? AGC and IF AGC is switched off btw. But i guess it is the init sequence in SDRangel, because SDRConsole does not show this behaviour.

    I also did some tests with the latest version of SDRConsole - see attached pictures - with two tone test and drive setting of 50 (this seems to correspond to tx gain -20 dB)

    One test is with TX offset +200kHz, the other without TX offset.

    I used this command to get the gain setting:
    iio_attr -o -u ip: -c ad9361-phy voltage0 hardwaregain

    Thanks & 73, Christian DO1ZL