ADALM-Pluto 2-tone Test at 144 MHz
Over on his YouTube channel Adam 9A4QV has been testing his ADALM-PLUTO SDR in the 2M ham band at around 144 MHz. In one of his videos he shows a 2-tone test. A 2-tone test is used to determine how well an SDR can handle two strong narrowband signals at once, without causing intermodulation and imaging problems. The two tones in his video occur with real world signals on the 2M band when two amateur radio operators are transmitting strong signals at the same time.
The video shows that the Pluto SDR has some intermodulation problems occurring when the two strong signals transmit at once. No problems are noticed when only one signal transmits.
Problems like this with the PlutoSDR may be expected as it was never designed to be a high performance receiver, but rather a tool for learning and experimentation. But it is still possible to use it as a more general purpose receiver if you are aware of the limitations.
Speaking of two tone tests. The ADI PLuto IIO-oscilloscope application has a transmitter section in which you can select one tone/ two tone/ db level per tone and some other options that are beyond any info I’ve found. Then use sharp and view the signal on the IF FFT window……. cool…
Something is generally wrong here! Have you ever seen sideband signals that are symmetrical? On my Pluto I can clearly see if they are LSB or USB. This is not normal. Did you do a reboot?
It looks symmetrical but is not, all the red part is the upper sideband and he positions on the left of it. However the yellow part looks like a distortion that normally occour with local signals when the gain is set too high and overloads the receiver.
Let’s see what Adam says.
oh i understand what you mean, even the red part alone looks symmetrical, that’s true
You can observe the same behavior (signal presentation) on the screen using the Colibri receiver in the following video during the same contest:
I got the point 🙂
You are probably more familiar with the USB/LSB signals on the HF. And yes, the signal looks a bit different listening the HF comparing to the VHF signals. This is only because of the signal strength. You can see that the signals are really strong. Such a strong signals you can find on the HF usually listening to the broadcaster.
To give you an idea how strong the VHF signal can be, if I used the same antenna as the Slovenian station (on the video) the signal on my SDR input will be -5dBm! As I was using just a dipole the signal was let say -25dBm or so. This is way too much for the SDR that I was using in the video. You can calculate the signal strength using the following parameters 1800W tx with 16dB antenna gain at the 70km distance on 144MHz LOS.
I made today another video that should be uploaded tonight where I am receiving the HF on 14MHz where the signals look as you have the idea plus strong signal on the same band that looks like the signal on this video.
Thank you for the explanation Adam, in the following video is the station IQ5NN that you showed at 144.205, i recorded it with gain set to zero because it was very closed to me, so you cannot see distorsion but the signal looks also symmetrical.
The other reason is that these guys are winding the radio to a maximum, mic gain and audio compression so it is difficult to get some descent, clean signal.