I don't think 'scope FFTs are anywhere near good enough for HF transmitter harmonic testing, and get worse as the frequency goes up. The job needs a real spectrum analyzer. The TinySA is very good value, some 70dB dynamic range if I recall, which is plenty for transmitter testing, and in comparisons I did with my professional SA it was always within a dB. All for somewhere in the $50 ballpark..
On Wed, Feb 28, 2024, 9:34?PM Eric W. Hansen <kb1vun@...> wrote:
High-band QMX rev2 (2024), 9V version, Firmware 017.? Having fun learning about this radio. LPF cutoffs are good, though LPFs 1 and 2 show stopbands a little high, about –30dB.? I get 4W out on 20m and 17m, less than 3W on 15m, 12m, and 10m.??
I thought I'd take a look at the output spectra, after reading several posts here about harmonics.? I don't have a tinySA, but I do have a 200MHz oscilloscope with good FFT capabilities.? I made the following setup:? QMX --> Dummy load --> 10X probe --> Scope.? The second harmonics on 15m, 12m, and 10m are higher than what I would expect, between 39dB and 42dB below the fundamental.? 20m and 17m are OK, 63dB and 52dB down, respectively.
My understanding is that the push-pull Class D amplifier is effective at suppressing even harmonics, prior to any filtering, so I'd like some ideas about where the high levels might be coming from and what I might be able to do to reduce them.
