Chris, In general I'd say that the rather high level sidebands on your signal are not reasonable at all, but I think I see what's *really* wrong: your SA settings. It looks to me like the tall signal is the DC spur, (a.k.a. "start spur") of you analyzer, and the signal very near center screen is the actual signal of interest. Try setting the CF and span of the analyzer so that the left end is at higher than zero freq, and I think you'll feel a lot happier.
BTW, It's important to avoid any DC component on the signals that you feed into the SA, as DC could severely overload the mixer in the SA with all manner of bad results depending on the front end design of the SA. Many SAs already have a DC blocking capacitor built-in, but not all. The output of the SN74AHCT125N will certainly have a strong DC component, so do pay heed to the comment immediately above. Also, it you want to filter the output of the SN74AHCT125N, be sure that the filter is not putting a DC short on the output of the logic device. If the filter does show a DC short at its input, add a DC blocking capacitor in series. A LPF should never do so, but once you get into BPSs or HPFs anything is possible depending on the filter's particular design. It may be that your class-D amplifier is objecting to a DC component on its input- yet another place where a blocking cap could be needed. Good luck! Dana On Wed, Jun 5, 2019 at 11:01 AM Chris Wilson <ch...@chriswilson.tv> wrote: > > > 05/06/2019 16:24 > > I am using a small transmitter called a U3S by QRP Labs as an exciter > for a high power Class D amp. I take the output at circa 951kHz direct > from CLK0 (it has no output stage built, I go direct from CLK0 on the > Si5351A and input it to a level converter based on a SN74AHCT125N IC. > The Class D amp needs a frequency input of X2 the desired output > frequency. I have experienced terrible gate and drain waveforms on two > Class d amps driven by this, so I put my SA on the output of the > SN74AHCT125N via a switchable 50 Ohm attenuator. I see the trace in > the first photo in the link below. Now I am the first to admit that > apart from a fear of blowing the SA front end, I am a total novice > with an SA. Are the high level additional frequencies away from the > centre frequency to be expected, and do LPF's work on square waves? I > di try a big high power 475 kHz LPF between the output of the > SN74AHCT125N and the SA and the second shot was the result. Obviously > that LPF is far from ideal though, but it was sat on the bench so I > trie it.... Thanks please keep the answers at a novice level :) > > http://www.chriswilson.tv/mf.zip > > https://www.qrp-labs.com/ultimate3/u3s.html > > -- > Best Regards, > Chris Wilson. > mailto: ch...@chriswilson.tv > > > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
