[time-nuts] Si5351A harmonics question

[Chris Wilson]

  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: chris at chriswilson.tv