[time-nuts] Si5351A harmonics question

[Robert LaJeunesse]

Chris, better to set your SA to 5 MHz span with 2.5MHz center. Adjust the input attenuator (start with lots of attenuation)  until the 995 kHz peak is just less than the always present peak at zero frequency. (Namely always keep zero Hz at the left side of the screen, or to the left of that so it's off screen and you only display positive frequencies.)

I'd also consider adding to your (AC coupled) RF probe a series resistor of 450 Ohms (470 in parallel with 10K is close enough) so as to not heavily load the '125. Yes, it attenuates by 20dB, but that's no problem for the SA. To be fair, a few pF cap across the resistors will flatten the frequency response at higher up. Use a 15pF trimmer and you can adjust it for a more accurate result.

Bob LaJeunesse

> Sent: Wednesday, June 05, 2019 at 11:33 AM
> From: "Chris Wilson" <chris at chriswilson.tv>
> To: "Discussion of precise time and frequency measurement" <time-nuts at lists.febo.com>
> Subject: [time-nuts] Si5351A harmonics question
>
>
>
>   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
>
>
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