[time-nuts] Si5351A harmonics question

[Bob kb8tq]

Hi

Not quite sure what this has to do with time and frequency but ok ……

A spectrum analyzer has a “zero frequency” response. You can see this when you 
do not have any input. It is clearly present on both of your plots. I suspect you are 
confusing it with an actual signal. 

Analyzers are quite happy to display “negative” frequency. If you are centered on 
a 1 MHz input with a 7 MHz sweep width, you will see + 1 MHz and - 1 MHz (in addition
to the 0 MHz blip) on the screen. 

You normally are not distracted by this stuff in textbook pictures because the analyzer
has been adjusted to only show a limited span.

They also can be over driven (to much input) and the distortion will start to show up on the screen.
One normal trick is to keep the highest “signal” blip below the “zero frequency” blip.

http://literature.cdn.keysight.com/litweb/pdf/5952-0292.pdf <http://literature.cdn.keysight.com/litweb/pdf/5952-0292.pdf>

=====

Any periodic waveform can be represented by a Fourier series. Put another way,
you can add harmonics together to get various waveforms (like square waves). 
The spectrum analyzer simply breaks a signal down into its harmonic components.

A square wave is made up of odd harmonics. ‘125 chip will put out a nice square wave.
Your first plot shows this quite clearly. 

https://en.wikipedia.org/wiki/Square_wave <https://en.wikipedia.org/wiki/Square_wave>

So, more or less, situation normal. Nothing alarming in any of the shots you show.

Bob



> On Jun 5, 2019, at 11:33 AM, Chris Wilson <chris at 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: chris at chriswilson.tv
> 
> 
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