Re: [time-nuts] Si5351A harmonics question

[Joe Leikhim]

Have you read this??

https://www.qrp-labs.com/ultimate3/u3info/u3pa.html

--
Joe Leikhim


Leikhim and Associates

Communications Consultants

Oviedo, Florida

jleik...@leikhim.com

407-982-0446

WWW.LEIKHIM.COM


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Re: [time-nuts] Si5351A harmonics question

[Bo Hansen]

Harmonics from a Si5351A is not a problem. As pointed out a LPF will do what it 
takes to get the fundamental frequency "extracted". The Si5351A is designed to 
clock/feed other digital circuits so that shouldn't be a problem in this 
particular case either.

In the digital clock domain spurs are, in most of the cases, not important. But 
in the RF domain things are VERY different. Unfortunately there seems to be a 
trend that only harmonics are important when in fact the in-band spurs are the 
important ones and what disturbs other stations and own receivers too.


In the RFzero  that also uses a 
Si5351A great attention has been devoted to keeping spurs and especially 
in-band down.
Bo

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Re: [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" 
> To: "Discussion of precise time and frequency measurement" 
> 
> 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: ch...@chriswilson.tv
>
>
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Re: [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 


=

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 


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  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
> 
> 
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Re: [time-nuts] Si5351A harmonics question

[Timestep Nuts]

That's what a square wave is !  A LPF will work on anything.  I'm not sure why 
you are using a class D amplifier though ?

Dave  G4IUG
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Re: [time-nuts] Si5351A harmonics question

[Dana Whitlow]

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  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
>
>
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> To unsubscribe, go to
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> and follow the instructions there.
>
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