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@chriswilson.tv
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@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@chriswilson.tv
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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 chris@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@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.
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
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@chriswilson.tv
To: "Discussion of precise time and frequency measurement" time-nuts@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@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.