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Author Topic: For W8JI: key clicks and amplifier non linearity  (Read 64649 times)
G3TXQ
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Posts: 1845




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« Reply #435 on: September 15, 2015, 05:09:56 AM »

Just to illustrate that last posting:

This is the original keying waveform:



Here's a close up of the leading edge:



and here's its spectrum:



Now add crossover distortion; here's the leading edge - you can just about see the distortion:



And here's the spectrum:



Checking afterwards, the same degree of crossover distortion applied to a 2-tone signal produced 3rd order products 30dB below PEP.

Perhaps not very scientific, but it seems to indicate that crossover distortion could produce significant artefacts.

Steve G3TXQ

« Last Edit: September 15, 2015, 05:12:31 AM by G3TXQ » Logged
W1BR
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Posts: 4194




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« Reply #436 on: September 15, 2015, 09:00:53 AM »

Steve

I suspect that key clicks are more complex than a two signal. But, I understand you point, and it shows that the cross over distortion is real.  The amount of energy in the side bands, and the occupied bandwidth, are subject to the waveform shaping.  While  a true square wave modulated signal has side band energy going out to infinity.

My question was based on a using a Class C amp (CB, transistorized, no bias) with a CW transmitter that perhaps had a reasonably shaped waveform.  I was curious as to how much the added distortion might affect the occupied BW, as far as noticeable artifacts (key clicks) to another station--in practical terms. I realize my question is rather quantitative, since it lacks specifics. It was more practical than theoretical.

Pete
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G3TXQ
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« Reply #437 on: September 15, 2015, 09:26:15 AM »

Pete,

With the tools I have I can't easily add class C distortion; the Audacity Crossover plug in only allows a limited range of distortion levels. I'll look to see if it might be possible to achieve the required effect by slicing and reconstructing the waveform.

What I didn't say in the earlier postings is that the effect of the crossover distortion was predominantly to increase the 3rd, 5th, 7th etc harmonic levels of the carrier. These components would presumably be eliminated by the output stage filtering. Looking in much more detail at artefacts close to the carrier which would not be eliminated by harmonic filters, the crossover distortion adds very little when the rise time is slow (10mSec), and a modest amount when the rise time is fast (2mSec).

I guess we're left to speculate what class C would do Wink

Steve G3TXQ
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DL8OV
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Posts: 1057




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« Reply #438 on: September 15, 2015, 10:20:38 AM »

This experiment has already done (many posts ago)

http://www.eham.net/ehamforum/smf/index.php/topic,104806.msg864042.html#msg864042

I supplied an OOK signal to an amplifier and varied the bias until the amplifier was operating at Class C, at which point the key clicks were described as "terrifying" by the guy with the receiver a few hundred metres away.

Peter DL8OV
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G3TXQ
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Posts: 1845




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« Reply #439 on: September 15, 2015, 01:40:20 PM »

Peter,

Do you know what the rise and fall times of the keying were before being applied to the output stage?

Thanks,
Steve G3TXQ
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JS6TMW
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Posts: 1257




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« Reply #440 on: September 15, 2015, 03:38:27 PM »

This experiment has already done (many posts ago)

http://www.eham.net/ehamforum/smf/index.php/topic,104806.msg864042.html#msg864042

I supplied an OOK signal to an amplifier and varied the bias until the amplifier was operating at Class C, at which point the key clicks were described as "terrifying" by the guy with the receiver a few hundred metres away.

Peter DL8OV

And thanks for reminding everyone, Peter. It was also my original query on a different thread that led to this vast outpouring of knowledge. Ironically, the transistor amp in question self-destructed before I could incorporate biasing, and even before I could generate a single dit or dah.

Steve in Okinawa JS6TMW/QRP
« Last Edit: September 16, 2015, 04:04:25 AM by EXW6BMZ » Logged
DL8OV
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Posts: 1057




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« Reply #441 on: September 15, 2015, 10:36:47 PM »

Rise time during my experiment was 4,5 milliseconds and I used a raised cosine. Output power was only at QRP levels and the receiver was a Yaesu portable rig handled by another ham a few hundred metres away in the car park. Before the start of the experiment I reduced my output power until he saw S9 so that overload was not an issue.

Based on the results of this experiment I would NOT run CW through a Class C Amplifier.

Peter DL8OV
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G3TXQ
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« Reply #442 on: September 16, 2015, 12:40:44 AM »

Peter - thanks!

Steve G3TXQ
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SWL2002
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Posts: 895




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« Reply #443 on: September 16, 2015, 03:38:44 PM »

Rise time during my experiment was 4,5 milliseconds and I used a raised cosine. Output power was only at QRP levels and the receiver was a Yaesu portable rig handled by another ham a few hundred metres away in the car park. Before the start of the experiment I reduced my output power until he saw S9 so that overload was not an issue.

Based on the results of this experiment I would NOT run CW through a Class C Amplifier.

Peter DL8OV

This is what I said way back only a few posts in to the original thread.  Amazing all the "theories" going around when the answer can be ascertained with a simple physical real world experiment like you did.  Bravo!

Don't use a class C amp for CW.

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W6RZ
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Posts: 365




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« Reply #444 on: September 17, 2015, 12:37:33 AM »

All the waterfalls I posted were from a real world SDR transmitter and receiver except the very first one in post #128. You can see the phase noise in them. Since I'm using a real receiver, I can also record the audio.

Here's 18 wpm CW with a 1000 Hz filter and fast AGC. About halfway through, I tune up 1 kHz.

The first clip has 8 millisecond raised cosine keying shape.

http://www.w6rz.net/noclicks.wav

The second clip has no shaping at all.

http://www.w6rz.net/clicks.wav

Here's what the audio looks like in Goldwave. You can easily see the keying shape at the beginning of an element.


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W6RZ
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« Reply #445 on: September 17, 2015, 01:14:05 AM »

Here are the clicks zoomed in. Looks like a single click duration is around 2 milliseconds.




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N3QE
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Posts: 5593




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« Reply #446 on: September 17, 2015, 03:53:51 AM »

Here are the clicks zoomed in. Looks like a single click duration is around 2 milliseconds.

Now that is the result of using a circa 1000Hz CW filter in the chain. You will never resolve a feature finer than a millisecond or two if you are using a 1000Hz filter.

If you had been using say a 6kHz AM filter, then the feature produced by a broadband click would be 0.2 - 0.3 milliseconds.

Tim.
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W6RZ
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Posts: 365




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« Reply #447 on: September 17, 2015, 04:44:48 AM »

Here are the clicks zoomed in. Looks like a single click duration is around 2 milliseconds.

Now that is the result of using a circa 1000Hz CW filter in the chain. You will never resolve a feature finer than a millisecond or two if you are using a 1000Hz filter.

If you had been using say a 6kHz AM filter, then the feature produced by a broadband click would be 0.2 - 0.3 milliseconds.

Tim.
Correctamundo. Filters are also subject to the Gabor limit. That is, you can't have a filter that has high frequency and temporal resolution at the same time. You can only have a narrow filter with a long time constant or a wide filter with a short time constant.

Here are the clicks with a 6 kHz filter.





Pretty close to your prediction. I guess it depends on exactly when the beginning and end of the pulse is defined.

Here's the audio file. I tuned away in 1 kHz steps. By the end of the clip, I'm 4 kHz away.

http://www.w6rz.net/clicks6khz.wav
« Last Edit: September 17, 2015, 05:03:52 AM by W6RZ » Logged
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