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Author Topic: For W8JI: key clicks and amplifier non linearity  (Read 64651 times)
K9AXN
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« Reply #390 on: September 11, 2015, 02:31:28 PM »

Steve,

I answered all of your questions and because I don't agree you assume they are unanswered. 

The answer to your questions lies in the configuration of your test setup using a crystal filter.  Compounding the problem is your gross misunderstanding of complex filter technology.

The use of and your expectations of that filter are the fundamental error in your test setup and the genesis of your bizarrely incorrect assertions.  Using a crystal filter the way you have is patently invalid.   

I suggested using the heterodyne method which will eliminate any filter ringing from the equation.  It is apparent that you won't use it out for fear it will disprove your theory; which it will. 

I also explained a rough way to calculate the Q of a crystal filter.

That being said Steve, I wish you well and have a good day.  Until you choose to use an valid test setup there is no point in continuing this conversation. 

Kindest regards Jim

     
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G3TXQ
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« Reply #391 on: September 11, 2015, 02:52:23 PM »

Jim,

This evening I've carefully measured again the leading and trailing edge responses of the filter. I've also looked at the carrier output from the filter with a slower, 2.5kHz, keying rate - constant amplitude with no visible modulation! It's absolutely clear from the filter's edge responses that its ringing cannot completely "fill" a 100uSec "gap" of the carrier input, let alone a 200uSec gap - there would be an amplitude variation of at least 20% during the "gap". So I'm sorry, but filter ringing cannot explain the constant amplitude carrier.

You've not answered my questions directly, so I can only assume you are avoiding them for some reason; therefore I see little point in continuing the debate.

Steve G3TXQ
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K9AXN
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« Reply #392 on: September 11, 2015, 04:12:19 PM »

Steve,

Use the heterodyne configuration --- get the filter out of the equation.  Remember the carrier did cease after 400us.  The quench period was 400us plenty of time to ring through a 100us gap.  What happened to the carrier after the 400us quench?  The carrier was gone?

Please explain that.  Just do the experiment correctly.

A good day to you and good luck either way.

Kindest regards Jim
« Last Edit: September 11, 2015, 05:53:22 PM by K9AXN » Logged
G3TXQ
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« Reply #393 on: September 12, 2015, 01:30:50 AM »

Jim,

I see that you still choose not to answer my questions, so let's see if you will answer one of your own.

Let me remind you from your #383:



Quote
Now lets look at #336 the, the photo you copied from a book.  You seem to understand the concept.  Now replace the red modulation line with a pair of two positive Square waves separated by an off condition.  What do you think it will morph to?  Do it in your head it's simple.  You don't have to draw it just explain it.

I'm not prepared to discuss filters further until you can answer your own question; it goes to the heart of the debate, and will reveal how much you understand about modulation theory.

Steve G3TXQ
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K9AXN
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« Reply #394 on: September 12, 2015, 08:45:02 AM »

Good morning Steve,

Just set the test up using the Heterodyne technique.  This will remove any ringing effects and with the results you can either learn something from it or if I'm wrong you can tell me how silly I was and I'll have learned something.

Been an interesting discussion from all the participants.  A whole lot better than talking about line cords and what you had for breakfast.

I did learn some things though, that would ordinarily bring attention to my age.  When I first saw the drawing of the "0 and 180 degree switching mixer", I thought "What the hell is this thing".  Had to stop for a moment to see it was what was called a Balanced mixer for the last 60+ years.  I'm a creature o the 60's --- graduated in 1964 and until this thread never heard of a 0/180 degree switching  mixer.  Kinda wonder what a Double balanced mixer will be called --- maybe a Grand High Exalted Mysterious device used as a stripper --- of side bands that is --- just jest.

On the lighter side, Steve.  Mathematical formulas are descriptions of very disciplined thought and are intolerant of the presence of external influences. 

A good example is the formula used to extract the pure reactance of an inductor 6.28xfxl.  In the real world it's correct but from a purely theoretical point of view it is incorrect because by inference it is used to calculate the pure reactance of an inductor disregarding all other influences.

Have some fall hang gliding waiting --- best time of year for me so have to say A great day to you and I wish you well.

Kindest regards to you and your family Steve.

Regards Jim

         
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G3TXQ
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« Reply #395 on: September 12, 2015, 08:51:12 AM »

Hi Jim,

I did more filter measurements this morning and can readily demonstrate that the ringing from my filter cannot "bridge" a 200uSec gap, nor even a 100uSec gap in the carrier.

But as I said in my last post, I'm not prepared to discuss that further until you answer your own question:

Quote
Now lets look at #336 the, the photo you copied from a book.  You seem to understand the concept.  Now replace the red modulation line with a pair of two positive Square waves separated by an off condition.  What do you think it will morph to?  Do it in your head it's simple.  You don't have to draw it just explain it.

Surely it can't be too difficult to answer your own question; to quote you: "Do it in your head it's simple".

Steve G3TXQ

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AC7ZN
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« Reply #396 on: September 13, 2015, 06:23:44 AM »

Keyclicks

The original question in this post was whether poor IMD performance in an amplifier could cause keyclicks.  We can gain insight on this from the spectrum of a keyclick.  This is discussed here.

Modern commercial transceivers don't usually have keyclicks so you may not have heard many CW signals that have them.  But when a signal has bad keyclicks the QRM it causes can be particularly annoying.

Much information on the nature of keyclicks can be gleaned by simply listening to the signal in a receiver:

1.  The clicks sound like clicks (obvious I know, but sometimes things are misnamed).
2.  They occur at the beginning and end of the transmission of a dit or dah (I'll call the dit or dah a code element).. 
3.  The keyclick at the beginning of the code element sounds just like the keyclick at the end (this is usually true but does not have to be--see below).
4.  Tuning off frequency, the CW can be outside the passband of your receiver but you can still hear the clicks.  For a strong signal with bad clicks they can be sometimes heard more than a kHz off frequency.
5.  The clicks occur both above and below the transmitted frequency and appear to be symmetric with frequency.
6.  They grow weaker the further off frequency you go until they drop into the noise floor.  There are no nulls where the click goes away and then comes back as you tune further.  The decay in click strength with frequency off the carrier is monotonic.

You might be able to discern more properties.

I think keyclicks are annoying to many hams because their brains automatically engage and try to decode them just like a telegraph signal.  But because the start and end click sound the same the information of when keydown starts and stops is not there, so the ham brain automatically raises the fact that something is wrong to the ham's conscious level.

It seems incongruent to talk about the spectrum of just a click: a spectrum is a frequency domain thing and is taken over a long time on a signal that is often repetitive so it seems inappropriate to say things like 'here is how the spectrum decays over time' or 'here is the spectrum of the click'.  But in the sixties scientists and engineers realized there is meaning in these statements...signals packed with information vary over time and short-term spectra that change frequencies as the signal varies provide valuable information.  A field of study called harmonic analysis studies these time-shifting spectra and interesting transforms such as the short term Fourier integral and wavelet transforms are used as tools.

We model the click at the beginning of a code element with a unit step function, which goes instantly to one at time zero and stays at one forever.  Our code element eventually goes off so we will discuss how this changes the spectrum of the simple unit step model we are using.

Here is the unit step:


The Fourier integral transform is used to find the spectrum of the entire unit step.  Here is the Fourier integral that converts a time function in seconds to a frequency function in hertz:



We multiply the time waveform by the sine and cosine of the frequency we want, and integrate the results to find the magnitude and phase of that frequency component.  Note the infinities in the integral limits: we must integrate the time waveform over all time. But we can simplify:  times before zero are all zero so we can move the lower limit up (as long as we are not interested in really, really low frequencies) to near zero. As we move the upper limit to smaller and smaller times, we find the only thing affected is the energy at or very near zero Hz.  So the 'leftover' portion of the spectrum must be what is making the keyclick.  Here it is, plotted as amplitude magnitude versus frequency.
 


I was surprised that the absolute magnitudes seemed so low, but all the energy in that waveform that gets through your receiver passband is packed into a very short time, so you will get a nice sounding click.  Note that the energy does drop the farther you go off frequency, and that the decay is continuous, as we observed.  When we modulate the carrier with this step, we get the click both above and below the carrier as we hear.  The spectrum at the end of the code segment is identical.

We can see that a low pass filter set to the right frequency will cut out most of this spectrum and that is what is used in the transmitter keyclick filter.

Anything that enhances the rising or falling edge slope will make the clicks worse.  For an amplifier, a really bad case of power supply sag, where the transmission starts out at very high power but quickly decays to a constant value due to droop in the DC supply could in theory enhance a transmission that already is close to having keyclicks.  The power supply droop caused by too-low capacitors in the DC supply would be too slow to make something as fast as a click, but poor (resistive and/or inductive) wiring from the DC supply to the finals might be able to do it.  So I think it may be possible for an amplifier alone to generate keyclicks in unusual cases. 

73,
Glenn AC7ZN
 




 
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G0HZU
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« Reply #397 on: September 13, 2015, 07:17:49 AM »

Glenn, your images don't post up on the forum for me. Could you try another method to repost them?

I was kind of hoping the thread would have died away if I'd stopped posting because I think it got seriously derailed by my initial comments. But it has kept going for several days now...

My comments were based on what W8JI said in this thread and from what I saw when I looked at his analyser plots he described earlier in the thread. They can be found here:

http://www.w8ji.com/occupied_bw_of_cw.htm

I really don't want to kick off the whole debate again but W8JI presents information in a strange way that is easy to misinterpret. For example, I interpreted from his comments and the above web page that he thinks the classic definition of 99% occupied bandwidth is independent of keying speed and it is only affected by the keying waveform.

Hence all the stuff I posted up about how to work out the (average) power of each spectral component in a continuous stream of dits using Fourier analysis and also the practical examples I gave using Fourier synthesis and some test gear. Occupied bandwidth is also known as spectral occupancy and I don't think W8JI is measuring this correctly in his plots of 99% OBW.

However, I do think that what/all he is actually trying to say is that the range of frequencies you hear clicks over can be limited by shaping the keying waveform. If he reworded his page to lose the reference to 99% OBW and also explained (better) what he was really trying to portray then I think it would save a lot of confusion.

Whilst his analyser plots on his page are of no use for displaying 99% OBW they do give some indication of the influence of the keying waveform. I think the whole page needs a makeover to make it less confusing.
« Last Edit: September 13, 2015, 07:23:42 AM by G0HZU » Logged
AC7ZN
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« Reply #398 on: September 13, 2015, 07:31:45 AM »

Thanks, but I don't know.  I've tried my own website, photobucket, and finally an oblique procedure using Google drive before I was able to see the images myself.  I almost gave up.  Can anyone help me with an absolute surefire method of making these display?  I personally feel eham should provide online space for these images because this seems to be an ongoing problem...

Thanks,
Glenn
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G0HZU
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« Reply #399 on: September 13, 2015, 08:03:25 AM »

I'm using photobucket and it seems to work for me OK Smiley

I  click directly on 'IMG' to the right of the image on photobucket where it says SHARE LINKS and it then says 'copied' the moment I click on the IMG logo

I then paste the clipboard text directly here

See below for an image of OOK sent at 1 dit per second. This has a 99% OBW of about 42Hz even with a square keying waveform.

But obviously, this waveform will click very badly over a very wide frequency range when listened to by a typical receiver because of the square keying waveform.  I've posted up this example to show that measuring a 99% OBW of 42Hz doesn't tell us much about keyclick performance.

Hence the reason I think W8JI needs to reword his analyser plots to lose the confusing reference to 99% OBW as a means to indicate keyclick performance for different keying speeds.

His plots don't actually measure 99% OBW properly anyway because his analyser display trace is losing detail/resolution in the critical area of the carrier. So they are worthless in terms of displaying 99% OBW anyway.

« Last Edit: September 13, 2015, 08:31:33 AM by G0HZU » Logged
W1BR
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« Reply #400 on: September 13, 2015, 10:46:36 AM »

I don't think there was any doubt that a square wave is composed of an infinite number of harmonics... and that the energy would occupy a vast spectrum.

The point that I see that Tom made was that the key clicks were evident during the transitional phase of the waveform.  I think anyone who has observed key clicks from an offending CW signal has noted that the interference is present to other stations during that time period. All of the math and arguments don't refute that observation.

Pete
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W6RZ
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« Reply #401 on: September 13, 2015, 12:04:28 PM »

Here's a nice online book about digital signal processing.

http://www.dspguide.com/

Chapter 8 discusses the DFT in depth.

http://www.dspguide.com/ch8.htm
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G3TXQ
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« Reply #402 on: September 13, 2015, 12:07:08 PM »

I've not thought about this much, but on the question of "does the keying speed affect the 90% OBW", it seems to me important to distinguish between the theoretical square-wave keying case and the situation where there is some shaping.

If we take G0HZU's spectrum - which I assume is close to square-wave keying - when we increase the keying rate the spectral lines will move further apart but retain the same relative amplitudes. Clearly, the bandwidth containing any particular power percentage will vary directly with keying rate.

But what happens if we shape the spectrum of the baseband keying signal? Imagine we could shape it so that the amplitude of the spectral components reduced linearly with increasing frequency, and were cut off totally beyond some upper limit. When we alter the keying rate of that signal the spectral lines cluster or separate, but always within the same, fixed, frequency-domain triangular envelope. Wouldn't the bandwidth containing a particular power percentage now be largely independent of keying rate?

I guess I'm saying that we need to be careful that what we deduce from the hard-keying case - or something close to it - is still valid for more practical cases.

Steve G3TXQ
« Last Edit: September 13, 2015, 12:33:25 PM by G3TXQ » Logged
SM0AOM
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« Reply #403 on: September 13, 2015, 12:40:20 PM »

You are quite right, shaping will change the amplitude distribution considerably.

If the optimum erfc(t) keying shape is used, the sideband amplitudes fall off very rapidly, with increasing order
and the occupied, expressed as the band containing e.g. 99% of the total power, and necessary, expressed as the amount of spectrum needed to transmit a certain keying speed with a certain quality, bandwidths become close to each other.

Without any shaping, the higher-order sidebands extend very far, and objectionable interference due to key clicks is created far outside the 99% "occupied bandwidth" and very far outside the necessary bandwidth of the A1A emission which has been long established by the CCIR to be 3 or 5 times the keying speed in Bauds.

Outside this "main lobe" formed by the first order modulation sidebands and their first 3 or 5 odd harmonics, there should ideally be no sideband energy. The width of this necessary frequency band is fixed by the keying speed, and the need for recreating the shape of the keying elements.

73/
Karl-Arne
SM0AOM

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G0HZU
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« Reply #404 on: September 13, 2015, 01:10:16 PM »

I think the 99% OBW is a function of both the keying speed and the key shaping. But if you fix the shaping and then reduce the keying speed by say, an order of magnitude then the 99% OBW will go down with keying speed.

But I think the important point is that the 99% OBW of a transmitter is NOT a good thing to measure if you are really interested in  how far away you can detect clicks on a receiver.

If W8JI deletes the references to 99% OBW on his web page and removes the 99% OBW marker text from his plots and just says:

"Look at the general shape of each spectrum plot for 5-40dps to see that the spectrum is limited by the keying waveform in the radio"

Then I think it will be less confusing. Otherwise, he will attract criticism for his poor use of his test equipment in his attempts to measure 99% OBW properly and also for his conclusions wrt 99% OBW vs keying speed..

It should be obvious that if he reduced the sending speed to 0.1dps on a radio transmitter that the 99% OBW of the transmission should go down. But the click phenomenon in a receiver will be the same in terms of how the clicks (and their level) sound although the click rate will obviously go down.

So I think the web page would ideally need some clarification wrt this.
« Last Edit: September 13, 2015, 01:15:53 PM by G0HZU » Logged
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