Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

donate to eham
   Home   Help Search  
Pages: Prev 1 ... 23 24 25 26 27 [28] 29 30 Next   Go Down
  Print  
Author Topic: For W8JI: key clicks and amplifier non linearity  (Read 64691 times)
W6RZ
Member

Posts: 365




Ignore
« Reply #405 on: September 13, 2015, 02:18:38 PM »

A quick test shows that occupied bandwidth is not a function of keying speed. It is only a function of the rise/fall time of the CW dit.

First set of four images are with an approximately 8 millisecond rise/fall time. 48 wpm, 24 wpm and 12 wpm.

Second set of four images are with an approximately 4 millisecond rise/fall time. 48 wpm, 24 wpm, 12 wpm.

The keying shape is the optimum raised cosine.

The frequency resolution is 300 Hz per division in all spectrograms.















Logged
W1BR
Member

Posts: 4196




Ignore
« Reply #406 on: September 13, 2015, 02:44:37 PM »

A quick test shows that occupied bandwidth is not a function of keying speed. It is only a function of the rise/fall time of the CW dit.
 

I'd hope so...  a square wave is comprised by an in an infinite number summation of sinusoidal wave forms.  The occupied bandwidth of the resultant side bands would, at least in theory, would go on forever.  Obviously, in practice, the tank Q and antenna bandwidth would limit the product levels and their spectrum.

but, as Tom noted:

A CB amplifier without bias will square the envelope rise at the bottom That is due to the lack of bias in a Class C transistor RF amplifier.[/b]

Thus, how much harmonic energy is produced at the lower drive levels, as compared to distortion products at the peak of the RF crest?

To quote further:


The average power in the sidebands goes up with keying rate, but the signal sideband width stays the same (since it is set by the rise and fall shapes). Faster CW with clicks is more annoying because the clicks come more often, but the distance we tune off and are bothered by the clicks remains the same as long as the shape and duration of the rise and fall stay the same.

ALC induced clicks are reduced with higher speed, which is another irony. This is because the ALC stays more constant with higher speeds. At slow speeds, the ALC can drop to zero at times, and the next element can have an exaggerated rise with overshoot.


That pretty much is in line with W6RZ's observation.

Pete  k1zjh
« Last Edit: September 13, 2015, 02:55:23 PM by K1ZJH » Logged
G0HZU
Member

Posts: 163




Ignore
« Reply #407 on: September 13, 2015, 02:56:56 PM »

Quote
A quick test shows that occupied bandwidth is not a function of keying speed. It is only a function of the rise/fall time of the CW dit.

First set of four images are with an approximately 8 millisecond rise/fall time. 48 wpm, 24 wpm and 12 wpm.

Second set of four images are with an approximately 4 millisecond rise/fall time. 48 wpm, 24 wpm, 12 wpm.

The keying shape is the optimum raised cosine.

The frequency resolution is 300 Hz per division in all spectrograms.

Oh for goodness sake....

Turn the keying rate down to 1 dit per second and measure the 99% OBW as per the way it is supposed to be measured.

Has it gone down?

To save you the time just go back and look at my plot using the Agilent signal analyser. It shows a 99% OBW of about 42Hz for 1 Hz OOK.


« Last Edit: September 13, 2015, 03:00:42 PM by G0HZU » Logged
W6RZ
Member

Posts: 365




Ignore
« Reply #408 on: September 13, 2015, 03:11:23 PM »

Occupied bandwidth is the same at 1 wpm.

8 millisecond rise time.



4 millisecond rise time.

Logged
G0HZU
Member

Posts: 163




Ignore
« Reply #409 on: September 13, 2015, 03:21:42 PM »

Quote
Occupied bandwidth is the same at 1 wpm.

Measure the 99% OBW. W8JI was trying to measure and report the 99% OBW to prove that it is independent of keying speed.  

The definition for 99% OBW is all over the internet and it is a function of keying speed and the keying waveform.

Would you like me to post up the equations for it?
« Last Edit: September 13, 2015, 03:24:55 PM by G0HZU » Logged
AC7ZN
Member

Posts: 90




Ignore
« Reply #410 on: September 13, 2015, 03:22:36 PM »

licks

Thanks to G0HZU for helping with the images.  This post is now stale (it was stale when I posted it) so you don't have to read it, but if the images are not coming through please let me know.

Thanks, Glenn

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.
 


Plotted in dB:



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
 


Logged
G0HZU
Member

Posts: 163




Ignore
« Reply #411 on: September 13, 2015, 03:42:03 PM »


Just for the record, here are a couple of images showing the definition of 99% OBW  and the equation from the ITU for A1A showing how to work it out based on both baud rate and risetime .









Does this help?




Logged
W6RZ
Member

Posts: 365




Ignore
« Reply #412 on: September 13, 2015, 03:50:51 PM »

That equation is based on relative rise-time. In other words, as a percentage of the dit time. As the keying rate increases, the rise-time gets shorter.

I'm showing you constant rise time. The rise-time remains the same at different keying rates.

Please consider for a moment what I'm showing you instead of immediately attacking it.
Logged
G0HZU
Member

Posts: 163




Ignore
« Reply #413 on: September 13, 2015, 04:11:58 PM »

But if you fix the rise/fall time then I can just lower the keying rate as low as I like to keep lowering the 99% OBW.

The relative build up coefficient will then fall to nearly zero and the equation simplifies to

99% OBW = 19 * Baud rate



« Last Edit: September 13, 2015, 04:15:00 PM by G0HZU » Logged
G0HZU
Member

Posts: 163




Ignore
« Reply #414 on: September 13, 2015, 04:29:08 PM »

Way back in the thread I mentioned that I did a quick and dirty excel spreadsheet to predict %OBW for a square key waveform. I think/hope it is accurate.

Maybe this will be interesting or useful.

It looks at the power sharing of a 100W OOK transmitter wrt the modulation and harmonics etc.

You can see by the 19th harmonic we are at 98.99% OBW. See the red arrow to show this.

This closely agrees with the '19' in the ITU equation.







So for 1Hz OOK the baud rate is 2 so the 99% OBW for square keying is

99% OBW = 19*2 = 38Hz according to the ITU equation.

This agrees with my spreadsheet in the /Hz column on the far right (arrowed in red).
It predicts the 98.99% OBW is 38Hz.



« Last Edit: September 13, 2015, 06:12:21 PM by G0HZU » Logged
G0HZU
Member

Posts: 163




Ignore
« Reply #415 on: September 13, 2015, 06:08:39 PM »

The spreadsheet also predicts the levels of each modulation sideband in terms of dBc.

If you look at the 1Hz OOK image below you can see that the dBc levels for each harmonic agree very closely with the prediction in the spreadsheet. See the Excel column with dBc data.

eg the 19th harmonic is about -30dBc in the plot and the spreadsheet predicts -29.5dBc.


Logged
SM0AOM
Member

Posts: 258




Ignore
« Reply #416 on: September 14, 2015, 03:12:31 AM »

Looking back some 400 posts, it appears that the "bottom line" of the argumentation is that the international radio engineering community and the ITU are all wrong regarding the relations between
rise time, keying rate and the bandwidth expressions for Morse emissions.

It is certainly "your call", but I do find it somewhat ineffective trying to convince especially the ITU of its shortcomings using the pages of eHam.net.
It is also highly improbable that any representative of the FCC, the ITU Radiocommunications Assembly or its Secretariat is following this debate and comes up with proposals to change its views on its own.

Instead, some guidance of a potentially more efficient way to accomplish these goals is hereby respectfully submitted:

It starts with first closely studying this document http://www.itu.int/dms_pub/itu-r/opb/res/R-RES-R.1-6-2012-PDF-E.pdf,
and then contacting the Office of Engineering and Technology at the FCC presenting your arguments.

If the FCC is convinced and finds your arguments worth-while, the FCC can make up a proposal for re-writing the affected parts of the Radio Regulations
(Appendix 1) and the related ITU-R Recommendations.
The FCC then submits this proposal to the ITU Secretariat through the US Department of State.
This is because only Governments are full voting members of the ITU.

If sufficient interest about this matter can be aroused internationally, the ITU Secretariat may work out Terms of Reference for a Working Party (WP) within the
ITU-R Study Group 1, who is dealing with spectrum efficiency and spectrum management matters.
Its objective will be to rewrite the relevant parts of ITU-R Recommendation SM.328.
All this is subject to the final approval of the Director of the ITU.

The designated members of this WP may then meet at their convenience, discussing the matters about the necessary, occupied and key-click bandwidths of Morse emissions
to their "heart's content" until a consensus within the WP has been reached about how the new wording of the relevant texts in the Radio Regulations and SM.328 should be.
Their findings have to be summarized in an input document to a future meeting of the ITU-R Study Group 1, where it will become an agenda item.

After a through discussion in the sub-, sub-sub and sub-sub-sub groups formed at this meeting, the proposal ultimately reaches the Plenary where it may be approved.
When finally approved by consensus or vote, the new text is finally incorporated in the ITU-R Recommendations, and by reference in the Radio Regulations.

A reasonable time frame for the whole process is in the order of 3-7 years, but if you are convinced that your views are right, it might be "worth waiting for".

73/
Karl-Arne
SM0AOM


Logged
G3TXQ
Member

Posts: 1845




Ignore
« Reply #417 on: September 14, 2015, 07:05:10 AM »

It's interesting to take the ITU equation and consider its derivative with respect to keying rate at various values of alpha.

W6RZ made the important observation that alpha is the "relative" rise time; for a fixed rise time - the situation in most radios I would guess - alpha is proportional to the keying rate B.

When the rise time is very short or the keying rate is very slow, alpha tends to zero; the occupied bandwidth then becomes directly proportional to the keying rate.

At smaller values of alpha the occupied bandwidth is less dependent on the keying rate; however for practical values of alpha it is never completely independent.

Steve G3TXQ
« Last Edit: September 14, 2015, 07:23:28 AM by G3TXQ » Logged
K9AXN
Member

Posts: 442


WWW

Ignore
« Reply #418 on: September 14, 2015, 08:16:47 AM »

Perfect explanation Ron!

Kindest regards Jim



A quick test shows that occupied bandwidth is not a function of keying speed. It is only a function of the rise/fall time of the CW dit.

First set of four images are with an approximately 8 millisecond rise/fall time. 48 wpm, 24 wpm and 12 wpm.

Second set of four images are with an approximately 4 millisecond rise/fall time. 48 wpm, 24 wpm, 12 wpm.

The keying shape is the optimum raised cosine.

The frequency resolution is 300 Hz per division in all spectrograms.
















Logged
G3TXQ
Member

Posts: 1845




Ignore
« Reply #419 on: September 14, 2015, 08:58:34 AM »

Is there actually any inconsistency between the ITU equation and W6RZ's plots? Taking W6RZ's rise time of 8mSec, the ITU equation predicts a tiny 2% change in occupied bandwidth when you drop the keying rate by a factor 4 from 48wpm to 12wpm - a change that it would be very difficult to resolve on those plots.

Steve G3TXQ
« Last Edit: September 14, 2015, 09:00:48 AM by G3TXQ » Logged
Pages: Prev 1 ... 23 24 25 26 27 [28] 29 30 Next   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!