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Author Topic: For W8JI: key clicks and amplifier non linearity  (Read 64225 times)
W1BR
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« Reply #15 on: August 16, 2015, 01:05:43 PM »

Good idea, but he should add an attenuator equal to the amplifier gain to make a realistic guess.

Pete
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W8JI
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« Reply #16 on: August 16, 2015, 02:08:13 PM »


I clearly stated rise and fall time, so what does infer?  Continuous carrier? 

So, you don't know the answer. The question was simple--how much effect does using a Class Amp have on the rise and fall time of a properly shaped CW waveform?  Is there enough distortion to affect the signal's BW?  It has been argued on here before that certain "CB" amps would not even be suitable for use on CW because of their poor two tone IMD performance.

Pete

Any signal changing amplitude must have sidebands. This means the CW signal has sidebands when the rising and falling edges occur. It is obviously not a single frequency except during the time when level is not changing.

When level is changing, it is actually an AM signal.

Bandwidth, or number of sidebands and frequency spread of the sidebands during the rise and fall, is determined by the slope and time of carrier power change while rising and falling.

If we ran that signal into a non-linear amplifier and changed the rise and fall shape, we would alter the bandwidth.

A class C amplifier will distort the rise and fall, usually flattening it off or squaring the edges. The square edges would greatly increase keying sideband bandwidth over something like a raised sine shape.

I think people are probably getting a bit too fussy if they think normal IMD of a linear amplifier can significantly increase bandwidth, but clipping of the leading edge (even at low power) or sharpening of roll over at the top edge can add significant bandwidth. A grossly non-linear amp, one that increases the change in slope of the envelope, can make bandwidth go way up.

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.

73 Tom
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G3RZP
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« Reply #17 on: August 16, 2015, 10:47:43 PM »

Many of the QRQ ops prefer a signal with 'hard' i.e. clicky keying, as being easier to read at speeds of 40 wpm plus. With the advantage that they can the n argue that as they run at QRQ in a contest and need hard keying, the fact that it keeps people away from 'their' frequency is a bonus!
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JS6TMW
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« Reply #18 on: August 16, 2015, 11:57:01 PM »

This is important to me because I plan to use a cheapy "CB amp" on the higher HF bands on CW with my QRP CW rig when needed. (I'm making a good lowpass filter for it so harmonics should not be an issue). It's impossible to know what is really going on inside without opening it and taking voltage readings while it's operating, but as far as I can tell from the schematic, it runs exactly the same for SSB and CW. In fact the only difference between the "SSB/CW" and "AM/FM" switch settings seems to be a time constant (delay?) for Transmit-Receive.

Steve AI6KX waiting for my Japanese license
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ZS6DX
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« Reply #19 on: August 17, 2015, 12:39:55 AM »

So Pope is proved right again......

Seems like the Pope has issues......  Sad

Interesting discussion! I have played around quite a bit with the rise and fall times of my CW keying, adjustable on many modern radios. I found that for "slow" CW, up to about 25wpm more gentle rise and fall times make for a very pleasant signal and I have had people comment on it.

When you go up in speed readability becomes an issue, dos/dashes seems to flow together and it becomes a strain to read. I sometimes run at 40-65wpm and then I prefer very sharp rise/fall times, sounds more harsh and the signal does get wider but it is crisp and readable.

I have not tried using CW on a non linear amplifier but I think it will mostly be ok if the radio/amplifier is clean.

Thanks or you informative posts, I have learned a lot from you!

73, Rudi de ZS6DX/V51VE
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SWL2002
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« Reply #20 on: August 17, 2015, 03:27:05 AM »


When level is changing, it is actually an AM signal.

73 Tom

Interesting, I wonder where we heard that before?

Quote from: SWL2002

If you are talking about what hams call "CW" (morse code) what you are calling CW or continuous wave is not CW or continuous wave at all.  You are turning a carrier on and off with some finite rise and fall time.  This is an amplitude modulated signal.  It also has a bandwidth defined by the speed and also the rise/fall times of the signal, among other things.


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JS6TMW
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« Reply #21 on: August 17, 2015, 03:49:36 AM »

I think just now the Pope is trying to decide how many angels can dance on the head of a pin.
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W8JI
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« Reply #22 on: August 17, 2015, 05:05:25 AM »

A CB amplifier without bias will square the envelope rise at the bottom. That is the main damage class C does.

If any amplifier saturates hard, it will square the top edge.

Generally if those two points are taken care of, everything else will be acceptable. The low end change is the difficult one to see on a scope.
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G3RZP
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« Reply #23 on: August 17, 2015, 05:08:56 AM »

However, even when limiting, the numbers showed that the bandwidth increase caused by intermodulation was minimal compared with the bandwidth caused by the modulation. The sharpened rise and fall times are not a characteristic of intermodulation but the non-linearity.

The Pope in question is Alexander......
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W8JI
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« Reply #24 on: August 17, 2015, 05:31:03 AM »

Right. IMD of the amplifier is not such a concern as any abrupt change in transfer function as level changes. You don't want to hose up the shape with something sharp.
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W1BR
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« Reply #25 on: August 17, 2015, 05:45:54 AM »

Now that I think about it a bit more... a dual trace scope could be used to sample and compare the waveform shaping before and after the amplifier.  But, I don't have a cheap SS amp so it is a moot point.

I'd also suspect that if the amp is using RF sensing the resultant hot keying and lead to problems.

Pete
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JS6TMW
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« Reply #26 on: August 17, 2015, 06:03:19 AM »



I'd also suspect that if the amp is using RF sensing the resultant hot keying and lead to problems.

Pete

Yes, I am worried about that. I have no experience with this kind of amp yet. I might have to slow it down. Not sure how the QRP rig  (Weber Tribander) works as far as keying goes either since it is now under construction.

I don't understand why W8JI assumes CB amplifiers are Class C, since they are mainly intended to boost AM signals (is there much SSB on the CB band???)

There seems to be a common opinion that just because these CB amps are illegal for CB use, they are also by nature producers of dirty signals. Oh, and I HAVE read W8JI's test of the HLA-150 using a two-tone input signal. Not very flattering. The RM KL300 I bought might have more headroom. Then again, it might be the same transistors with a new nameplate.
« Last Edit: August 17, 2015, 06:36:22 AM by EXW6BMZ » Logged
G3RZP
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« Reply #27 on: August 17, 2015, 06:54:00 AM »

Quote
I don't understand why W8JI assumes CB amplifiers are Class C, since they are mainly intended to boost AM signals (is there much SSB on the CB band???)

Surprisingly, very heavily clipped AM through a Class C stage is still understandable. But you don't need Class C for poor IMD. There was an experimental RAF VHF TX in WW2 that never got anywhere but used a Class C 8018 (selected 807 for full output at 120MHz) fed with the modulated signal. It's probably that legacy that leaves aircraft VHF sounding to me like 90% distortion and 10% unmentionable!

Even 'Linear amplifiers' are not necessarily so. G3SJX reviewed the Expert 1.3K-FA amplifier in July RadCom. He found the 3rd order IMD at outputs of 1.3 to 1.5kW to be  -20 to -26dB relative to PEP - that's -13 to -20dB relative to one tone of a 2 tone signal. At 1kW, it was -30dB  rel PEP  i.e. -24dB relative to tone.

Not exactly brilliant
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W1BR
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« Reply #28 on: August 17, 2015, 08:40:36 AM »



I'd also suspect that if the amp is using RF sensing the resultant hot keying and lead to problems.

Pete

Yes, I am worried about that. I have no experience with this kind of amp yet. I might have to slow it down. Not sure how the QRP rig  (Weber Tribander) works as far as keying goes either since it is now under construction.

I don't understand why W8JI assumes CB amplifiers are Class C, since they are mainly intended to boost AM signals (is there much SSB on the CB band???)

There seems to be a common opinion that just because these CB amps are illegal for CB use, they are also by nature producers of dirty signals. Oh, and I HAVE read W8JI's test of the HLA-150 using a two-tone input signal. Not very flattering. The RM KL300 I bought might have more headroom. Then again, it might be the same transistors with a new nameplate.

Well,  many of the cheaper CB amps are running with no forward bias on the devices, and are designed for AM amplification. I believe Tom had written up the "advantages" for doing so (as in the reasoning why it "worked" for CB lids, and not that it was technically desirable!!,)  at the expense of destroying adjacent channels, etc.

Also, I am not sure that a lack of forward bias is actually true Class C for transistor devices; by definition they'd need to have some reverse bias voltage placing them well into the cut off region. Non linear for sure, but I am positive that the devices are fully into the Class region.

Gain compression for CW signals may slightly affect the waveform, but again, how much of a problem does it cause unless the waveform rise or fall times become to close to being a square wave?  Most CB amps have some forward bias, albeit with poor regulation, but is that more of a concern for SSB waveforms vs. CW waveforms?

Pete
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DL8OV
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« Reply #29 on: August 17, 2015, 09:47:53 AM »

Gentlemen, I have some test results for you:

Transmitter - Homebrew rig with RD16HHF1 final producing about 5W into a leaky dummy load, variable output power and variable bias.

Receiver - FT817 with untuned mobile whip at about 200m range. Transmit power was adjusted to give an S9 signal on the receiver to avoid overload. For the purposes of the test 'Signal Width' is assumed to be the two points either side of my signal where I measure S5 or four S points down on the carrier.

1) Biased for Class A and transmitting a clean sine wave the signal width was reported as 400 Hz and no key clicks were heard.

2) Biased so that clipping could just be seen on the output waveform signal width was reported as 400 Hz with very slight keyclicks, maybe too small to notice unless you were listening out for them.

3) Biased for Class B with 50% of the waveform on the scope signal width was reported as 700 Hz with heavy key clicks. They could also copy my 14 MHz signal on 28 MHz despite using a low-pass filter on the TX.

4) Biased for Class C with 40 % of the waveform on the scope signal signal width was reported as 1,3 KHz and the key clicks were described as 'terrifying'. Signal level was confirmed as S9 so this was not due to receiver overload.

Peter DL8OV



« Last Edit: August 17, 2015, 09:56:57 AM by DL8OV » Logged
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