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Author Topic: For W8JI: key clicks and amplifier non linearity  (Read 64184 times)
W1BR
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« on: August 15, 2015, 08:46:03 AM »

Tom

Here is a poser for you... a lot of hams have expressed opinions that a linear's IMD performance somehow affects key clicks. Since IMD is products caused by undesirable mixing of multiple signals, it would seem that a single tone CW signal would NOT require an linear amplifier.

On the other hand, rise and decay time of the waveform might be adversely affected at the extremes between Class C and true linear service.  That may cause key click artifacts when a properly shaped CW waveform is amplified in a non linear stage?

The question remains:  How much affect does the linearity of an external amplifier have on a CW signal?

Pete
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SWL2002
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« Reply #1 on: August 16, 2015, 05:32:04 AM »

Tom

Here is a poser for you... a lot of hams have expressed opinions that a linear's IMD performance somehow affects key clicks. Since IMD is products caused by undesirable mixing of multiple signals, it would seem that a single tone CW signal would NOT require an linear amplifier.

On the other hand, rise and decay time of the waveform might be adversely affected at the extremes between Class C and true linear service.  That may cause key click artifacts when a properly shaped CW waveform is amplified in a non linear stage?

The question remains:  How much affect does the linearity of an external amplifier have on a CW signal?

Pete

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.

In the simplest form, "CW" consists of a pure sine wave multiplied with a square wave that's either 0 or 1, corresponding to the keying of the carrier. As with a mixer, or amplitude modulation, multiplying two signals generates frequency components that are the sum and difference of each frequency component of the multiplicands. In the simple form described above, where the switching waveform is a square wave, the bandwidth can be very large, because a square wave consists of an infinite series of odd harmonics.  Real transmitters filter this square wave to some extent, and perfect square waves can't exist in practice anyway. The slower the transition from "on" to "off" is made, the less bandwidth is required. If you send faster, then there are more transitions per second. Since each transition requires a certain amount of energy away from the carrier frequency, faster speed means more sideband power, that is, more bandwidth.

It is amazing that many (most?) Hams do not know or are not taught this.  I guess it should not be surprising since most Hams nowadays are appliance operators.

As to the effect of using a Linear or class C amplifier on the "CW" signal, I will leave that up to you to deduce.
« Last Edit: August 16, 2015, 05:59:56 AM by SWL2002 » Logged
N3QE
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« Reply #2 on: August 16, 2015, 06:30:27 AM »

I'm not Tom, but I know for a lot of half-century in old ham transmitters, the keyed class C stages often went through a region of instability every keying transition and the resulting parasitic squawks were just everywhere - clicks not just kHz away but up into VHF.

The handbooks always wrote this up as if it were the sparks from the key contacts, recommending filters on the key line, and I think they got this wrong. I'm sure the sparks on the key contacts caused local very much local RFI but the key filters recommended by the handbooks, made the instabilities on keying transitions last longer.

I don't think this is applicable to any modern equipment but I could be wrong. Maybe Tom has some thoughts on neutralization instabilities that happen as grid current starts to get drawn with capacitive divider neutralizing circuits.

Incidentally, my experiences with parasitics in ham equipment cause my company's lawyers to get the heebie-jeebies when it comes to parasitics in other equipment. They claim that the half century of postcards I've gotten from OO's, in no way make me qualified to testify on the subject in court :-).
« Last Edit: August 16, 2015, 06:33:44 AM by N3QE » Logged
G3RZP
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« Reply #3 on: August 16, 2015, 06:47:52 AM »

Firstly, to be really pedantic about that which we usually refer to simply as CW, let's clarify it as having the ITU-R designation 100HA1AAN.

A Class C stage can 'sharpen' the rise and fall times depending on the circuit. If, for example, a 'clamp' tube is used without a VR tube in series with the screen, it will be in Class A at the start of the pulse and the effects will be minimal. If it is biased just to cut off with grid leak bias making up the rest of the necessary Class C bias, again, there won't be much sharpening. If the bias is derived solely from a bias supply, then you will see sharpening, the amount depending how far back the tube is biased. So a true linear amplifier should not make any difference, although the presence of an ALC spike could have an effect.

In theory, any amplifier COULD also add a chirp to a keyed signal by virtue of AM to PM conversion: in reality, it will be so small as to be negligible, and only noticeable with special test gear, but I mention it for the sake of completeness, since pedantry is the order of the day. That's with it acting purely as an amplifier, without N3QE's spurious oscillations (they can do - oh, yes they CAN - but not inevitably!) taking place during the power ramp up/ramp down.
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W1BR
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« Reply #4 on: August 16, 2015, 06:58:45 AM »


 
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.
 

As to the effect of using a Linear or class C amplifier on the "CW" signal, I will leave that up to you to deduce.

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
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W1BR
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« Reply #5 on: August 16, 2015, 07:08:01 AM »

Firstly, to be really pedantic about that which we usually refer to simply as CW, let's clarify it as having the ITU-R designation 100HA1AAN.

A Class C stage can 'sharpen' the rise and fall times depending on the circuit. If, for example, a 'clamp' tube is used without a VR tube in series with the screen, it will be in Class A at the start of the pulse and the effects will be minimal. If it is biased just to cut off with grid leak bias making up the rest of the necessary Class C bias, again, there won't be much sharpening. If the bias is derived solely from a bias supply, then you will see sharpening, the amount depending how far back the tube is biased. So a true linear amplifier should not make any difference, although the presence of an ALC spike could have an effect.

In theory, any amplifier COULD also add a chirp to a keyed signal by virtue of AM to PM conversion: in reality, it will be so small as to be negligible, and only noticeable with special test gear, but I mention it for the sake of completeness, since pedantry is the order of the day. That's with it acting purely as an amplifier, without N3QE's spurious oscillations (they can do - oh, yes they CAN - but not inevitably!) taking place during the power ramp up/ramp down.

Hi Peter

I should have been a bit more precise:  tube finals are bit more involved, since there must be some sort of protection when drive is removed; most likely grid block or cathode keying, or a clamp tube (which I haven't seen used for CW.)  There was usually some method of providing an RC time constant, either on the grid bias or in the keyed cathode to help control the rise and fall time. Fancier rigs had sequential keying to control chirp problems, etc.

I was more concerned about statements that were made in this forum in the past concerning the use "CB" amplifiers that are being sold on the ham market.  Their two tone IMD characteristics aren't that great, and it was claimed that they were entirely unsuitable for CW because they weren't linear enough for that service.  I've never seen any actual measurements that prove or disprove that theory. I suspect the waveform rise and fall times will be affected to some degree, but how much does that increase the key clicks to a point that is noticeable?

Pete

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SWL2002
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« Reply #6 on: August 16, 2015, 07:14:02 AM »


 
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.
 

As to the effect of using a Linear or class C amplifier on the "CW" signal, I will leave that up to you to deduce.

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

Obviously this went over your head or you are having reading comprehension problems again.  I thought it was so clear that it was not worth stating.  Unless you are keying the POWER SUPPLY of a class A,B,C,D,E,F, or whatever amp, the amplifiers linearity will affect the "CW" signal.  The more IMD the amp generates because of non-linearity, the wider the "CW" signal becomes (beyond the bandwidth attributed to rise/fall times, keying speed, carrier phase noise, etc...).  You are putting an AMPLITUDE MODULATED signal into the "linear" amplifier in question.  What do you think happens when an AM signal is put into a non-linear amplifier?  The same thing happens when a keyed "CW" signal is put into a non-linear amplifier.

The answer was laid out in front of you, but you still could not see it.  Pitiful...
« Last Edit: August 16, 2015, 07:29:02 AM by SWL2002 » Logged
G3RZP
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« Reply #7 on: August 16, 2015, 07:15:54 AM »

Pete,

Depends on the bias. Because a bipolar will start turning on at a low voltage, I suspect the sharpening won't be noticeable - especially if driven with a rig with lousy key clicks like so many! MOS without bias, quite possibly. I'd worry more about harmonic attenuation to Part 97.....

See your messages....

Peter G3RZP
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W1BR
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« Reply #8 on: August 16, 2015, 07:17:56 AM »

Indeed, harmonics are an issue, but that can be taken care with proper output filtering.

Pete
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G3RZP
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« Reply #9 on: August 16, 2015, 08:18:14 AM »

Quote
The more IMD the amp generates because of non-linearity, the wider the "CW" signal becomes (beyond the bandwidth attributed to rise/fall times, keying speed, carrier phase noise, etc...).

The amount is so negligible as to be hardly worth considering. Refer to ITU-R Rec SM.1138, where the necessary bandwidth for a 25 wpm signal in fading is considered to be 100Hz. 25 wpm means with the standard word, dots 100ms long:  the 10-90% rise and fall time specified in ITU-R Rec. SM328-11 requires an occupied bandwidth (99%) of 66.4Hz. So take a 10 Hz modulating frequency: there is a carrier with two 10Hz sidebands each 6dB down, and drive the amplifier to clipping. Then the worst case 3rd order IMP each side will be -15dB rel carrier at 20Hz: the worst case 5th order will be -29dB and the 7th at 40Hz away will be around -38dB: so all still within both the necessary bandwidth and indeed the occupied bandwidth and lower than modulation components themselves.

A reference on IMD in limiting amplifiers  may be found in  S.F. George and J.W.Wood, 'Ideal Limiting' Part 1, U.S. Naval Research Laboratory, AD266069, October 2nd, 1961.

Phase noise would not intermodulate, although the 3dB lower AM component could. However,
since at a 10Hz offset, one would expect the AM component to be at least 20dBc/Hz down, the IMPs will be much lower than those for the modulation sidebands.

Thus to all intents and purposes, unless the CB amplifier has a special noise generator built in, it is OK for CW. Or all those transmitters built over the years with acceptable performance by people like Collins,  Marconi, STC , KW, Labgear etc all got it wrong - as did the ITU.

harmonics - yes. Just needs filtering but the CB amps don't bother! One point that needs thinking about with those amps is stability when feeding a load which is not 50+j0 at the operating frequency. There was an article in Electronics Design magazine on that back in the mid 1970s.
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SWL2002
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« Reply #10 on: August 16, 2015, 08:45:26 AM »

Quote
The more IMD the amp generates because of non-linearity, the wider the "CW" signal becomes (beyond the bandwidth attributed to rise/fall times, keying speed, carrier phase noise, etc...).

The amount is so negligible as to be hardly worth considering. Refer to ITU-R Rec SM.1138, where the necessary bandwidth for a 25 wpm signal in fading is considered to be 100Hz. 25 wpm means with the standard word, dots 100ms long:  the 10-90% rise and fall time specified in ITU-R Rec. SM328-11 requires an occupied bandwidth (99%) of 66.4Hz. So take a 10 Hz modulating frequency: there is a carrier with two 10Hz sidebands each 6dB down, and drive the amplifier to clipping. Then the worst case 3rd order IMP each side will be -15dB rel carrier at 20Hz: the worst case 5th order will be -29dB and the 7th at 40Hz away will be around -38dB: so all still within both the necessary bandwidth and indeed the occupied bandwidth and lower than modulation components themselves.

A reference on IMD in limiting amplifiers  may be found in  S.F. George and J.W.Wood, 'Ideal Limiting' Part 1, U.S. Naval Research Laboratory, AD266069, October 2nd, 1961.


No where did I make a judgement on whether the levels were worth considering nor did I make any comments on whether a CB amp would be acceptable.  I was talking about the result, not whether the result was negligible of not.  Those are two different things.  

Quote

Phase noise would not intermodulate, although the 3dB lower AM component could. However,
since at a 10Hz offset, one would expect the AM component to be at least 20dBc/Hz down, the IMPs will be much lower than those for the modulation sidebands.


I did not say that phase noise would inter-modulate.  I said that the CW bandwidth is affected by multiple things one of which is carrier phase noise.

Quote

Thus to all intents and purposes, unless the CB amplifier has a special noise generator built in, it is OK for CW. Or all those transmitters built over the years with acceptable performance by people like Collins,  Marconi, STC , KW, Labgear etc all got it wrong - as did the ITU.

harmonics - yes. Just needs filtering but the CB amps don't bother! One point that needs thinking about with those amps is stability when feeding a load which is not 50+j0 at the operating frequency. There was an article in Electronics Design magazine on that back in the mid 1970s.

Yes, they might be acceptable for CW, but many use them for SSB service also, and that is not OK if the CB amp is biased class C or whatever.  Some CB amps are so poorly built, I would not even use them to amplify a true continuous wave signal (not what Hams call "CW").

  
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G3RZP
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« Reply #11 on: August 16, 2015, 09:01:40 AM »

So Pope is proved right again......
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DL8OV
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« Reply #12 on: August 16, 2015, 10:19:51 AM »

There is one thing I am not sure of. If I generate a morse code signal with a 5mS rise and fall time then feed this into a Class A amplifier I will see a larger output signal which still has a 5mS rise and fall time, a faithful reproduction. If I do the same exercise using a Class C amplifier will I still see that 5mS rise and fall time on the output? A little voice in the back of my head keeps on telling me that the output will have crisp sharp edges and be a source of key clicks because a Class C amplifier can be either 'on' or 'off'.

It's tempting to take my homebrew rig and rebias the P.A. into Class C just to see what happens.

Peter DL8OV
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G3RZP
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« Reply #13 on: August 16, 2015, 10:56:12 AM »

it's going to depend on how Class C it is. Let's say you are driving a silicon bipolar. Then there's  going to be conduction starting at around 0.4 to 0.5 volts vbe. Now if the 5mS is 10 - 90% (which the ITU feels a bit short unless you are talking of 50wpm), then the transistor will be starting to conduct at 2 to 3ms. Of course, it won't be an exponential rise and fall.....

Now take a tube biased to say -100 volts and starting to conduct at say -20 and running to +20 volts. Your 5ms is down to about 1.5ms. Duty cycle is around 80 degrees and efficiency up there with the Gods. Well, not quite.

The imperfections in all this mean, as usual, that a few rough calculations can give you an idea and then empiricism has advantages. Simulation works well too, PROVIDED you have an accurate model, and much of the time, you don't really have that. Especially in IC design, tying the CAD guys down to what the devices REALLY do at RF and what all the process corners really are is a bit like picking up liquid mercury from a Teflon crucible while wearing boxing gloves with your hands tied behind you!

Cynical? Who, me?
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DL8OV
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« Reply #14 on: August 16, 2015, 01:03:16 PM »

Well, now I have something to do on a wet Monday, tweak the bias on my P.A. Driver until I see < half an AC cycle on the output then try sending some CW into a dummy load***. Another local ham will be listening for my signal (and any keyclicks) using an FT-817 and a mobile whip.

Much more fun than feeding the lot into SPICE and seeing what the computer spits out the other end.

Peter DL8OV

*** This dummy load leaks, on purpose.
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