#384 

Several years ago I made a video demonstrating and illustrating how square waves are composed of odd-harmonic phase aligned sinewaves, and the shape gets more "ideal" the more harmonics you add.

Yes, I had thought it would be pretty much a sine wave with little harmonics given technology has advanced some in the the hundred years. I would figure the FCC would mandate as 'raised cosine' pules form.

They'd be more likely to regulate occupied BW, and leave the techniques up to the mfrs.

​@@laserhobbyist9751I think it was a joke.

​@@GustavBertram Indeed it must have been

@@gonebamboo4116 I've always heard the FCC has zero humor, but to see it in action... 🤣

An external keyer shouldn’t make any difference compared to a straight key at the same speed.

Lots of practice over the years trying to emulate the work of Forrest Mimms.

Thank you, Alan! Is there a course/book perhaps somewhere that I could use? 73! Adrian, YO6SSW

@@w2aew Mims

I would say 3-5ms is a good start.

Yes, that is the definition of the BW of a signal in Part 97. I chose to go a little deeper.

It will slow the edges - but making a narrow BPF at the RF frequencies would be difficult. It would be best to do it at an IF and then upconvert to the RF output.

The carrier is not modulated with a tone, it is a simple unmodulated RF signal that is switched on/off. So, really it is an extreme case of slow, pulsed, amplitude modulation - specifically ASK (amplitude shift keying). But, when the carrier is "on", it is not modulated. The tone that you hear is generated *in the receiver* by mixing the incoming signal with another oscillator called a BFO (beat frequency oscillator). What you are hearing is the difference between the incoming signal frequency and the BFO frequency. More accurately, this mixing is happening at an IF frequency - where the incoming signal is converted into an IF frequency, and the BFO signal is mixed with that, and the resulting difference is the tone you hear. The BFO signal might be slightly above the IF frequency, or slightly below the IF frequency. In some radios, which "side" the BFO is one may depend on which band you are on. When you hear different tones calling, it means that each of them are transmitting a carrier that is slightly different in frequency. My friend Vince VE6LK recently did a video on this. You can find it here: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-FFsvMFfag0w.htmlsi=R5yh2_oBHeoRqlSZ

@@w2aew Thank you, that is certainly in alignment with what I was thinking and you confirmed the different tones also as I was thinking that they must be just a little off frequency. My ICOM has an auto-tune feature for CW and I guess I am just right at their frequency and others seem to be off. Also, I do know that in pileups I move slightly off to be heard. Thanks again for the response and the referral video. BL

Yes.

No. The RF risetime is much, much, much longer than a single cycle of the RF, so the moment it is switched "on" with respect to the RF zero-crossing doesn't matter.

Yes. Anytime a signal moves *quickly* in the time domain, it occupies a lot of bandwidth in the frequency domain. So, if you start with a 0 volt signal, and then immediately turn on a full amplitude RF carrier, you will generate a lot of wideband RF energy at the point of turn on - because you're transitioning from a DC value immediately to a sharp rising signal at the zero crossing of the sinewave. In CW transmissions, the turn on/off transitions occur over thousands and thousands of RF cycles, each becoming gradually larger/smaller. This is what tends to limit the spectral content and occupied BW.

Still the same. Occupied BW will be a function of speed and shape of the RF rising and falling edges, and speed of keying.

The spectrum will continue to be narrower as the rise/fall are slowed further. A narrow BP filter would do the same.

@@w2aew My expectation would be that with slowest possible rise-fall we should reach Shannon limit, while currently with these 300Hz bandwidths we are >10x above Shannon limit.

@@BarsMonster It would seem so to me. As I would do about 10 letters/minute I could tolerate a 15 ms rise time or more.

@@BarsMonster A few reason for this are: I'm looking much further down the skirts (36dB down), and I'm still using fairly fast rise/fall w.r.t. pulse width.

you're likely right.

The effect would be audibly about the same as what you hear through those two filters.

A CW Signal that rises and falls too slowly generates ringing making the signal unpleasant to listen to if not outright uncopiable. A CW Signal with a Rise/Fall Time too fast generates Key Clicks with gets under the skin of your neighbors and earns you a not so savory reputation. So it follows that Code Speed dictates the rise/fall Time one needs to use. Today's radio equipment is frequently set with the Rise/Fall Time set too 11:43 fast. Radios with the Rise/Fall set too slow is very rare.

​@@WestCoastMoleah thx.

​@@w2aew:) thx

James the rise and fall Time has to be sharp enough to give a distinct note. It takes time for your brain synapses to fire. If there is to much decay time one note runs into another. Remember as speed increases the time between dits as dahs decreases. For me anything above 55 WPM is just a blur to me. I can't pull apart dits dahs at those speeds. But there CW Operators in Europe that can do more than 150 WPM. The only guy that I know that can do those speeds is Barry Kutner W2UP. He used to be able to do 120 WPM in his prime. Age has caught with Barry, he is almost 70 and says his max speed is 80 WPM. As for me I'm more than satisfied with doing 32 WPM in a contest.

Even an RC will provide some measure of spectrum containment.

@@w2aew On a side note, with every video you release the urge to get at least a novice license increases. The problem for me back then was that the DARC OVs in my region were a bit stiff. Not many people to talk to and with the rise of the internet, radio was already on it's way out over here.

Still basically the same.

Now that's not being very neighborly, is it.

Would make things much worse (going from zero to full carrier synchronously). Typically, the carrier is ramped up over many thousands of carrier cycles so that there are no abrupt changes. Consider the example here. At 14MHz, each carrier cycle is 71ns. So, during a 4ms rise time, there are 56,000 carrier cycles.

When I say synchronously, I'm thinking that the carrier is enabled and disabled at the zero crossing, which wouldn't have the effect that you describe. Is this even possible? I don't know offhand.

@@Roy_Tellason I understand what you mean by synchronously. But it really wouldn't apply in this case. Typical RF envelope rise and fall times are 4-5 orders of magnitude longer than the RF cycle - meaning that the modulated waveform increases gradually, cycle by cycle over the course of 10-100 thousand cycles before reaching full power. In this scenario, having this ramp-up start synchronously would have no effect on the resulting BW. If on the other hand, you're suggesting that the carrier be switched "on" at it's zero-crossing, that would be an equivalent of an extremely fast rise/fall time, resulting in a huge amount of occupied BW during the turn on/off times.

@@Roy_Tellason It does not matter at all. Only the shape of the envelope matters. AM modulation maps the baseband spectrum of the modulating signal (i.e. the spectrum of the envelope, or IOW the spectrum of the shaped Morse pulses) to the sidebands below and above the carrier.

Yes, a proper test would be to apply a mask the progressively drops the further out you go.

@@w2aew There is many ways to gather data. The approach in the video is very good and gets to the point in a clear and concise way. One reason I follow your content.

take the test. use an app. it's easy. fun and social

That would slow down the rising and falling edges of the RF pulse.

I wonder how the rise and fall time affect the sound?

@@jamescollier3 See my reply to your other comment.

​@w2aew because the high frequency components are reduced, is that right ?

Yes

Modes that contain redundant signal content (like both upper and lower modulation sidebands) like AM and FM would the widest. Some form of QAM, whether using a single carrier or multi-carrier OFDM would be much better.

@@w2aewThanks for taking the time and posting this clip. Your analysis not only have significance for CW/Morse code but also in CDMA (code division multiple access) That is in a CDMA band we have particular frequencies (f1, f2, ... fn) that are chosen so that they are not aliasing that we use for sending data. For example, one phone is assigned that if say f2, f6, and f8 all to be on at given time it would be a "1" and if all of them are off it is a "0", and if onlysome of them are on is noise. Now how fast we turn these frequencies on and off and with what profile (here you used straight line to turn your frequency on and off, but you could use say exponential or say sinc function profile) would expand or contract the spectrum used. You could send the square wave modulating signal to a low pass RC filter and see the effect on the spectrum. You could also observe the location of the RC filter pole ( ie, its bandwidth ) on the expansion of spectrum. Also, you could use a band pass filter on your modulated signal ( signal that goes to antenna ), and after demodulation see profile that that band pass filter created for your signal.

It would be determined by the composite noise, etc of the signal. But in an ideal sense, yes.

@@w2aew I witnessed this conversation in the early 1970s between my father, a physicist, and his "uncle-in-law", W1DI, who was my elmer. My father could not wrap his head around the idea that a carrier would have a bandwidth just by turning it on and off. Neither could I until watching your video.

Excellent explanation and demonstration!

I’d certainly be interested in stopping by. Email is the channel name at ARRL dot net.

Yes, this is true for a continuous key-down. But, for an Morse code CW transmission, the carrier is modulated on and off, thus does have modulation sidebands and occupied a non-zero amount of BW.

If it's a CW sine wave signal of infinite duration... which is what you seem to be implying... then AFAIK, mathematically (Fourier transform, and sinc function) the 'bandwidth is infinite, since the sinc function has no zeros... but if the 'CW' signal is of finite duration...then the signal gets turned off, at some point in time....then mathematically, it definitely has a 'bandwidth.' It's a pulse or burst. The longer the duration, the smaller the bandwidth, since bandwidth is inversely related to the duration for a single 'pulse."

It’s been a while since I checked them out.