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Author Topic: confused about "CW"  (Read 2002 times)
KJ6OYJ
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Posts: 6




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« on: November 01, 2012, 10:36:26 PM »

Hello Esteemed Elmers!

Been using PSK31 on my Icom 718 in SSB.   But I see there are other modes on the 718 -- CW and RTTY.  Got me thinking about how the 718 transmitted/received those modes.

Did a lot of Web and book reading about CW, I'm still confused.  Here’s what I’ve come up with -- please make comments/corrections!

CW
“True” CW is unmodulated carrier keyed on and off (preferably in Morse Code Cheesy).   When received in AM mode, it sounds like clicks or low noise.  When received in CW mode, the rcvr turns on its BFO to inject a single tone 800 Hz apart from wherever the signal is in the IF stage.  The resulting beat frequency (during the carrier-on times of the signal) is detected and makes audible tones in the AF output.  The carrier-off times are silent. 

MCW
Modulated CW is, strictly speaking, True CW where the carrier-on part is modulated with an audible tone.  If received on AM, the keyed tones are heard.   On CW, you will hear a combination of the tone and the beat frequency.    Some people define MCW as steady carrier with simple Morse code tone modulation -- detectable on AM.

CW/AFSK 
If a 800Hz tone is keyed and transmitted in SSB mode, then the resulting suppressed-carrier signal looks a lot like True CW -- a keyed carrier with no modulation. 

RTTY
This is 2 different carrier frequencies in a sequence -- each one keyed on and off.  Like 2 CW signals near each other, so all of the above receive ideas should apply.


Questions --

1.  What is it like when True CW is received in SSB mode?   Why?
2.  Same question for MCW (as defined above).
3.  If I hook up a key to my Icom, put the Icom in CW mode, and key Morse code, what is the transmitter sending?  Is it True CW? 
3a. Same question for Icom RTTY mode (assuming I had some RTTY gear to connect to the Icom).
4.  If I use Fldigi on my PC to send Morse code, with the Icom in SSB mode, what is the transmitter sending?  If it’s sending what looks like True CW, then the receiving radio must be in CW mode?
4a. Same question for sending RTTY via FLdigi in SSB mode.
5.  The Icom has settings for Side Tone -- what is that and how does it apply to the above points?
6.  In the typical CW QSO these days, what transmitter and rcvr modes are used?
7.  Same question for RTTY.
8.  Why is MCW (as defined) not permitted below 50MHz?

Thanks!

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Doug McC.
San Francisco
KJ6OYJ
G3RZP
Member

Posts: 4742




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« Reply #1 on: November 02, 2012, 08:53:34 AM »

1.  What is it like when True CW is received in SSB mode?   Why?

If the 'true CW' is on the nominal carrier frequency, it will be zero beat with the SSB carrier reinsertion, and you'll get much the same effect with the rx in AM. Tuned so that there's an 800 Hz difference between the CW signal and the carrier, you'll hear the on/off keyed CW.

2.  Same question for MCW (as defined above).

Tuned for 'zero beat' with the carrier reinsertion (CIO), you'll hear an on/off keyed tone. Incidentally, MCW can be keying the carrier (and thus the audio) or just keying the audio tone into the tx.

3.  If I hook up a key to my Icom, put the Icom in CW mode, and key Morse code, what is the transmitter sending?  Is it True CW?

It may be sending 'true CW' on the same carrier frequency as in SSB, or it may be sending on an frequency offset - some rigs have separate carrier frequency generators for CW receive and CW tx.

3a. Same question for Icom RTTY mode (assuming I had some RTTY gear to connect to the Icom).

Depends on the rig. It could be sending the mark and space frequencies offset by some amount from the carrier, or it could be the carrier and the  shifted frequency.

4.  If I use Fldigi on my PC to send Morse code, with the Icom in SSB mode, what is the transmitter sending?  If it’s sending what looks like True CW, then the receiving radio must be in CW mode?

If you put a pure audio tone into an SSB tx, you get out a single RF signal indistinguishable from a carrier except it is offset from the nominal carrier by the frequency of the tone. A number of early transceivers did precisley this for CW: it's not very good, because any harmonics produce multiple signals unless the frequency is high enough that the SSB filter cuts off the sidebands casued by the harmonics. Then if you are working true transceive with TX and RX on the same nominal carrier frequency, the received tone is a bit too high for comfortable listening and reading Morse.


4a. Same question for sending RTTY via FLdigi in SSB mode.

The mark and space frequencies are offset from the carrier by their audio frequency


5.  The Icom has settings for Side Tone -- what is that and how does it apply to the above points?

Side tone is usually an audio oscillator that follows the keying and allows you to listen to your sending. Accurate sending 'blind' is not so easy. When using separate rx and tx, the rx was often reduced in sensitivity to enable you to listen to the actual sent signal, although you had to be careful to avoid overloading. This was fine when rx and tx were on the same frequency, as in amateur practice: where this wasn't the case as in marine radio operations, the audio oscillator  was usually in the tx and fed to the rx so the operator could hear his sending.

Operating through a satellite and listening to the morse coming back with a delay is a very good way of completely screwing up your sending because of the delay in the feedback loop to the brain!
 
6.  In the typical CW QSO these days, what transmitter and rcvr modes are used?

CW

7.  Same question for RTTY.

RTTY


8.  Why is MCW (as defined) not permitted below 50MHz?

Bandwidth. If modulated at 1kHz, it's 2kHz wide (assuming no modulation harmonics) In the days of 500kHz at sea, transmissions were MCW so an off tune receiver would still hear them: if in CW, if the tx was off frequency or was zero beat with the BFO, the result might not be audible. This was especially useful in the automatic alarms.
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N2EY
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Posts: 3895




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« Reply #2 on: November 02, 2012, 01:29:11 PM »

In the days of 500kHz at sea, transmissions were MCW so an off tune receiver would still hear them: if in CW, if the tx was off frequency or was zero beat with the BFO, the result might not be audible. This was especially useful in the automatic alarms.

Yes, but there's more to it. The use of MCW on 600 meters also meant that ANY receiver in use could receive the signal, whether or not it had a BFO. This came down from the days of spark and crystal-detector receivers.

73 de Jim, N2EY
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K0ZN
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Posts: 1553




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« Reply #3 on: November 02, 2012, 05:50:21 PM »

If you hook up your key to the key jack in your rig, put the rig in the CW position and push down on your key, you are sending CW !  i.e. the key is simply a switch that turns the transmitter on (sending an unmodulated carrier when the key is closed). The transmitter sends out an RF signal during each part/element of a Morse character.....dits or dahs.... when the key contact is NOT closed (key up) there is no signal transmitted. It is just like turning on a light bulb with a wall switch.... when the switch is closed (key down) there is light because power can flow through the bulb.

If the rig is in the SSB mode, CW sounds like CW !  The detector circuitry "hears" the carrier as a signal and converts it like any other signal to audio. The biggest
difference is in the receiver bandwidth. SSB requires a wide bandpass to process a human voice. CW signals can "pass" through an extremely narrow band pass giving
them much better signal to noise ratio.....which is why CW "gets through" better than SSB or other wideband modes when there is a lot of static, etc. A description of
circuits that can "process"/detect SSB signals in the SSB mode is complex; there is more than one way to go about it. Suggest you research that as a separate subject.

MCW is essentially just AM with a single tone modulation rather than voice audio being used to modulate the amplitude of the carrier.  In this day and age it is kind of an archaic spectrum hog. There is no legitimate need for it on the ham bands. MCW signals are essentially just as wide as an AM signal because they are an AM signal. You can send MCW simply by putting a rig in the AM position, transmitting but not saying anything into the mike, while keying an audio code practice oscillator into the microphone. You could whistle Morse code into a microphone while transmitting on AM and it would be MCW. Obviously, a "real" MCW transmitter would just wire the tone modulation in to the audio drivers without a microphone so there could/would be just the tone and no voice or other audio.

If you want to hear "real" MCW signals you can listen to the Aircraft non-directional beacons and marker signals below the broadcast band, roughly between about 180 and 500 KHz with your rig in the AM mode. If you don't hear any on AM, try the CW mode and tune slowly. Typically, they are not very strong due to low transmitter power, but you can often hear a few depending upon where you live. A long single piece of wire to the center conductor of your coax connector usually will suffice as an antenna. With a good LF antenna, you can sometimes DX some of them for a couple of hundred miles on some winter nights when atmospheric noise is low in the LF region.

(to other "experts":  I fully realize the foregoing is not technically "perfect"; please save the technical nitpicking of this post.  I was trying to keep this SIMPLE .  
 This is not an IEEE paper or technical article !! )

73,  K0ZN
« Last Edit: November 02, 2012, 06:10:10 PM by K0ZN » Logged
G3RZP
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Posts: 4742




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« Reply #4 on: November 03, 2012, 04:59:53 AM »

Aircraft Non Directional Beacons (NDB) seem to be disappearing at an enormous rate in Europe and probably in the US too. The Russians have quite a lot still east of the Urals apparently: that, it is said, is why they weren't in favour of a ham band in 470kHz region.

So if you don't hear them, don't worry.
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STAYVERTICAL
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Posts: 875




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« Reply #5 on: November 04, 2012, 12:22:21 AM »

I believe there is a push to remove NDB and many other types of conventional aircraft beacons and ILS systems.
The new paradigm is to use GPS and local airport terrestrial GPS/error reduction systems.
This will reduce costs and maintenance requirements.

Personally, when that big solar flare waiting in the Sun to fry/disrupt our satellite systems comes knocking, I hope I am on the ground.
It happened with Loran etc, and it is now happening with aircraft navigation.

Talk about putting all your eggs in one basket.

73 - Rob
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STAYVERTICAL
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« Reply #6 on: November 04, 2012, 12:35:52 AM »

To give my input to the original question:

When a CW signal leaves your antenna it is indistinguishable from any other CW signal - no matter how it was produced - direct or audio sine wave.

The same is true for an RTTY signal.

People use the CW mode setting when using CW because that is usually where the narrow filters for CW are available.
If you use audio methods for producing CW, you must use the SSB mode, so you may or may not have access to the narrow filters.
The same applies for RTTY produced by audio tone means.

If a radio has a dedicated RTTY setting, this usually means it can directly shift an R.F. oscillator by the appropriate shift.
This is a direct method of producing RTTY which some purists will prefer over audio means.
It generally requires "keying" an RTTY port on the rig with either two logic levels or on/off keying.

Sidetone is the audio tone you hear when you are sending CW, so you know what you are sending.
It is frequently adjustable in both level and frequency to personal taste.
Sometimes it also tracks the offset which is automatically applied to your rigs dial in CW mode.
This offset is to compensate for the fact that if you zero beat a CW signal, you will hear nothing, since the offset is 0Hz.
Common offsets are 700 and 800 Hz which is the audio frequency coming from the speaker.
Typically, CW filters are centred around these frequencies as a result.

MCW means modulated CW, and is simply putting an audio tone (AM) on a CW signal.
As others have said, it means you do not need to have a BFO (Beat Frequency Oscillator) to receive the signal.
For example aircraft use AM transmission on VHF so unless they engage a BFO, they may not receive beacons easily.
As others have said it was typically used on MF at sea, and still used for beacons.

73 - Rob
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W5DXP
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WWW

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« Reply #7 on: November 04, 2012, 07:32:29 AM »

“True” CW is unmodulated carrier keyed on and off (preferably in Morse Code Cheesy). 

Just a bit of history: It was first called CW because it wasn't dampened like a spark gap signal. CW (continuous wave) is somewhat of a misnomer since Morse Code is never a continuous wave. (A continuous wave carries zero information.) It is true that a continuous wave is an unmodulated carrier but when the continuous wave is used for Morse code it is no longer unmodulated or continuous. Morse code is a coding method which requires modulation. What we call the CW mode is actually a form of AM (amplitude modulation) OOK (on-off keying) where the amplitude changes from zero to some value which introduces sidebands just like AM. A Fourier series analysis of the CW mode will reveal that the sideband frequencies for square-wave modulation actually extend out farther than voice AM and thus we are forced (by the FCC) to decrease the bandwidth and eliminate CW key clicks by slowing the rise and fall times of the CW keying square wave.

When I was a Novice in 1953, there was another Novice on 40m whose key-clicks could be heard from lower band edge to upper band edge due to the way he was modulating his "continuous wave" with Morse code.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
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