Why do they transmit a square wave, instead of just short and long dashes of a single frequency? Only the top of the square wave is the signal, the bottom is the 'negative' of it.
They are not transmitting a square wave. The square wave is the drive waveform that is modulating the transmitter. If you look at the square wave shot, you will see longer "high" periods (dash) and shorter "high" periods (dots).
In classic "CW" (continuous wave, aka. Morse Code), it would be keying the transmitter on and off. When the transmitter is on, it is transmitting a pure sine wave at the Tx frequency (10.140015 MHz). When the transmitter is off, it is transmitting nothing (duh).
Looking at the schematic, the kit in question is transmitting continuously and modulating the carrier to be slightly higher or lower in frequency based on the square wave modulation drive.
For a direct conversion receiver[1], (heterodyne[2] works the same way, but uses intermediate frequencies to improve selectivity) the receiver has an internal oscillator that is tuned to approximately 1000 Hz difference from the transmitter. By mixing the two signals in the receiver, it demodulates the transmitted signal into a 1000 Hz tone when the transmitter is transmitting and nothing when it is not transmitting (CW) or higher and lower pitches with the kit from the article.
The CW on/off modulation is what you hear (simulated) in the WW2 movies with the beeping ("dit" and "daaah") sounds.
Because it would be very difficult to decide if your box was not transmitting or just unable to be heard at any given time. The square, spread over a large time allows you to say with confidence, "yes I have received and yes its the bottom/top of a dot/dash".
Are you involved with this as an amateur, or taking a stab at coming up with a reasonable answer? I would like to know the actual reason, and so far Google hasn't been helpful.
I am starting to think the square wave is an artefact from the circuit design.
Hi
I am the designer of this circuit ( http://www.hanssummers.com/qrsskit ) and can answer that question. I've just seen the thread on this.
There are various possible "modes" in QRSS and certainly plain on/off keying as you suggest, which is in fact just very slow morse code, is one of them.
The mode I am transmitting here can be known as FSK/CW. CW (Morse Code) in a Frequency Shift Keyed (FSK) way. As you say, the top level is the "key down" ordinary morse code.
The reason for this is NOT just an artefact of the circuit design! The circuit is designed this way for a reason! I could just have easily designed it such that it keyed the signal on and off, like real morse code but much slower.
There are several reasons why FSK/CW is more suitable than plain on/off keying. 1) and 2) below are the most important.
1) Readability: Many kinds of interfering signals on the band are just a plain carrier. It might be some mixing product coming out of your computer monitor, or a harmonic signal of some oscillator in your neighbour's TV, or plain carrier transmissions from great distances. A weak carrier, drifting in and out of visibility, can look VERY much like a weak on/off keyed CW signal in QRSS. Experience of QRSS experimenters has shown that it is much easier to read the "squarewave" FSK/CW style transmission. It appears to be more resilient to interference and weak signal propagation conditions.
2) Chirp. This is a temporary shift in frequency which occurs on key down, in an on/off keyed CW transmitter. That makes the end result much harder to read, less resilient to interference, and just plain ugly. See some examples at http://www.hanssummers.com/qrss/qrssqrv.html . This "chirp" can be very hard to get rid of in such simple circuits. It may only be a few Hz of frequency shift but it is very ugly and visible on QRSS transmissions. Having the transmission continuously ON and just shifting its frequency slightly, avoids the chirp problem.
3) Drift: as the oscillator components and crystal heat up, the frequency can drift a few 10's of Hz. On key down, components will be transmitting some hundred or so milliwatts of power and can slightly warm up, changing the frequency ever so slightly but visibly. Again, a problem that is not so easy to resolve in such a simple circuit. Keeping the transmission permanently ON solves that problem too as once the steady state thermal equilibrium is reached, everything stays there.
I hope this helps to explain it - you can read more about my QRSS experiments at my web page http://www.hanssummers.com and let me know if you have any more questions
Hmm. I seem to have answered the wrong question. I answered "why does it look like a square wave on a spectrum analyzer over a very long time instead of just appearing and disappearing?". Even that I did badly.
