morse code signals are only carrier waves, right? so a transceiver transmits a 14 MHz wave, unmodulated, or nothing at all. but i can't hear 14 mhz.

i know my radio plays a 700 Hz "side tone" when i transmit (i think) but when i listen to others, the tone can change, so i don't think the tone is being generated locally.

so what am i hearing when i am listening to morse code?


A CW receiver works by first tuning an oscillator to the dial frequency, minus the difference frequency. This is called the local oscillator or "LO". The LO is then mixed with the LO. The difference frequency is usually the same as the sidetone frequency, so the sidetone you hear when transmitting is at the same pitch as a properly tuned signal being received. It's typically between 400 and 700 Hz, and in most modern radios is configurable.

This mixing, with the appropriate filtering, has the effect of "shifting down" the RF in frequency, so for example a carrier at 14,000,000 Hz makes a tone at 700 Hz, and a carrier at 14,000,100 Hz makes a tone at 800 Hz.

This is identical in operation to USB, the only difference being CW mode usually has a narrower filter, and applies a fixed offset (in this example, 700 Hz) to the LO. This is why you can also hear CW transmissions in Morse code in USB mode, if you tune 700 Hz below them.

  • $\begingroup$ thank you, this helps a lot. you also answered my other question, which was the purpose of the "tune" dial- it has nothing to do with antenna tuners. $\endgroup$ – SRoberts Dec 5 '20 at 15:38
  • $\begingroup$ also, i assume if i don't hear the sidetone directly on transmit, the receiver must be receiving what i am transmitting (probably on-chip, as not to throw off the AGC)? $\endgroup$ – SRoberts Dec 5 '20 at 15:41
  • $\begingroup$ @SRoberts turns out i had it wrong. Sidetone is the thing you hear, so it's not at 700 MHz for sure. But it's the thing you hear on transmit. $\endgroup$ – Phil Frost - W8II Dec 6 '20 at 4:03
  • $\begingroup$ Any receiver that can receive CW as an audible beat frequency can also receive SSB -- in other words, not only your SSB radio can receive CW, but your CW radio can receive SSB if carefully tuned. $\endgroup$ – Zeiss Ikon Dec 6 '20 at 15:23
  • $\begingroup$ @ZeissIkon I'd say that's only partially true, as often the filter in a CW receiver will be too narrow for SSB. You might still get intelligible speech, but it will sound distorted. $\endgroup$ – Phil Frost - W8II Dec 7 '20 at 13:48

There are several methods to allow hearing CW as a audible sidetone on receive. Regenerative and direct conversion receivers often use the same method as they use for LSB/SSB receive and tune above the actual CW spectrum. Some superhets and SDR radios actually zero beat the CW signal with the main VFO, and then inject a separate BFO oscillator into a separate audio modulator. This allows using a low-pass CW filter at one of the IF stages instead of an offset bandpass filter.

CW-R uses the USB/SSB receive path instead of LSB. Weird, because when you tune upwards in frequency with the VFO, the CW audio sidetone goes lower in frequency.


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