I'm throwing together bits-and-pieces towards my rig on a less than shoestring budget. It is possible to apply a take-off from the key and use it to drive a piezo-buzzer. Of course, there won't be any tone control locally. But I can live with that, or use a divider from ground to provide minimal tone control.

Does the side-tone oscillator serve any purpose other than allowing the operator to listen to the code being transmitted?

  • $\begingroup$ I'd suggest removing the first bullet ("Do I really need…") because it's basically subjective, and once you get comprehensive answers for the second you have the information to answer the first for yourself. $\endgroup$ – Kevin Reid AG6YO Nov 17 '14 at 5:40
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    $\begingroup$ How small is your shoestring budget? A 555 Timer IC can be your side tone oscillator with a few other components (couple of resistors, capacitors) and batteries and I assume that you will have headphones or a speaker already. Buy a dozen 555 Timer ICs for less than US$5. $\endgroup$ – K7PEH Nov 17 '14 at 19:40

I am assuming two things in my answer and both may be wrong assumptions:

  1. Your shoe-string budget rig is a transceiver, probably QRP, maybe even single band.
  2. Rig is CW only (typical of small shoe-string budget rigs).

Thus, do you really need a side-tone oscillator so that you can listen to your own sending (assuming CW).

In my opinion, the answer is yes. I learned CW when I was 9 years old (or, 58 years ago) and today, my main operating mode that I use every day, is CW. My copy speed is a comfortable 20 to 24 wpm error free but I can do contests up to 35 wpm and copy call signs (usually) at 38 to 40 wpm. But, one thing I can't do is send without using a side tone so that I can hear what I send. I assume you are just beginning at CW so therefore, I am pretty sure you can't effectively send without hearing what you send.

You ask if you can use a buzzer or some external oscillator. Answer is yes you can. Now, whether there would be interference in driving two separate circuits from a single switch (that is, using your single telegraph key with the transmission as well as separate side tone oscillator depends on the circuits themselves. A high-impedance interface to the side-tone oscillator should be safe (~10 times higher than the impedance of the keying circuit of the transmitter unless that impedance is also high).

You will be missing a nice feature of the built-in side tone oscillator and that is the ability to zero-beat a signal. Actually, there are people who can do this accurately without a side-tone generator but I can't (though, haven't tried in years). But, if you are a beginner at CW, then maybe zero beating a signal is not the most important feature. If you were to participate in CW traffic nets (as I do) then the net control would often require zero beat (or, ask for it). They will send out QNZ which means "Zero Beat your frequency with mine".

If you fail to zero beat a signal it is possible that you may not be heard if you send to him (or, her). The reason is that he may have squeezed down his filters to a point that you have to be very close to his frequency in order to be heard. For example, when I am on the hunt for CW stations, I keep my filter fairly wide at 1 KHz. But, once the QSO starts I narrow down my filter to anywhere between 200 and 400 Hz. If you are more than 400 Hz away in that situation then I probably would not hear you. Now, I use a K3 that has nice sharp roofing filters: 200 Hz, 400 Hz, 1000 Hz, 1800 Hz, and 2700 Hz. Note that my rig does not do AM since I took out my 6000 Hz AM filter.

As I said, my rig is an Elecraft K3 and it has a nifty feature. It can do a zero beat on the incoming signal by the mere push of a button (called the Spot button). I also have an LED display that shows how far off (low or high side) from center frequency that the signal is which is kind of nice. And, I have the P3 pan adapter that allows me to sort of zero beat visually on the screen. Nice rig but often never a first rig.

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    $\begingroup$ Why do you need to be able to receive what you just transmit? Can't you put an LED and a buzzer in the tx circuit so you can tell what you're transmitting? (I might not be understanding the problem, but I'd appreciate if you'd clarify). $\endgroup$ – Aaron Esau Sep 12 '18 at 23:13
  • $\begingroup$ @Arin It can be useful to receive your transmitted signal - or, more properly, a much-attenuated version - to ensure signal quality. So many problems can occur, especially with a home-made transmitter, that monitoring your transmissions is a good idea. But, as W8II points out in another answer to this question, this is different from a sidetone, which monitors a copy of the input, not a copy of the output. $\endgroup$ – Brian K1LI Mar 24 '19 at 13:15

side tone
1. (Telecommunications) sound diverted from a telephone microphone to the earpiece so that a speaker hears his own voice at the same level and position as that of the respondent

Collins English Dictionary – Complete and Unabridged © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003

This is of course referring to the more familiar sidetone in a telephone, but the sidetone in a CW transmitter is defined analogously. So by definition, the sidetone oscillator has no purpose other than to make audible what is being transmitted. I suppose some radio designs may have some other use for an audio frequency oscillator for which the sidetone oscillator can be dual-purposed, but there is no general reason this must be so.

I would also note that if you have a piezo buzzer that produces a tone from DC, then it contains an oscillator. Thus, you haven't really eliminated the sidetone oscillator, you've just used the one in the buzzer. If it's cheap and otherwise meets your requirements, then I can think of no reason not to use this solution.

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    $\begingroup$ Your statement: "So by definition, the sidetone oscillator has no purpose other than to make audible what is being transmitted.". Not true, a CW op will often use sidetone to zero beat the RX CW station of interest. This is done by matching incoming tone with sidetone. Of course, this assumes that the sidetone is generated by the rig and not some external buzzer or oscillator. I am pretty good at doing this but probably slipping out of practice with my Elecraft K3 & KX3 where zero beat is a button push. I am a 90 percent and more CW operator -- every day. $\endgroup$ – K7PEH Nov 17 '14 at 22:07
  • $\begingroup$ @K7PEH Are you sure that's sidetone? $\endgroup$ – Phil Frost - W8II Nov 17 '14 at 22:11
  • $\begingroup$ In the earlier days before transceivers were common, a receiver BFO was used but this could not as easily be used to zero beat. With a transceiver it is easier. And, today, it is called Sidetone generator and has been called that in the modern rigs I have owned. $\endgroup$ – K7PEH Nov 17 '14 at 22:19
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    $\begingroup$ Well, maybe we or I am confused as to your terminology. When I say side tone generator, I mean the circuit in a Transceiver that generates an audible tone that you hear when you close the key (dots and dashes). For example, say this tone is 600 Hz then, in my K3 for example, when I adjust the side tone to 600 Hz the center frequency if my TX signal is 600 Hz away. Say TX is 14050 KHz as shown on VFO then my sidetone may be 600 Hz away (down or up depending on CW setting on rig). If I hear the same tone, or close to it on the other guys CW signal then I know I am close to zero beat. $\endgroup$ – K7PEH Nov 17 '14 at 22:40
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    $\begingroup$ I think this answer is focusing on the circuits (the presence of an oscillator) whereas the question is more about the black-box / user-interface perspective — "what uses does this feature of the rig serve?" $\endgroup$ – Kevin Reid AG6YO Nov 17 '14 at 23:12

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