The following was written by Al, KN1O in response to a question regarding the difference between CW-N & CW-R settings.
The main reasons to use CW-N vs. CW-R are:
1) In some situations, switching helps deal with QRM. For example, if you set the received tone to 700 Hz and there’s another station that gives a 1200 Hz tone, switching to the other setting will put the interfering station at 200 Hz. Depending on your ears and the filter, dealing with one situation may be easier than the other.
2) Some folks prefer to have the rig set up so that, as they turn the main tuning knob higher in frequency, the tone of a received CW signal goes higher. Others prefer the opposite.
And I confirmed by tuning to 10.000.00 for WWV and beat note does not change when switching between CW and CW-R.
If I am in CW and “zero beat” WWV the readout is showing 9.999.40 and then if I switch to CW-R I hear what I believe is a beat note of about 1.2 kHz. So what is going on?
Thinking about how a modern transceiver probably works; an audio side tone is generated (typically 600 Hz that you hear when transmitting. The frequency indicated on the radio’s readout is the transmit carrier frequency. In receive I suspect that the same side tone is modulated onto a carrier and then the carrier is suppressed (probably using the same circuitry that is used to generate SSB) thus creating an RF carrier that is 600 Hz either higher or lower in frequency than the transmit frequency. Assuming that you want your transmit frequency to be the same as the station you wish to communicate with then you want to tune your radio accordingly. But how do you do that?
As mentioned, the received signal is being fed into a product detector along with the carrier mentioned above that is 600Hz away from the frequency the radio is tuned to. When the receiver is set to the other station’s transmit frequency you will thus hear a 600Hz note.
There are several ways to properly tune to the desired station. One is to match the pitch of the transmit side tone to the received signal which is fine if one is musical and has a good sense of pitch. That will get you pretty close. Another approach is to use an audio spectrum analyzer app on a smart phone. Another method is to use the narrow CW filter and tune to get a peak reading (the filter peaks at around 500 Hz so again you will be very close). But there is another method and that is to use the CW and CW-R feature on many radios.
On Icom radios, they call the Upper CW mode “CW-Reverse,” or CW-R. Here’s the explanation of how it works from the IC-756PROIII manual:
CW reverse mode
CW-R (CW Reverse) mode receives CW signals with a reverse side CW carrier point like that of LSB and USB modes. Use when interfering signals are near a desired signal and you want to change the interference tone.
If you have properly zero beat the signal of a station with which you are in contact, then the other station’s signal will sound the same in either mode. (if it doesn’t sound the same then you are not tuned to that station’s frequency!) An interfering signal will, however, sound much differently, and, as shown, may be out of the passband altogether.
If you don’t zero beat properly, though, being in different modes will affect the way that the other station hears you. Say, for example, that both you and the other station are in normal CW mode (LCW on the TenTec). The other station’s transmit frequency is set to 7030 kHz. Your sidetone frequency is set to 500 Hz (meaning that the BFO frequency is 500 Hz above the receive signal), but you set your transceiver to hear a 600 Hz tone. To hear that 600 Hz tone, you will have to set your transmit frequency to 7030.1 kHz.
If you’re in UCW mode, or CW-R mode as Icom calls it, then the BFO frequency will be 500 Hz below the received signal, and to hear a 600 Hz tone, you’ll have to set your transmit frequency to 7029.9 kHz. In the first case, the other station will hear a 500 Hz tone when receiving your signal. In the second case, he’ll hear a 700 Hz tone.
To minimize the frequency difference between the two stations, it’s a good idea to check the setting of your sidetone frequency and zero beat as closely as possible with the other station. I would also avoid shifting between CW modes while in contact with another station, or if you do change modes, don’t change your transmit frequency while in contact. Use the RIT control to change the frequency of the tone you hear.
And since I have mentioned the product detector here is some information that may help explain the differences in how a modern transceiver detects CW (or SSB) vs. a receiver that has a simple diode detector. From Wikipedia:
The simplest form of product detector mixes (or heterodynes) the RF or IF signal with a locally derived carrier (the Beat Frequency Oscillator, or BFO) to produce an audio frequency copy of the original audio signal and a mixer product at twice the original RF or IF frequency. This high-frequency component can then be filtered out, leaving the original audio frequency signal. The product demodulator has some advantages over an envelope detector for AM signal reception.
- The product demodulator can decode overmodulatedAM and AM with suppressed carrier.
- A signal demodulated with a product detector will have a higher signal to noise ratiothan the same signal demodulated with an envelope detector.
On the other hand, the envelope detector is a simple and relatively inexpensive circuit, and it can provide higher fidelity, since there is no possibility of mistuning the local oscillator.
“A product detector (or equivalent) is needed to demodulate SSB signals.” This last statement is not completely true as envelope detector (diode detector) receivers with BFOs were used for years to detect SSB. However one had to “ride” the RF gain control to reasonable balance the level of the RF signal with the BFO signal. If the BFO signal was to strong the resulting demodulated audio was very weak and if it was too weak the SSB signal sounded very mushy. Also, it was a challenge to properly tune an SSB signal because the receiver might not be properly tuned to the SSB signal but you could still make the audio sound correct by adjusting the BFO. The audio might sound OK but the received signal was outside the peak of the IF passband. It isn’t as hard as it might sound to tune an SSB signal with an envelope (diode) detector but it did require a bit of finesse on the operator’s part.