Elecraft's auto-spot and CWT features -- available on the K3/K3S/KX2/KX3 -- are very useful tools for CW operators, especially those not experienced in pitch-matching. Here's a bit of history on where these features came from and how they work.
CW Spotting History
When a station finishes a CQ in CW mode, the operator faces the challenge of copying someone who's calling back. Callers may be weak or obscured by QRM; the op can usually deal with both problems by narrowing the filter passband. However, callers may also be off frequency. A calling station may be using a wide filter passband themselves, not attempting to carefully match their VFO frequency to that of the CQing station. The result may be no QSO, even when propagation is excellent.
In the Days of Yore, a frequency offset between stations didn't always matter. Sometimes both stations used crystal-controlled transmitters, so operators had to patient tune around after calling CQ.
As a 14-year-old novice I embraced this operating style for a year or so, armed with a dozen or so crusty FT-243 crystals for my Heath HW-16. I nearly wore out the socket swapping them in and out. After calling CQ, it was not unusual to find a caller 30 or more kHz away! (Away from "where" was a poorly answered question, as my Hallicrafters receiver dial wasn't exactly digital.)
Fortunately I soon acquired an outboard VFO, a life-changing addition to my station. Jealous friends doubled up on their paper routes to pay for their own. Girls suddenly paid more attention to me.
These days virtually everyone has a VFO, along with the expectation that they won't have to tune theirs very far, if at all, to tune you in. Not only that, they're stable and well calibrated, not like the beasts we had to skillfully tame. Progress!
Manual Spotting (SPOT switch)
Once I had a VFO I quickly learned to do *manual* pitch matching. Older rigs did't provide a way to do that explicitly, so you'd improvise. Basically, you had to coerce a very weak signal out of your own transmitter, say by turning on only the driver, then tune the transmit VFO until you could hear your signal on your own receiver -- superimposed on the calling station, at the same pitch. This is what we call spotting.
Of course spotting is a lot more convenient these days, as many rigs include a SPOT switch. This function is easy for a modern transceiver designer to add, because the radio's firmware is quite capable of turning on only the CW sidetone without transmitting.
That is the purpose of the SPOT switch on all Elecraft transceivers. Tap SPOT, and you'll hear your sidetone pitch. Most people can do a good job of adjusting the VFO such that the CQing station's pitch matches that of the SPOT tone. This ensures that when you call them, you'll be close to their own frequency.
Tuning Aids: Filtering (APF), PLL (NE567), and Spectral (CWT)
Since not everyone has an inherent musical ear, various hardware-enhanced means of tuning in CW signals have been developed.
The simplest method is to just narrow your receiver passband so much that, if you can hear a station calling CQ at all, you're guaranteed to be "right on top of him." This assumes that your transceiver enforces alignment between its transmit and receive pitch...true of all Elecraft gear.
Narrow filtering has gone through decades of evolution. Some filters were based on op-amps (active filters), while others were based on LC filtering, conscripting humongous toroidal cores scavenged from telco equipment. I acquired my stash of these from a haphazard mound of old switching racks, decaying in an abandoned aircraft hanger on the Bermuda U.S. Navy base. (That irresistible junk pile was also a mother load of TO5 transistors, multi-pound electrolytic capacitors, and tetanus, but that's another story.) Typically the toroids were 88 millihenries -- a huge value for a high-Q inductor, permitting resonance in the low audio range.
Later, such filters migrated to digital signal processing, in the form of switched-capacitor ICs or DSPs. You can still buy these switched-capacitor chips, like the MF10, from various sources. It's instructive to roll your own tunable filter, just for fun.
Whether passive or active, the goal of filtering is typically to achieve a narrow passband, say 250 Hz or less. With DSP, nearly perfect filters with "brick wall" passbands can be created. But these have the disadvantage of ringing like a bell when pinged by a CW signal or noise, making copy difficult.
One solution incorporated into the K-line and KX-line is the Audio Peaking Filter (APF), which provides a 30-Hz bandwidth at -3 dB, but broad skirts, preventing ringing from occurring. As our customers will attest, APF works like magic on weak signals obscured by noise.
Another forerunner to DSP techniques was the audio phase-locked-loop, using inexpensive ICs like the legendary LM567. When locked on a signal that matched its center frequency, the circuit would turn on an LED, alerting the operator that the VFO was now properly tuned.
With the DSPs in our K-line and KX-line radios, we can provide a much more powerful tool: CWT, or "CW Tuning Aid." When enabled, CWT turns the upper portion of the rig's S-meter into something of a mini spectrum analyzer. The pitch of the strongest signal in the passband is analyzed by the DSP, then represented as a single segment of the bar graph. For CWT-enhanced manual spotting, the operator simply tunes the VFO slowly until the center CWT segment is flashing along with the keyed signal.
Manual tuning with CWT can also be used in FSK-D and PSK-D modes as described in the owner's manual.
Closing the Loop: Auto-Spotting (SPOT + CWT)
The Elecraft K3/K3S/KX2/KX3 take CW tuning another step forward by providing a way to *automatically* retune the VFO frequency to match that of a received signal. How does this work?
When CWT is turned on, firmware treats the SPOT switch as AUTO-SPOT. The DSP analyzes the incoming signal, and with a bit of processing, determines its exact audio pitch. From there all that's needed is a bit of math to offset the VFO to match this pitch to the CW sidetone.
There's another subtlety, though. Since a CW signal is generally being keyed on and off, the CWT algorithm has to ensure that it doesn't "take off," chasing a signal that's not there. To avoid this, we keep track of the energy in the passband, and slew the VFO incrementally over an average of about 0.5 second, moving only when the target signal is present.
How to Use Auto-Spot
I encourage you to give the auto-spot feature a try. It's best to start with a fairly narrow passband, say 400-600 Hz; narrower if there's a lot of QRM. Find a signal, turn on CWT, then tap SPOT to tune it in. A second tap of SPOT may get even closer, especially if there's a lot of band noise.
Auto-spot can also be used in Elecraft's PSK-D mode, i.e. for PSK31/PSK63. As with CW mode, just turn on CWT, tune in a prospective signal, and tap SPOT. Since PSK auto-decoding requires very accurate tuning, it's best to set the filter bandwidth to 50 Hz, then let auto-spot dial things in down to the last 2 or 3 Hz. If you have text decode turned on, you should start seeing text characters scroll by after auto-tuning has completed. Tapping a second time or fine-tuning the VFO a bit in 1 Hz steps may improve copy.