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.
73,
Wayne
N6KR
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