[Elecraft] zero beat detector with KX1?

[Leigh L Klotz, Jr.]

On Wed, 1 Dec 2004 9:11 am, Eric Ward wrote:
> I was interested in WA5ZNU's recent post about tri-state and side tone 
> levels on the KX1.

Eric,

My proposal requires hardware and firmware changes to the KX1 and was 
somewhat telegraphic as it was aimed at Elecraft.

N6KR has I am sure already figured out a better way to accomplish my 
ask...I think all of the interest in CW zero beat on the list may help 
raise the priority, though.  (One problem is that my proposal has some 
current draw as it requires biasing...maybe there is a way around it, or 
maybe there is another way to achieve the desired result of easier CW 
tuning on the KX1.)

Here is my long description of the issue, with background in response to 
your question.

When you have a QSO, you generally you want to be on the same frequency 
as the OM.  (Ok, ignore split and repeaters.)  With SSB you tune about 
until the OM or YL sounds good.  Our brains do this fairly easily.  With 
CW though, you need to do a little work.

With separate transmitters and receivers, hams used to "spot" the signal 
with a low-power TX and listen at the same time, and match the pitch.
Even so, after calling CQ you often had to tune up and down a few KC to 
find an answer.  A transceiver, though, shares enough parts internally 
that there are shortcuts. Usually transceivers use the same pitch for 
the CW sidetone as the transmit offset, mimicing the sound that the OM 
will hear if both of you are both transmitting on the eame frequency.  
(You effecively listen at an offset, and this offset is the CW tone you 
hear, plus or minus the carrier frequency.  That is what CW and CWR are 
about.  If you were receiving and transmitting on the exact same 
frequency then you would hear a 0Hz tone, i.e. nothing, so the BFO 
offset takes care of that.)  Since the RX and TX are coupled by a fixed 
offset in a transceiver, the question devolves to how to set the RX 
frequency quickly and accurately.

Assuming a fixed offset (say, 600Hz default on the Elecraft KX1 and K2 
rigs) then your task when tuning is to set your RX frequency such that 
the OM's CW tone sounds like 600Hz.  Then you will be assured of 
transmitting on the same frequency as the OM.

The trick is for you how to do this.
Here are three ways:

1. Tune for loudest signal
This, coupled with a bit of pitch recognition, is what most hams do.  It 
is pretty easy and works unless your KX1 variable crystal filter is set 
too tightly, say below 500 Hz, in which case the maximum volume point 
shifts a bit and you need to use RIT to get it back to the filter 
center.  So it is slightly less convenient to do this on the KX1 than on 
the K2, which lets you preset the BFO setting for each filter width to 
accomodate for this shift.

2 Tune until you hear 600Hz audio.
2a This is easy if you have perfect pitch.
2b If not, you can use a music tuner (see my previous message).
2c Use one of the PLL-based devices that N0SS has been providing and 
various folks have designed, to light up an LED.

3. Turn on the sidetone and tune until they are both the same pitch.  
This works FB on the K2 and many other rigs, and you just tune until you 
hear the OM "disappear" behind the sidetone or until you hear the 
minimum wah-wah-wah beating sound.

Most folks use #1 or #3, but you can't use #3 on the KX1 because the KX1 
mutes the sidetone during operation, and I believe it is because there 
weren't enough volume levels available on the KX1 sidetone to allow it 
to be adjusted to a comfortable level for TX sidetone and a louder level 
for matching that 599+ OM.  So, you cannot press the sidetone test menu 
and tune the RX and listen for the two tones (sidetone and the OM you 
are listening to) to match.  This is the issue that my previous message 
about the PIC chip was designed to address.

Also, the sidetone is lightly-filtered square wave, thus having strong 
3rd and 5th harmonics.  Some hams are tone-deaf enough to try to beat 
the OM's signal at 6*3=1800Hz, or to accidently beat QRM nearby against 
it.  The extra levels could be used to generate more of a sine wave, by 
stepping the output voltage during the sidetone generation, but the 
amount of code space and effort this would take is probably not worth 
it, and I suspect analog filtering with an additional capacitor would do 
the same job more cheaply.

If you got this far, you have a lot of patience!

73,
Leigh WA5ZNU