Let's see if I can make this clearer.
Let's suppose you are listening to a CW signal that is on exactly
14020.00 kHz. Let's also suppose that the sidetone pitch in your K3 is
set to 500 Hz. Leaving aside all the frequency translation stuff that
goes on inside a superhet and just talking about the frequencies you
observe from outside the radio, what actually happens in the receiver is
that its effective BFO frequency (which digital-mode amateurs today
often refer to as the "suppressed carrier frequency") is tuned to
14020.50 kHz. What you hear in your speaker or headphones is the 500 Hz
beat between the BFO and the actual transmitted signal. Note that in CW,
the K3's dial displays the actual CW frequency, not the suppressed
carrier frequency as it does in SSB.
You can verify this in practice by tuning in a CW signal until its pitch
exactly matches your sidetone pitch, then changing modes to LSB. In
order to hear the same thing you hear in CW, you will find that you have
to tune the dial frequency higher by an amount equal to your sidetone
pitch, at which point what you hear in LSB will be the same as what you
heard in CW. Actually, there is a VFO OFS setting in the K3 (under
CONFIG:CW WGHT, 5 button) that will do this retune automatically for you
when you switch modes, if that is how you want the rig to behave.
OK, so with the suppressed carrier tuned to 14020.50 kHz, the centre of
your filter bandpass is on 14020.00 (assuming you have left FC equal to
the sidetone pitch). Let's suppose your filter is set to 400 Hz
bandwidth, so that it passes audio frequencies between 300 Hz and 700
Hz. The frequency at the bottom of the audio range (the "lo-cut"
frequency) is 300 Hz below the suppressed carrier, i.e. 14020.20 kHz.
The frequency at the top of the audio range (the "hi-cut" frequency) is
700 Hz below the suppressed carrier, i.e. 14019.80 kHz. Note that the
lo-cut audio frequency is the higher frequency at RF, and the hi-cut
audio frequency is the lower frequency at RF.
The reason the AF and RF bandpasses are inverted with respect to one
another is that normal CW in the K3 is on the lower sideband, i.e. the
suppressed carrier or BFO is above the frequency you are listening to.
In CW-REV, the suppressed carrier is below the frequency you are
listening to (i.e. the receiver is in USB), and in that case the AF and
RF bandpasses are no longer inverted with respect to one another.
Hope this helps.
73,
Rich VE3KI
On 2014-02-12 2:44 PM, Scott wrote:
Thanks Rich, that is a very clear and thorough explanation! I cannot
relate all to my level of understanding, but your explanation is
complete so I can study it. In really basic language, what I understand
is that adjusting the width will only affect the upper limit until I get
it narrow enough so that both are in an acceptable range.
However, I am still left with some doubts: particularly, why does the
upper limit change when I adjust the lo-cut, and the lower limit change
when I adjust the hi-cut? Thus, if I want to narrow my bandwidth to
avoid a station above the desired one, I must adjust the lo-cut (I can
see this happen on the P3). This occurs on all bands on CW.
You are good at giving explanations—I hope I am not trying your patience!
Thanks,
Scott AA0AA
Date: Tue, 11 Feb 2014 14:06:56 -0500
From: Richard Ferch <[email protected] <mailto:[email protected]>>
To: "[email protected] <mailto:[email protected]>"
<[email protected] <mailto:[email protected]>>
OK, if you're talking about CW, the K3's normal CW is on lower sideband.
In CW the filter behaviour gets tangled up with the sidetone pitch
setting, which is also the default FC (centre frequency) for the filters
in CW mode. Suppose the sidetone pitch/FC is 500 Hz and the BW
(bandwidth) is 2000 Hz. Given those numbers, the calculated hi-cut and
lo-cut frequencies would be 1500 Hz (FC + BW/2) and -500 Hz (FC - BW/2).
That's a minus sign on the lo-cut frequency, meaning it's on the other
sideband. The firmware should not allow that; it needs to place the
lo-cut frequency on the proper sideband far enough above zero to give
adequate carrier and opposite sideband suppression. IIRC, the low-pass
cutoff is adjustable in the latest firmware - I believe it can be set to
100 Hz, 200 Hz or 300 Hz.
So, with the low-pass cutoff set at 200 Hz, FC at 500 Hz and BW at 2000
Hz, the actual filter bandpass will be from 200 Hz to 1500 Hz. Adjusting
the Width control would only affect the upper limit, as the lower limit
is fixed by the low-pass cutoff. If the Width setting is sufficiently
narrow for the calculated lo-cut frequency to be at or above the
low-pass cutoff (100, 200 or 300 Hz), the effect of the Width control
becomes symmetric about the centre frequency, as you might have
originally expected.
Because CW is on the lower sideband, adjusting the Width control only
affects the lower RF (=higher audio) edge of the bandpass - the upper
edge is fixed by the low-pass cutoff.
Does this help make sense of your observations?
73,
Rich VE3KI
>Scott Monks <[email protected]
<mailto:[email protected]>> wrote:
>
>Now before you guys think I too am complaining--no, I just
have a curious mind!
>
>Two questions that I am sure you more experienced can help me
with:
>
>1. When I begin narrowing the DSP down, on filter 1 (2.7, std,
offset -0.84) only the lower freq side moves.? The DSP switches
to the 2.1 filter, reacting the same, but when it narrows to
about 1.10 (within filter 2; 2.1, offset 0.0) it changes so that
both low and high move equally.? Even narrowing continues
through the 700 and 400 Hz filters.? The reverse occurs in the
same way.?? I have checked the offsets with the K3 program, and
they are set as required.? On the K-3 display the "bar-thing"
always seems to move equally on both sides.? Is this normal?
>
>2. When seen on the P-3, on hi-cut-lo, if I move the lo side,
the higher freq side moves, and if I move the hi side, the lower
freq side moves.? On the K-3 display the expected side moves.?
Again, is this normal?
>
>I have the last production software installed.? This was first
noticed when I got the P3 and a "big screen", when in a contest
I wanted to shift the receiver up to avoid a disturbing signal
and found I had to move the hi side to move the lo side!? Now I
am accustomed and just watch my screen to make sure I am moving
outside of the interfering peak.
>
>These only "bother" me because I know there must be a reason
but I don't why it happens!? If curiosity is going to kill the
"cat", don't tell me and I will just continue to be happy with
my partial K-line!
>
>Thanks for your patience,
>Scott? AA0AA
Scott AA0AA; XE1/AA0AA
[email protected] <mailto:[email protected]>
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