[Elecraft] [Elecraft_K3] curious behavior of the K3/P3, etc.

[Richard Ferch]

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 <ve3iay at storm.ca <mailto:ve3iay at storm.ca>>
>     To: "elecraft at mailman.qth.net <mailto:elecraft at mailman.qth.net>"
>     <elecraft at mailman.qth.net <mailto:elecraft at mailman.qth.net>>
>
>     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 <cq_dx_de_aa0aa at yahoo.com
>         <mailto:cq_dx_de_aa0aa at yahoo.com>> 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
> cq_dx_de_aa0aa at yahoo.com <mailto:cq_dx_de_aa0aa at yahoo.com>
>
>