[Elecraft] K3: FM Filter for AM?

[Guy Olinger K2AV]

You will have to choose which filter is used ALL THE TIME for SSB TX.

You COULD currently define the 2.1 filter as 2.7 and set an offset to
center it properly.  BUT it will not be cleaner as you suppose.  The
narrower filters have more envelope delay near the skirts, and that
creates distortion and widened bandwidth. You will have distortion
products that are outside the filter bandwidth.  You should also note,
that the choice to transmit CW through the 2.7/2.8 filter was at least
partly made with a CLEANER signal in mind by not adding distortion.

The K3 is NOT an analog radio.

The CW signal is not made by turning an oscillator on and off,
modulated by the rise time of a controlling voltage.  It is a stream
of numbers which describes a pristine CW signal.  For ALL transmitted
output, the DSP drives an analog to digital converter whose output is
the TX IF signal which is upconverted and amplified exactly the same
way for all TX output. There is no balanced modulator to create the
SSB signal.  After d/a conversion, only SSB style high linearity keeps
a signal pristine.  Envelope delay in narrower filters will BROADEN
the CW envelope, not narrow it. A CW signal generated in the K3
fashion cannot be narrowed except in the firmware. The rise time
option picks between number streams used as input to the DAC, not
between timing capacitors in an analog on/off circuit.

Transmitting a clean signal at a legal bandwidth is FAR more
complicated than where the skirts are.  It is a different issue to use
an FM roofing filter to pass AM bandwidth transmitting, but there is
still the issue of legal bandwidth for an emission and the K3 needing
to maintain type acceptance.  Strictly speaking, Elecraft may not
publish methods, assist, provide advice, parts or software which make
it possible for a type accepted rig to operate outside the type
restrictions and legal modes in the countries involved.

Users on the Elecraft repeater may bemoan not being able to do this or
that, but the hard deck is that Wayne has to follow the rules or lose
type acceptance.  10 kHz ESSB is not legal on HF, and Wayne can't
provide the means to allow a K3 to do so.  Much of his means to keep
things legal is also embedded in proprietary code.  Also, trying to
get him to lay it all out so we can feel "happy" about being
restricted to FCC regs, is asking him to lay out his "stuff" which
then can be copied by competitors.

This ain't your daddy's analog radio.  Like there is no RF gain in a
K3.  The only reason there is a knob called RF gain was the marketing
determination that it would not be accepted unless it quacked, walked
and swam like an analog radio, and had all the same knobs. I am sure
that TenTec's experience with this in the Orion series was known when
the decisions were being made.  The knob controls variable IF gain.
But the pot is not in the RF/IF string.  It's followed by an analog to
digital converter that converts the pot position to a number and sends
the number on to the MPU.  RF gain can be "chunk adjusted" by turning
preamp/ATT on and off.

So the K3 knobs and twiddles LIKE an analog radio, with an analog
face, but only a skin deep resemblance.  The key to understanding the
K3 is shedding the old analog understandings in favor of learning the
digital innards of the K3.  Learning to read the K3 schematics and
spending some time in them is useful.  I realize that's a bit worse
than RTFM, but it does get you out of the mode of thinking of the K3
as a series of individual digital emulations of individual analog tube
circuits.

Just how the firmware deals with the digital equivalent of  "AGC off"
is another story.  I think that AGC off should just be an extreme
setting of AGC on, where LIM sets the loudest audio you want to hear
in any mode any setting.

73, Guy.


On Fri, Oct 8, 2010 at 12:39 PM, Doug Faunt N6TQS +1-510-655-8604
<faunt at panix.com> wrote:
> In fact, 2.1 was one of the original possibilities for the TX filter
> and was removed.
> 73, doug
>
>
>   From: "Don Cunningham" <wb5hak at martineer.net>
>   Date: Fri, 8 Oct 2010 11:31:04 -0500
>
>   I agree, Bob, and wouldn't 2.1 kc make our transmit signal even cleaner??
>   I'm afraid I don't see the "status quo" either.
>   73,
>   Don, WB5HAK
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