[Elecraft] K2 keying bandwidth significantly reduced

[Guy Olinger, K2AV]

You mean like comparing Minnesota and Bermuda in dead of winter?  :>)

----- Original Message ----- 
From: "Rick Tavan" <tavan@tibco.com>
Cc: "Elecraft" <elecraft@mailman.qth.net>
Sent: Thursday, September 11, 2003 11:49 AM
Subject: Re: [Elecraft] K2 keying bandwidth significantly reduced


> Compare this response to that of Yaesu when hams began reporting
clicks
> from the FT-1000 a few years ago.
>
> /Rick N6XI
>
> Wayne Burdick wrote:
>
> >I was going to wait for Eric to come back from vacation to
collaborate with me
> >on this, but I had a brainstorm and decided to give it a try. Seems
to work
> >well. We'll have to get a few builders to try the modifications,
but here's a
> >brief summary for those who may be interested (results are
discussed at the end):
> >
> >Hardware changes:
> >
> >1. U10A on the control board is reconfigured as a second-order
low-pass filter
> >rather than a simple R-C shaping network. This requires one new
capacitor and
> >three component value changes, including replacing D3 with a
resistor. The
> >result is an approximately symmetrical, rounded trapezoidal
waveform with about
> >4 ms rise and fall times. The original circuit had a characteristic
R-C
> >exponential decay waveform on the falling edge, which was a
principle cause of
> >the observed bandwidth.
> >
> >2. A large capacitor is added from pin 2 of U8 on the Control board
to ground.
> >(The MAX534's output buffers are stable with any capacitive load.)
This reduces
> >the slew rate of the output buffer, further rounding the corners of
the keying
> >envelope. The final rise/fall times are about 4-5 ms.
> >
> >Firmware change:
> >
> >A change in transmit signal sequencing is required to take
advantage of the
> >slower rise/fall times. (If the modified hardware is used without
the new
> >firmware, the waveform will be distorted and there will be no
improvement in
> >keying bandwidth.)
> >
> >Results:
> >
> >Initial tests show at least a factor of two reduction in the
bandwidth of keying
> >sidebands, using a method similar to that posted recently by Earl,
K6SE. The
> >signal also sounds very clean (and looks very clean on the scope).
Waveform
> >symmetry is preserved over the full power control range. I haven't
done any
> >extended testing yet, i.e. using spectrogram.
> >
> >One other point of interest. The bandwidth-limiting technique
suggested by
> >W8JI--routing the CW signal through a narrow-band filter--would be
very
> >difficult to implement on the K2. The transmit signal path cannot
be
> >conveniently be routed through the CW filter, even with the KSB2
option
> >installed. It would require 10 or so additional parts, and probably
a couple of
> >coax jumpers, and could degrade the ultimate rejection of both the
CW and KSB2
> >filters on receive. The solution I described here (turning the
shaping network
> >in to a 2nd-order LPF) is the only simple method I have found
during two days of
> >lost sleep and head-scratching. In fact it's probably similar to
what W8JI ended
> >up doing on his other radios, although I couldn't find a
theoretical description
> >of his actual modifications, and don't have the FT1000 schematics.
> >
> >Assuming this change passes muster with our short list of
high-power contesters,
> >we'll phase it into the K2 and offer some type of mod kit. It's
likely that this
> >and a few other minor changes will be included in next K2 firmware
release.
> >
> >At present there are two many unknowns for me to give you a date on
this. Please
> >DO NOT call Elecraft about it--we'll announce it, as usual.
> >
> >73,
> >Wayne
> >N6KR
> >
> >
> >
>
>
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