[Elecraft] K2 keying bandwidth significantly reduced

[Rick Tavan]

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
>
>  
>