Posted by
Geoffrey Mackenzie-Kennedy on
Feb 07, 2010; 3:09pm
URL: http://elecraft.85.s1.nabble.com/K3-in-a-cw-pileup-needs-work-tp4523884p4529478.html
Hello Juan,
Thanks for your e-mail. I will reply in full after I return home this
evening, off-list if my answer is long.
The symptoms, and the cures suggested, strongly suggest that it is a well
known type of IMD problem caused by the IF hardware - which includes the
roofing filter.
I understand the AGC recovery problem that you mention, but do you know if
the in-passband IMD products of the FT1000MP were actually measured? To
measure IMD products with both test signals inside a CW bandwidth passband
usually requires crystal controlled generators for reasons of low phase
noise, especially if the receiver's in-passband odd-order dynamic range is
large. Increasing bandwidth to allow much wider spacings of the test signals
usually paints a false picture, because the signal handling capability of
narrow bandwidth crystal filters is usually worse than that of the wider
filters.
73,
Geoff
GM4ESD
On Sunday, February 07, 2010 at 12:57 PM, Juan EA5RS <
[hidden email]> wrote:
Are you guys sure this is a frequency domain problem?
Has anyone measured those IMD products under the mentioned conditions and
concluded from the results that they are the culprit for the mushy (somewhat
vague description) audio that makes impossible to copy the different
callsigns?
Some W6s measured in a lab test setup a similar effect on the FT1000MP years
ago and found that the problem was the AGC recovering too fast (in between
symbols) making all signals sound as loud as others. The solution was to
increase the time constant. The loudest signal would then set the radio gain
level. Then characters in-between from weaker signals sounded weaker making
them distinct (or maybe buried).
I cant think why you would ever need a recovery time constant any shorter
than say 150 ms, while shorter time constants are normally used -reading
Clifton I think the FAST setting in the K3 is 73 ms-. At 60 WPM, a dit is
20ms long if I am not mistaken, and an intersymbols spacing (7 time units)
to full gain recovery seems the shortest time that would be necessary for
the receiver to recover full gain.
73 de Juan EA5RS
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