[Elecraft] K3 Noise Blanker problem?

[Guy Olinger K2AV]

Hi Larry,

[This response is posted only to Elecraft Reflector. Posting to more than
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To Larry and all other fellow noise sufferers equipped with K3/K3S
transceivers...

Short Version:

See if the AM BC crud goes away when the NB LEVEL has IF set to OFF. The
crud is a necessary possible artifact of IF blanking in the K3/K3S. This
"IF" process in the K3/K3S is at root the same as 1950's noise blanking in
tube gear, but with modern improvements.

Long Version:

The K3 or K3S (referred to as K3 in the rest of the post) has two separate
noise blanker processes: IF and DSP. Depending on settings with a hold on
the single NB button [LEVEL] you can have none, just IF, just DSP, or both
processes running. If they are both on, they work in series. The IF NB
works first, in broadband circuits before the roofing filters. The DSP NB
works second, after the RX low IF analog to digital conversion. The DSP NB
has algorithms doing its magic, and is buried in the number soup that is
SDR.

The IF NB is basically the same NB in the tube ham receivers I used as a
teenager. A strong pulse is detected early in the RX string, and is used to
cut off the incoming broadband signal for the duration of the pulse. There
are refinements to that process, but at root blanking cuts off some part of
the RF/IF string for the duration of the noise pulse. It is possible for
the RF peaks of non-noise signals to trip the blanking threshold.

The modern, mostly digital K3 still uses the ancient IF blanking because it
still works for certain kinds of noise. Usefully, by messing with
preamp/attenuator settings and the width and delay of the blank, the oldie
but goodie blanking can be "tuned" to better blank certain kinds of noise.
We get to "tinker" with the NB and discover the best setting for a certain
kind of noise.

But the IF NB still has inescapable aspects of blanking, drawbacks when I
was a teen and the same now: modulating the RF or pre-filter IF with the
blanking pulse, and MUST have sidebands and intermodulation from the
process. Some of those artifacts of blanking will sound like chewed up
signals that are NOT in the passband. This is because the blanking pulse
has to be applied in a broadband place where the noise pulses are still
extremely sharp and narrow.

Anything loud making it through the front end bandpass filters can be in
the NB IF process blanking artifacts. On 160 meters that easily involves
strong local AM broadcast. In CW contests with many strong CW signals
around the band, you can hear beeple bopple from intermodulating CW signals
or crud from "spread out" QRN. The thinking? The crud, while possibly
irritating, is far less of a problem than the line or spark noise that is
being blocked. The K3 IF blocking does have sophisticated amenities added
to the RF/IF blanking which improve on the old tube type versions which
generally were either just on or off with no adjustments.

I have found that a pernicious recurring line noise on 160 around my QTH is
well dealt with using NB, IF set to narrow-3 and DSP to 1-7 or 2-7. This
will produce a variation in effectiveness with frequency, usually with a
noise null or two somewhere in the CW frequencies 5-10 kHz wide. The
overall noise drop in these nulls is 5, 6, 7 S units, dropping down to the
level of regular band noise without the buzz. There will be some blanker
trash that comes and goes, but I am rescued from giving up on copying
anything not S9 or louder and am able to operate the contest at 93% or so,
placing my run frequencies in the K3 NB noise null.

To those trying to get rid of a certain noise, remember that there are
21+21+(21x21) combinations of IF and DSP blanking. That's 483 combinations,
not counting variations with preamp/normal/atten settings.

Hope this has been helpful.  73,

Guy K2AV





On Tue, Dec 8, 2015 at 10:56 AM, Larry - K1UO <k1uo at tds.net> wrote:

> Hello all..    This AM on 160 meters there was a station CQing on 1.820.6
> with a fundamental freq signal strength of S9 on my K3.  I also found (with
> the NB on) that I could hear him Q5 but very weak on 1.810.6 and the same S
> 0 reading but perfectly Q5  on 1.830.6.  I turned the noise blanker off and
> of course the images disappeared.  Is this a know problem when using the NB
> or is this just with my radio after a recent factory alignment?   I have
> had a K3 for years and never noticed anything like that before but then
> again this AM was very quiet on 160M and I had the NB on because I could
> hear a distant tick tick of an electric fence.
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