Joe et al:
I experienced the same thing with my KNB2. A "sort of" fix, for me, was
to increase the size of both timing capacitors so as to lengthen the
blanking pulse. I increased C11 from .001 uF to .0047 uF, and C12 from
.0068 uF to 0.033 uF. In each case, this represented an approximately
5X increase. This seemed to help quite a bit, particularly on power
line buzz, but I am sure there are further improvements that could be
made. I believe that the basic design of the KNB2 needs to be changed,
but I haven't the time to play with it now.
What got me started was when I had just completed the KNB2, and I set
about to test it's operation. I found that the blanker did not respond
to my traditional "noise source" (more about that in a minute) but did
seem to work when scratching the top of Q22. So, the KNB2 works, in
it's own way, but it seems to be effective for only a limited range of
noise types. Immediately after completing the KNB2, I tried my
"traditional" tests (below) and found that the KNB2 was totally
ineffective. I thought it was broken. However, after communicating
with Gary Surrency, he put me onto the "scratch Q22" method, and of
course, it was just fine.
So, what was the difference? Why had my traditional noise blanker test
failed? Some background follows:
Several years ago (before I owned a K2) I was searching for a
controllable noise source to use for testing and aligning noise blanker
systems. I thought about what a noise pulse was, and decided that the
most important characteristics of noise were (1) very short duration,
high amplitude signals, and (2) very wide bandwidth.
After a few tries involving use of Model-T ignition coils, door buzzers,
some other ways to create "artificial noise", I hit upon the idea of
using a sweep generator. If you think about how a sweeper works, it
sends out a signal that covers a very wide band, and the dwell time on
any particular frequency is very short. If the sweeper is set so that
it's output signal covers the entire range of the receiver being tested,
then depending on the sweep rate, the time that the swept signal is
present in the receiver's IF passband is very small - it appears
basically as a noise pulse. At the same time, because some blankers
work on the idea of "out of band" noise, the swept signal also meets
that requirement.
For example, for a HF receiver, I might set the sweeper to start at
100KHz and sweep up to maybe 50 MHz. For a VHF or UHF set, I might set
the sweeper to run from 20 MHz below the receiver tuning range to 20 MHz
above. In any case, you can see that when using rapid sweep rates the
signal is on any particular frequency for only a very short time
(microseconds to milliseconds, depending on sweep rate). As a bonus,
the output level controls of the sweep generator allowed me to adjust
the "noise" pulses from very weak (less than 1 microvolt) to extremely
strong (+10 dBm) at the receiver antenna jack. I was using a old HP
8601 that I found on eBay for less than $50, but most any sweeper
covering the range will do, and home-made units are not that hard to
make, particularly if all you want is "swept noise" and precise control
over frequency and output level are not required. By this I mean you
don't need "frequency counter" type readouts, and the output level need
not be precisely calibrated.
What I was actually trying to do was simulate 160 meter LORAN pulses and
"Woodpecker" over the horizon radar signals, which a few years ago were
among the most egregious "QRN" ever heard. By adjusting the sweep
rate, I could simulate signals "ticking over" once or twice a second (an
idling engine type noise), to "power line buzz" which occurs at 120 Hz.
This worked like a charm, and was great for adjusting noise blankers,
such as the ones in my Drake "C" line. As years passed, I found the
setup worked with pretty much any rig having a noise blanker, such as
the Kenwood, ICOM and Yaesu sets, as well as most everything else. In
fact, this setup has worked on anything I have tried so far, except for
the KNB2! To really give a "real world" test, I could also mix in
signals from conventional signal generators to see how much the blankers
"hashed up" the wanted signal while doing their thing. I used a scope
to look at either / or the receiver IF signal or the audio output. It
was all very interesting and informative, and I learned quite a bit
about blankers in the process.
So, what's with the KNB2? Possibly one reason the Elecraft KNB2 fails
this test is because it was designed after the LORAN A stations and the
"Woodpecker" were history. I don't know this for a fact, it is just
supposition. However, I maintain that a good noise blanker should be
able to handle this type of interference, because who knows when some
administration or military group somewhere will decide to use the HF
bands for pulse (LORAN) or swept signal (Woodpecker) type transmissions
again. To me, a good blanker should be able to handle ignition noise,
power line leaks, electric appliances, lightning, static discharges from
raindrops or snowflakes hitting the antenna, the "Woodpecker" and more.
No blanker can completely clean up all noise, and some will distort the
intended signal to a greater or lesser degree while operating, but all
should produce a significant reduction in noise. Natural lightning is
perhaps the most difficult, because the pulse duration can be quite
long, but I believe putting up with a "hole" in the desired signal for
several milliseconds (or tens of milliseconds?) is preferable to having
to listen to those very loud crashes.
My present rig, other than the K2, is a Yaesu FT-1000 Mark 5. It has 2
blanker settings, each of which is adjustable over a wide range of
levels. It is interesting to hear even raucous power line noise or TV
set horizontal oscillator signals disappear as adjustments are made.
However, I will also say that at the more aggressive settings, the Yaesu
blanker can "hash up" a signal pretty badly. It becomes a balancing
act to use enough blanker to get rid of the noise while at the same time
maintaining enough intelligibility to copy.
Just some idle ramblings. I will go home now. I hope some of you find
this interesting.
- Jim, KL7CC
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