[Elecraft] KNB2 Noise Blanker comments from N6KR

[wayne burdick]

And now a word from the designer ;)

The KNB2 was designed largely around noise found at my QTH in Belmont. 
I have pretty horrendous line noise at some times of the year, as well 
as the usual collection of noisy light dimmers, etc. (My family was 
amused by my running around, turning lamps up and down during the 
process.) I also put the rig into various vehicles, including a very 
noisy 1983 VW Vanagon. The blanker proved effective on all these 
sources.

The KNB2 does have limitations. It sits at a wideband part of the RX 
path, so in some cases it will be compromised by very strong in-band 
signals. Fortunately this isn't a problem for most operators. It also 
had to meet certain size and cost goals, limiting complexity; there 
wasn't room for additional delay filtering, amplification, or more 
serious gating hardware.

Beyond achieving an acceptable level of noise rejection, the KNB2 had 
to satisfy one other criteria: to not degrade the dynamic range of the 
receiver when it is turned on. It sits right at the post-amp output, 
where a change in impedance could be problematic. So, one of the final 
tasks was to come up with an RF band-pass filter for the blanker that 
would present a decent SWR to the post-amp. The resulting filter has a 
resistive divider rather than a capacitive divider (see the KNB2 
schematic). This filter topology didn't have quite as long a delay as a 
filter with a capacitive divider, but it made the post-amp happy and 
still worked on all the noise sources I had available.

There are several approaches we could take to improve the KNB2's 
performance. The one thing they have in common is adding a lot more 
parts -- to the point that the entire module would have to be 
surface-mount. For example, we could insert a narrowband diplexer at 
its input. This would terminate the image at the post-amp, slightly 
improve headroom of the RX strip, and reduce the swath of spectrum seen 
by the blanker to about 50 kHz rather than the full width of the RF 
bandpass filter.

I'm not sure when we'll be able to update the KNB2 design, but in the 
meantime, I'll be happy to answer questions for anyone who wants to try 
their hand at it. As must be obvious by now, we've worked hard to 
integrate promising modifications in a timely fashion.

73,
Wayne
N6KR

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