[Elecraft] [K2] NB2 heard a "spark gap" like signal 8k lower than the RX frequency

[Wayne Conrad]

Don,

The preamp was off, and the KNB2 was using the high thresshold.  Knowing 
that, are the symptoms still consistent with IMD?

73, Wayne Conrad KF7QGA

On 08/09/11 08:13, Don Wilhelm wrote:
> Wayne,
>
> The KNB2 can introduce IMD in the receive path, especially when using
> the low threshold and the wider blanking pulse (NB2). I believe that is
> what you were hearing.
> If you had the preamp on, the effect would be increased. Normally there
> is no need to turn the preamp on for 40 meters unless you are using a
> low efficiency antenna.
>
> 73,
> Don W3FPR
>
> On 8/9/2011 11:01 AM, Wayne Conrad wrote:
>> On my K2, in CW mode, and when I crank up the noise blanker by selecting
>> NB2 (and the default noise-blanker threshold, "Hi"), I sometimes hear a
>> strong signal some distance away from my receive frequency. The signal
>> I hear, when this happens, is not a tone. It's more like a spark-gap
>> buzz. Today, when the receive frequency was 7.034, I heard a CQ on
>> 7.026. The signal on 7.026 was quite strong but otherwise sounded
>> normal. FL1 (1.50) was selected. AF2 is installed but was off. If I
>> turned the noise blanker off, or select NB1, the interference goes
>> away. It doesn't happen often, and is seldom strong enough to be a
>> problem, but it does make me wonder if something is out of whack, or if
>> I'm operating the radio improperly. Is it just the nature of the
>> noise-blanker?
>>
>> 73, Wayne Conrad KF7QGA