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

[Don Wilhelm]

Wayne,

That depends on just how strong the off-channel signal was.
That signal could have been strong enough to overwhelm the IF AMP on the 
KNB2 board and cause it to respond to that input - remember that the 
KNB2 sits prior to the crystal filter.  If it was responding to the 
keying of the other signal, there would be gaps in the signal path.
That effect would not be what is generally considered IMD, it would just 
be an interruption to the signal that you were trying to receive, 
because that is what a noise blanker is supposed to do - it creates 
holes in the signal.

73,
Don W3FPR

On 8/9/2011 3:40 PM, Wayne Conrad wrote:
> 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?
>>>
>>>