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

[Wayne Conrad]

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

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[Don Wilhelm-4]

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
>

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[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

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[Don Wilhelm-4]

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?
>>>
>>>

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[Wayne Conrad]

Don,

Thank makes perfect sense.  Thanks!

73, Wayne Conrad KF7QGA

On 08/09/11 14:37, Don Wilhelm wrote:
> 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.
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