[K2] Excessive PSK IMD with new K2/100

[dberkley]

Just built a KPA100 for my K2 S/N 6274.  When originally running PSK on the barefoot K2 had no problem achieving IMD around -30 dB (measured with PSKmeter).  The K2 had a KAT2 tuner which was removed, and replaced by an external autotuner when the KPA100 was installed.

With the new KPA100 in place I can't get the IMD below about -24 dB and it seems to be independent of power setting or actual output power (including bypass mode at 5 watts which is where I set the original K2) and remarkably independent of audio drive, which wasn't true with original K2.  There is a broad minimum in the IMD when the ALC barely flickers on the 3rd bar.  Optimal performance is on 20m but there is little difference on other bands (when the audio is adjusted).

Any thoughts on what has changed?  Is there any obvious way to improve IMD, although I guess > -20 dB is in the barely acceptable range?

73, David
K2MUN

[Don Wilhelm-4]

David,

The KPA100 can be coupling back into the KSB2 option.
Remove the right side panel and tuck the ribbon cable and speaker cable
up into the space at the top of the KPA100 instead of letting it dangle
near the KSB2 board.

I will not guarantee that it will help, but it often makes a difference
on SSB, and can also help on data modes.

73,
Don W3FPR

dberkley wrote:

> Just built a KPA100 for my K2 S/N 6274.  When originally running PSK on the
> barefoot K2 had no problem achieving IMD around -30 dB (measured with
> PSKmeter).  The K2 had a KAT2 tuner which was removed, and replaced by an
> external autotuner when the KPA100 was installed.
>
> With the new KPA100 in place I can't get the IMD below about -24 dB and it
> seems to be independent of power setting or actual output power (including
> bypass mode at 5 watts which is where I set the original K2) and remarkably
> independent of audio drive, which wasn't true with original K2.  There is a
> broad minimum in the IMD when the ALC barely flickers on the 3rd bar.
> Optimal performance is on 20m but there is little difference on other bands
> (when the audio is adjusted).
>
> Any thoughts on what has changed?  Is there any obvious way to improve IMD,
> although I guess > -20 dB is in the barely acceptable range?
>  
>

_______________________________________________
Elecraft mailing list
Post to: [hidden email]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
 http://mailman.qth.net/mailman/listinfo/elecraft   

Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com

[dberkley]

Don,

At first I thought your suggestion didn't affect anything since I already had dressed the leads fairly well.  However, in bypass mode (less than 10 watts) the IMD is now back to -30 dB.  Also, after running for about 15 seconds, the IMD at 25 watts now drops to about -27 dB.

So there is now a noticeable difference between bypass and KPA100 mode and, I suspect, a difference in my SSB performance as well.  Any other good thoughts like that one?

Tnx and 73,  David
K2MUN

Don Wilhelm-4 wrote

David,

The KPA100 can be coupling back into the KSB2 option.
Remove the right side panel and tuck the ribbon cable and speaker cable
up into the space at the top of the KPA100 instead of letting it dangle
near the KSB2 board.

I will not guarantee that it will help, but it often makes a difference
on SSB, and can also help on data modes.

73,
Don W3FPR

dberkley wrote:
> Just built a KPA100 for my K2 S/N 6274.  When originally running PSK on the
> barefoot K2 had no problem achieving IMD around -30 dB (measured with
> PSKmeter).  The K2 had a KAT2 tuner which was removed, and replaced by an
> external autotuner when the KPA100 was installed.
>
> With the new KPA100 in place I can't get the IMD below about -24 dB and it
...
_______________________________________________
Elecraft mailing list
Post to: [e-mail]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
 http://mailman.qth.net/mailman/listinfo/elecraft   

Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com

[Don Wilhelm-4]

David,

The only other thought I have is that perhaps the KPA100 PA transistors
are not operating at exactly the right point of their characteristic
curve.  The only thing that will change the portion of the curve for the
PA transistors (i.e. the operating point) is the bias.  The bias setting
given in the KPA100 manual is good for most applications, but that does
not say it cannot be optimized for a particular set of transistors used
in a specific application.  I would suggest that you alter the bias
setting *slightly* on either side of the manual suggested setting to see
if you obtain any improvement.  Good luck with your experiments.

73,
Don W3FPR

dberkley wrote:

> Don,
>
> At first I thought your suggestion didn't affect anything since I already
> had dressed the leads fairly well.  However, in bypass mode (less than 10
> watts) the IMD is now back to -30 dB.  Also, after running for about 15
> seconds, the IMD at 25 watts now drops to about -27 dB.
>
> So there is now a noticeable difference between bypass and KPA100 mode and,
> I suspect, a difference in my SSB performance as well.  Any other good
> thoughts like that one?
>
> Tnx and 73,  David
> K2MUN
>
>
> Don Wilhelm-4 wrote:
>  
>> David,
>>
>> The KPA100 can be coupling back into the KSB2 option.
>> Remove the right side panel and tuck the ribbon cable and speaker cable
>> up into the space at the top of the KPA100 instead of letting it dangle
>> near the KSB2 board.
>>
>> I will not guarantee that it will help, but it often makes a difference
>> on SSB, and can also help on data modes.
>>    
>

_______________________________________________
Elecraft mailing list
Post to: [hidden email]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
 http://mailman.qth.net/mailman/listinfo/elecraft   

Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com