[Elecraft] K2: K2/100 birdie @ 3589

[Geoffrey Mackenzie-Kennedy]

Hi John,

Glad to hear that after all of your bad luck your K2/100 is now working 
well.

The strong birdie at 3589 kHz was also present in my K2/100 before I 
embarked on a birdie killing project a few years ago, which involved a fair 
amout of difficult surgery. This particular birdie is the result of the 
fundamental of the LO mixing with the fundamental of the BFO. In my K2 I 
found evidence that the mixing was taking place in the first mixer, the 
TUF-1 diode ring.

Using whole numbers and assuming that the IF is 4914 kHz, therefore the 
BFO's fundamental is 4914 kHz, when the receiver is tuned to 3589 kHz the 
fundamental frequency of the LO will be 3589 + 4914 kHz,  i.e. 8503 kHz. 
Should the BFO and LO fundamentals mix, one product will be at 8503 - 4914 
kHz, i.e. at 3589 kHz.

There is, unfortunately, no simple fix to this problem as far as I know, 
because due to the lack of shielding and lead filtering the fundamentals and 
harmonics of all the oscillators in the K2 can be found in places where they 
should not be inside the K2, which results in its birdie problem.

Although a receiver spurious response is not involved in this particular 
case, and all superhet receivers have spurious responses to a greater or 
lesser extent, if a mixing product generated by two or more internal 
oscillators should be "heard" by a receiver's spurious response, then this 
would result in a birdie. In the majority of cases it is the lower order 
harmonics of the oscillators which create birdie problems.

73,
Geoff
GM4ESD


John VK7JB wrote on Tuesday, November 02, 2010, at 11:50 AM:

> Hi All,
>
> I've just finished K2/100 #6998.  After blowing the PA transistors of the
> base K2 by accidentally dropping the frequency probe into the works, I've
> completed the KPA100, installed it and all is working very well.  Made
> several SSB contacts tonight and I couldn't be happier.  I have a great
> sense of achievement :)
>
> I'm curious about a prominent birdie, zero beat at 3589khz.  It's present,
> and more prominent, with the antenna disconnected and has its origin
> internally.
>
> There are some postings about this birdie from around 2008, but I don't 
> see
> any resolution.  I can live with it, even though it's inconveniently 
> located
> right where I like to work on SSB phone.  Did anyone work out where this
> comes from and whether or not there is a simple fix?
>
> Also, I've calibrated the K2 reference oscillator several times according 
> to
> the N6KR method.  The result is that my frequency display is 20hz off
> frequency after the rig has warmed up:  eg it reads 9999.98 for 10000.00 
> WWV
> signal.  Am I correct in understanding this is about as good I can expect
> given the DAC parameters of the K2?  I can't seem to improve on it further
> by repeating it.
>
> 73,
>
> John
> VK7JB