[Elecraft] K2 -15Mtr birdies?

[Geoffrey Mackenzie-Kennedy]

Ron,

You asked if it is normal to hear birdies, the short answer is "not always".
Most superhet receivers, but certainly not all designs, will be affected by
birdies.  Unfortunately many if not most of the receivers designed for the
amateur market use an "open plan" layout without shielding nor decoupling
filters perhaps for reasons of cost, and that will inevitably result in
birdies being heard.  Unfortunately the K2 does have a birdie problem.

You asked if there was something you could do about the weak birdies you
found at 21028.63 kHz and 21036.20 kHz.  Again my answer is yes, but I must
warn you that getting rid of these and the other birdies found in the K2 is
not an easy task, and requires "surgery".  Some years ago I got rid of the
birdies in my K2/100, but it required the addition of some shielding,
breaking some DC supply and control traces and inserting L-C filters, moving
some bypass ground points, etc etc.

Knowing the tuning rate of a birdie vs. the VFO tuning rate is useful
information.  One type of birdie, and there are different breeds, appears
when a spurious response of the receiver (the so-called image is one of
many) "hears" an oscillator, or a harmonic of an oscillator.  The tuning
rate of this type of birdie will sometimes provide a clue as to which
oscillator or mix of oscillators is responsible, and which of the receiver's
responses is "hearing" it.  To cite a simple example, if the BFO frequency
is 4.914 MHz, there are three possible birdies ganging up at 21.294 MHz, due
to the BFO's fundamental, BFO x 3 and BFO x 9 being heard by three different
spurious responses.  The birdie tuning rates in this case are 1/3, 1 and 3
because the third harmonic of the LO is involved.

Please do not hesitate to ask if I can help in any way.

73,

Geoff
LX2AO


On May 11, 2012 at 04:40 +0200, Ron VE8RT wrote:


>I hadn't noticed this before as I don't normally operate in this part
> of 15 metres.   As I tuned across 15 metres I noticed a couple of
> birdies, switching between the antenna and dummy load didn't make
> them go away.  They're weak, at the bottom of the S meter, but I
> don't know if this is normal, or if there is something I could do
> about them.  On CW mode the first is at 21028.63 KHz and the tone
> changes rapidly, maybe 100Hz or more for every 10Hz change on the
> tuning.  The second one is at around 21036.20 KHz.  Is this normal,
> and is there a fix for it?  Maybe there are others on other bands
> that I'm not yet aware of.