[Elecraft] K1 birdies - revised

[Morrow, Michael A.]

From: t.wells@blueyonder.co.uk 
 
> Note CAL OSC shows the VFO to be in the range 3008 to 3096 on 
> all 4 bands (4 band filter unit). Also note, on transmit into
> 50 ohm dummy load, the bands may not be well calibrated
> - maybe 2-3KHz off.

Tony,

You aren't saying that there's that much difference between the
receiver cal and the transmitter cal are you?  I think not, but...

In my opinion, the K1 should be calibrated such that the display
shows TRANSMITTED frequency, not RECEIVER frequency.  A separate
calibrated receiver set for LSB or USB will display the K1 
transmitter output frequency when the calibrating receiver is
zero-beat with the K1's output signal.  The CAL/OPF function
can then be used to make the K1 show the value shown on the 
calibrating receiver.  (Generally, most of today's commercial ham
transceivers introduce a CW offset in the digital display when
in CW mode, so that is why the CW mode should NOT be used for this
calibration.)

The K1 receiver operates in LSB mode for all bands (as can be proved
by the pitch of a received signal increasing as the K1 frequency is
increased).  If you calibrate the K1 to display transmit frequency
after you've set a CW offset of, say, 600 Hz using RF board C13,
then the receiver frequency (for zero-beat) will actually be 0.6 kHz
HIGHER than that shown in the K1 display.

> There are two birdies of concern. They are:
> 
> Birdy No. 1 in 10.100 Mhz Band - zero beat at 10170.7 
> (display 70.7)

I have the 170 kHz VFO option whose coverage begins around 10995
kHz, so I can't receive 10172 kHz on my K1.  Even if I could
get this frequency, it is WELL OUTSIDE the 30m ham band and I
wouldn't be concerned about it at all.

I don't know what's generating it.  There aren't any combinations
of VFO and BFO frequencies, nor their harmonics, that should be
generating a spur there.  I guess it's like the spur I find on my
K1 at 14094 kHz.  I can find no combination of internal frequencies
that should be generating it.  But it doesn't bother me since it is
well outside the normal CW band.

> Analysing the above, at a VFO reading of 3013 my dial reading
> should be 10172KHz. This is only 343 Hz off the 2nd harmonic
> of BFO Xtal 4915.

I think you mean 343 **kHz** from the second harmonic.
 
> Birdy No2 in the 21MHz Band 21072.5 (display 72.5) . This is 
> definately a VFO related harmonic.

As I wrote a couple of days ago, this is **definitely** the
seventh harmonic of the VFO.  As near as I can tell, baring use of
extra interstage shielding and traps, this birdie is un-avoidable.
It's actually not all that strong when an antenna is connected to
the K1.

In a related area, but in sort of a useless info category, here
is how one may determine the frequency of the birdie at the
lower band edges:
(1)  Measure the oscillation frequency of the front panel MPU
     crystal (I used a separate receiver).
(2)  Divide by four.
(3)  Multiply by the harmonic number of a 1 MHz signal at the
     band edge.

Thus, the amount that this signal is below the band edge varies
from band to band.

Example:
My MPU crystal operates at 3999.5 kHz.  One fourth of that is
999.875 kHz.  At the 21 MHz band edge, I'll be receiving the 21st
harmonic.  So, the frequency of the weak signal just below the
band edge will be on 20997.4 kHz.  Since my K1 is calibrated to
display  transmit frequency for a 0.6 kHz CW offset, my display
when zero-beat with this band edge signal will be 20996.8 kHz.
And that's where I actually find the signal.

I can now appreciate the design feature of the front panel design
such that the MPU crystal is deliberately required to oscillate
below its nominal 4 MHz frequency.  If it oscillated at or above
4 kHz, then these harmonics would be *IN* the lower CW bands
segments.

73,
Mike / KK5F