[Elecraft] K1 DDS VFO Retrofit?

[Mike Morrow]

Brad wrote:

> I'm ordering a K1 ... and I would like to integrate a DDS VFO

Hi Brad,

That's an interesting project, and one about which I've heard no earlier
discussions.  I'll admit when I first saw the K1 at Dayton 2000, I had built
three of Dave Benson's DSW mono-band DDS QRP rigs and I was a little
disappointed that the K1 was also not a DDS design.

However, I found that my K1's L-C VFO frequency stability is far better than
I had hoped.  In the K1 design, the only real advantages that would result
from using a DDS VFO are stability and tuning rate control.

John wrote:

> .. you would want to communicate the K1's band position back
> to the processor controlling the DDS (or vice versa) ... I suppose
> you could just have a band button on your DDS controller and push
> it when you pushed the K1's band button, but it seems like it would
> be nice to get it all integrated.

The K1 VFO simply tunes from about 3.1 (low band end) to 2.9  (high band
end) MHz, on ALL bands, so all a DDS VFO would have to do is tune the same
range.  The K1 MCU measures only the raw VFO output, and then displays a
derived dial frequency from that measurement and the info that the operator
has stored for the band in use and the display calibration constant.

There are some DDS disadvantages as a K1 retrofit:

(1)  The output of a DDS VFO (as opposed to a PLL slaved to a DDS) will not
be purely on the one desired output frequency.   There will be spurs, and
these spurs on the receiver local oscillator (LO) can degrade receiver IMD
performance  (SFDR/IMD issue).  Likewise, transmitter spurious output could
increase.  The K1 uses hetrodyne oscillators in the transmitter and the
receiver stages, and that will multiply the spur problem.

(2)  Most older DDS chips consume a fair amount of current.  The one in the
new KX1 may consume less than 10 mA, but the older one in the DSW units
consume about 30 mA.  That would increse receive current consumption by
about 50 percent in the K1.  (I don't know what the spec is on the DDS kit
you cited.  I saw no specs on this at the NJQRP website.)  Plus, you'll have
to power the controller that does NOT come with the DDS kit (see item (5)
below).

(3)  Aside from stability, the greatest advantage of a DDS is its frequency
agility.  It can change frequency very quickly from the receiver LO output
frequency to the transmitter carrier frequency, thus no hetrodyne stage is
needed to generate the transmitter frequency from the receiver LO.  Or, as a
receiver LO, frequency can be changed from below the IF to above the IF as a
means to change receiver sideband.  Simply replacing the K1 VFO with a DDS
VFO would completely waste most of advantages of using a DDS.

(4)  The K1 panel RIT/XIT control would no longer work, since it controls
bais on a varactor in the L-C VFO.

(5)  Big Problem:  The DDS kit you mention has NO control circuitry.  You'll
have to design and install an dial encoder and PIC control circuit to
control the DDS kit, or control it with an external PC.  The first option is
non-trivial for most of us.

(6)  If you are proposing more radical DDS use (direct generation of the
transmitter carrier frequency and receiver LO frequency while bypassing the
various mixers and oscillators now used to generate these signals), then
you'll have a terrible time communicating between the K1 MCU and your DDS
control processor to coordinate DDS bandswitching, RIT/XIT, band display,
transmit/receive switching, etc.  This is part of the problem that John
cites above.  The K1 MCU does NOT generate a frequency control word, it just
counts the raw VFO output.

I think I'd go for a KX1 over the K1 if you really want DDS control.
Otherwise, you could be in danger of turning a K1 into a Franken-Elecraft
monster.

And all of this is just my opinion.

73,
Mike / KK5F