[Elecraft] [KX1] Anybody build a KX1 for 15/20M?

[Mike Morrow]

Eric wrote:

> Has anybody tried to build a KX1 for 20 and 15m?  Is it even possible?

It's pretty close to being impossible.

The heart of the KX1 is an AD9834 direct digital synthesis (DDS) chip.
The maximum frequency that DDS chip must generate cleanly is the RF
operating frequency plus the IF, so that's about 21.5 + 4.9 = 26.4 MHz,
for 15m operation.  To generate that, the DDS must be clocked at least
three times that frequency...or 79.2 MHz.  The DDS in the KX1 is clocked
at 50 MHz by crystal oscillator Z2.  If you did change the clock chip Z2
to 80 MHz (assuming you could locate such a replacement), you'd still
have a problem...the AD9834 **maximum** clock rating is 75 MHz.

If somehow you overcame these fundamental DDS limitations, then you'd
need to tackle the required firmware modifications in the KX1 MCU, a
PIC16F876A micro-controller.

After that...the job of revising the KX1 RF design will be needed,
including any changes in the Q6 PA circuit to optimize PA performance
at 21.1 MHz.

So I say that modification of the KX1 for 15m operation is "pretty close
to impossible" or, at least, certainly impractical.

I can appreciate the desire for 15m operation.  When band conditions
allow (as they often do now), it's the very best QRP band.  That's where
the excellent K1 excels...its performance on 15m as part of its intrinsic
design is wonderful, and its overall ham-band CW RF performance in every
respect except frequency stability easily outstrips the KX1.

I know...referring to the K1 doesn't help if all you have is a KX1.  But
the K1 doesn't get the good press on this list that it deserves...still,
more than 13 years after it was introduced.

73,
Mike / KK5F