[Elecraft] BPF/VFO-ALC Mod - Before & After

[Larry East]

While looking at the output of my K2 with a neat little spectrum analyzer 
probe, I discovered a spur at about 18MHz when operating on 10M that was 
less than 40 dB below the carrier. All other spurs and harmonics on all 
other bands were more than 40 dB below the carrier (dBc) -- the only 
marginal case being the 30M second harmonic which was 41-42 dBc. I 
contacted Gary S., and he pointed out that this is exactly the problem (the 
18MHz spur) that the BPF/VFO-ALC mod is intended to cure. So ... I rummaged 
thru my parts bins for the necessary parts and installed the mod tonight. 
(Gary sent a PDF file to me describing the mod -- don't know if it is on 
the Elecraft web site yet -- and very few parts are required.)

Here are before and after results (measurements performed at 28.100 MHz):

Before the Mod
--------------

Carrier    ~18 MHz
  Power       Spur
-------------------
   3W        37 dBc
   5W        38 dBc
  7.5W       41 dBc
  10W        46 dBc

This meets FCC requirements for spurious emissions at <=5W (at least 30dBc) 
and for >7W (at least 40dBc) power levels, but is marginal between 5W and 
7W. However, using this K2 at power levels below 6-7W to drive an external 
amp would probably not meet the FCC's requirement of 40dBc (I have a 
homemade amp, but I didn't hook it up for measurements).

After the Mod
-------------

Carrier    ~18 MHz
  Power       Spur
-------------------
   3W        48 dBc
   5W        50 dBc
  7.5W       52 dBc
  10W       >55 dBc

Well within FCC requirements even when driving an external amp! (Assuming 
that the amp is reasonably linear...)

So ... now your know. Your results may vary.

(I estimate the uncertainty on the above measurements to be no greater
  than +/- 2 dB.)

73,
Larry W1HUE/7