[Elecraft] Trying to improve the K2's IIP3; mission (almost...) impossible!

[John, KI6WX]

The ARRL test report for the K2 (Feb 2004 QST) shows an input IP3 of +21 dBm
on 20 meters at both 5 and 20 kHz offset frequencies.  This measurement
implies that the two-tone third-order dynamic range (DR3) should be 101 dB
at both offset frequencies (assuming no other effects).  The measurements
show that it is actually 91 dB at 5 kHz and 97 dB at 20 kHz.

We should be more concerned with the DR3 numbers than the input IP3 numbers.
The DR3 is what determines how strong the nearby signals can be without
generating any interference at your receiving frequency.  So why is the K2
dynamic range so much below the expected level?  Is there anything we can do
to improve the DR3?

The main additional contributor to the K2 DR3 is the local oscillator phase
noise.  I've taken extensive measurements of its phase noise on all of the
major bands.  At a 20 kHz offset frequency on 20 meters, the oscillator
phase noise is -130 dBc/Hz.  This noise plus the IP3 mixing will limit the
K2 to a predicted DR3 of 98 dB.  This is close enough to the measured 97 dB
that component variation and measurement error can account for the entire
difference.

At a 5 kHz offset on 20 meters, the oscillator phase noise is -118 dBc/Hz.
This will produce a predicted DR3 of 91 dB, which agrees exactly with the
measurement.

The only way to improve the DR3 of the K2 is to reduce the oscillator phase
noise.  I have been able to reduce the oscillator phase noise on 20 meters
by about 5 dB with some component changes.  I am looking to reduce it
another 5 dB.  If I can get there, the K2 DR3 will be better than 100 dB at
20 kHz and about 97 dB at 5 kHz.

The current K2's DR3 is limited by the oscillator.  Substituting a TUF-1H
for the mixer will have no significant effect on its dynamic range.  This
means that a high input IP3 K2 will not have any performance advantage over
a standard K2.  If the oscillator phase noise is reduced, than a high IP3
mixer and improved post-amp would increase the DR3 beyond 100 dB at a 20 kHz
offset frequency.

-John
 KI6WX

----- Original Message ----- 
From: "D.W. Harms PA2DW" <eqtc@xs4all.nl>



> IIP3 of mixers at their lower frequency ranges, are something like 15dB
> above the 1dB compression point. The TUF1 (or TOP-1 as sometimes used by
> Elecraft, not sure what the maker is!?!) has +1dBm compresion point, so it
> has a total IIP3 of +16dBm.
> Adding the losses of the bandpassfilters and switches brings the IIP3 at
the
> claimed IIP3 of 21 dBm (preamp off).
>
> The TUF-1H (or TFM-1H) has +14dBm of input compression point what turns
out
> to be +29dBm of IIP3 at HF bands! Adding the input filter loss - you can
get
> something like +34dBm of (compelete RX) IIP3, but only when the postmixer
> amplifier and the XTAL filter are linear enough!
> What value is good enough for the postmixer stage? With +29dBm of
mixer-IIP3
> and 5dB of conversion loss its OIP3 is +24dBm. Now if the postmixer
> amplifier has an IIP3 of +24dBm, it will lower the overall IP3 by 3dB!!!
So
> you need a postmixer amplifier that can do at least +29dBm IIP3 to get the
> promised +34dBm IIP3 of your RX!
> I am not sure what the K2 postmixer amp is capable of, nor did I measure,
> but perhaps someone knows that?
> There are many great designs of linear IF amps, all consuming 5-10W, so
that
> almost equals the output of the barefoot K2. Not a good idea hi!
>