[Elecraft] K2 AM-receive mod idea?

[Ron D'Eau Claire]

The K2 does a great job with AM just as it is. In fact, it will usually
receive short wave AM better than most "AM" short wave receivers. That's
because the K2 only "detects" one of the AM sidebands and supplies the
carrier locally. 

The big problems with AM received on a conventional AM receiver are
selective fading and interference from other stations. The K2 offers
superior reception of AM signals on both counts. 

In selective fading the carrier and sidebands QSB at different rates -
upsetting the ratio of carrier to sidebands and producing severe
distortion that comes and goes with the QSB. In the K2, the carrier
received over the air is not used. The local BFO signal is used instead,
so there is no issue with the carrier fading in relationship to the
sideband. 

A conventional AM signal has two redundant sidebands - one on each side
of the carrier. A signal close enough to interfere with either sideband
will be heard in the speaker. The K2 only uses one sideband. Since each
sideband is identical, nothing is lost. (Well... half of the signal
isn't used so the signal strength at the detector is half, but half of
the noise being picked up with the signal isn't used either, so the
signal-to-noise ratio is just as good. Sensitivity is not affected at
all by using only one sideband). What is GAINED is that if there is an
interfering signal on one side of the AM station, you can just switch
sidebands with the K2 and eliminate it. 

Some top-of-the-line AM receivers uses "synchronous detection" to get
around the selective fading and distortion problem. Those receivers
inject a locally-generated carrier just like the K2 does with its BFO.
But when both sidebands are being received as in a conventional AM
receiver, the phase of the local carrier is critical to avoid
distortion. Having the local carrier on the same frequency isn't good
enough - it has to be exactly 'in phase' with the on-air carrier as
well. That's because it has to have exactly the same phase relationship
to both sidebands. That makes the AM receiver much more complicated.
Getting rid of one sideband eliminates that problem. 

So what's the downside of using the K2 for AM reception? Selectivity!!
The K2 is designed for optimum selectivity for voice. The OPT1 filter is
only a smidgen over 2,000 Hz wide! That's pretty narrow for anything but
voice, and it constricts even the voice a bit. Of course, it gains the
ability to dig signals out of the noise better that way, but you can't
get the sort of fidelity a wider IF filter would provide. 

Ron AC7AC
K2 # 1289