[Elecraft] K3, using AM Mode RX with 250 Hz filter

[David Hachadorian]

For an upcoming Frequency Measurement Test, I'm trying to use a K3  in 
AM Mode RX-only with a 250 Hz CW Filter. My plan for the FMT is to use a 
GPS Disciplined Oscillator as a BFO, and by measuring the frequency of 
the beat tone, I will be able to accurately determine the mystery 
transmit frequency, without worrying about the dial calibration or drift 
of the K3 itself.

It works fairly well with the standard 2.7 KHz filter, but, the WIDTH 
control only reduces the DSP bandwidth down to 1.35 KHz. 1.35 KHz is 
pretty wide if there is QRM near the mystery signal. I tried using the 
XFIL button to manually switch in the 250 Hz filter, but signals don't 
seem to pass through when I do that, and then when I touch SHIFT or 
WIDTH, the roofing filter switches back to 2.7 KHz.

Is there a way to accomplish what I'm trying to do?  My alternative is 
to set up a narrow digital filter in the tone recording software, but it 
is still disconcerting to hear a bunch of QRM around the mystery signal 
in real time.

Thanks in advance.

Dave Hachadorian, K6LL
Yuma, AZ