[Elecraft] K3 configuration question

[Lyle Johnson]

>> I had the same thought when ordering my K3, but I chose 
>> the FM filter instead of the AM filter.  That will give 
>> me sufficient bandwidth for good quality AM RX as well 
>> as FM TX/RX.
> 
> Absolutely.  It is a shame that the K3 firmware currently 
> blocks ESSB or AM transmit when the FM filter is installed 
> in place of the AM filter.  Fortunately, if you're not using 
> FM transmit,  it is possible to lie to the firmware and tell 
> it FL1 is 6 KHz.   

While you can do this, I suggest you not do so.

Here's why.

The K3 uses a DSP IF at 15 kHz.  The roofing filter is the primary means 
of suppressing the 15 kHz image on transmit and receive.  In wideband 
modes (ESSB, AM and FM), this is especially true.

The wider bandwidth of the 13 kHz FM filter results in poorer 
suppression of the image.  In the FM sub-bands, it is suppressed well 
within the definition of good engineering practice.  Further, FM is not 
considered to be a weak-signal mode.

SSB is often a weak signal mode, and ESSB and AM operations are 
typically within the image offset of potential weak signal operations. 
Thus, if you transmit in other than an FM sub-band using the FM filter, 
you risk interfering with nearby weak signal operation.

We are studying the use of the FM filter for such purposes, but we have 
not yet completed our characterization of the use of the FM filter for 
non-FM use.  When we do, and if the image suppression is sufficient to 
allow you to be a "good neighbor" when running your K3 to drive a legal 
limit amplifier such that the transmit image is in a nearby weak-signal 
area, we'll directly support its use in the K3 firmware.

Until then, please consider the potential interference you may cause if 
you use the FM filter in this manner.

73,

Lyle KK7P