I purchased a SignaLink USB interface to use with my KX3 for digital work. After purchasing it, I read about some mods which could be performed on the SignaLink to make it have better frequency response. I did the mods and decided to see what its frequency response was after the mod. So, I downloaded Spectran and set it up. When I saw the low-end response curve, I was pleased. But then, as I played with the KX3, I noticed something strange: On SSB, whenever the FL2 filter kicked it, there was a gap from about 1350 to 1650 Hz where the signal was suppressed by about 30 dB. To make sure that this was not the SignaLink, I removed it from the circuit and just looked at the audio out from the KX3. Below are the results:
First, I checked to make sure that the RX EQ was flat, and then set the Passband tuning to Lo/Hi Cut. The low frequency was set to 50 or 150 Hz, and the upper limit was set to 4000Hz. -- wide open. With Spectran running, I gradually narrowed the bandwidth. When it reached 2800 Hz and FL2 kicked in, the gap suddenly appeared. It stayed there as long as FL2 or FL3 was engaged. When I began to open the frequency response up wider, as soon as the FL1 filter kicked in, the gap disappeared. (See the first figure below.)
Just to be sure it had nothing to do with the Passband tuning control, I set it to "Nor", and tried it again. This time, the center frequency was set to 1500 Hz and the bandwidth was varied from the maximum down to about 150 Hz and then back to Maximum. The same gap was there whenever FL2 or below kicked in. (see the second Figure below).
Why is there a gap in the audio response right where I need a boost in the frequencies?
(See below the pictures for an answer!)
The reason there is a gap is: It's supposed to be there! The KX3 uses a method of demodulation known as the Weaver mode. In this mode, any unbalance in the I/Q signals can be cancelled out, giving better reception. The Weaver mode automatically engages at 2.9 KHz bandwidth unless you set it to 2.4 KHz in the PBT mode menu. (Of course, in the Data: Data A mode, the Weaver mode does not occur at all). My radio was set to engage the Weaver mode at 2.8-2.9 KHz transition, and it made for harsh listening (to my ears) when I set the bandwidth on SSB to 2.8 KHz or less.
So, one little adjustment and now I can hear better on SSB.