The KX3 is another potential solution for those who need an I/Q interface from a K3 or K3S to a computer. If you already have a KX3 that you normally only use for portable operation, you might give this idea a try.
The output from the KX3 is wideband (~200 kHz) analog I/Q, not digital, but that’s fine if your computer has a high-performance sound card interface. The KX3 will work well in this application since it has very good RMDR (receive mixing dynamic range) thanks to its low-noise synthesizer, strong mixer, and front-end band-pass filtering.
Recommended one-time setup:
1. Connect the KX3’s antenna input to the rig’s IF OUT jack (standard on the K3S; comes with the KXV3 option on the K3).
2. VERY IMPORTANT: Set the KX3’s power output to ZERO watts to avoid problems should you accidentally transmit into the IF OUT jack. The IF OUT jack does have a carrier-operated relay, but we’ve already tested it. You don’t need to.
3. Use FREQ ENT on the KX3 to set it to 8.215 MHz, matching the K3/K3S IF. This will switch the KX3 to the 40-meter band.
4. Other recommended settings at the KX3:
- Set mode to USB
- Normalize the filter passband (NORM)
- Turn the preamp on to ensure a good noise figure driving the computer’s panadapter or other application
- In the menu, turn RX ISO ON, set PREAMP gain 20 dB, and make sure RXIQ is ON.
- Set rig’s AF GAIN to 0 (you won’t be listening to the signal on the KX3, though you could, just for fun)
5. Connect the RX I/Q jack on the KX3 to your computer sound card input.
6. Run your favorite software application(s) that need I/Q data. (Note: If you see symmetrical signal images in a software-based panadapter display, chances are the RX I/Q connector isn’t fully plugged in at the KX3 end.)
7. The above setup may require additional tweaking depending on what applications you’re using. You might need to adjust the KX3’s VFO frequency to match the crystal filters used in the K3/K3S, for example. Since the KX3 itself can be controlled from a software application over USB, it would be easy to automate settings using a bit of custom software.
If you try this, please post your settings and results to the list for the benefit of others. We might summarize things in an application note. It’s an excellent use of any KX3 that’s otherwise stashed away in a go-bag.
73,
Wayne
N6KR
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