[Elecraft] IQ output of KX3 to PX3

[Mike Lichtman via Elecraft]

I received this bulletin from Elecraft a while ago when I was trying to diagnose a problem. I think it has what you need. 73 Mike KF6KXG
ELECRAFT KX3 Application Note
Determining I and Q signal output levels
Revision A, February, 2013
Copyright © 2013, Elecraft, Inc.; All Rights Reserved
Background
The KX3 provides a way to connect to popular Software Defined Radio (SDR) programs. Most of these
programs utilize a PC-based soundcard to import the I and Q signals into these programs. This Application
Note discusses the I and Q signals provided by the KX3 for this purpose.
Determining the I and Q signal levels in the KX3
Equipment required:
• Dual channel Oscilloscope with at least 10mv per division sensitivity
• Signal Generator with -73dbm output. An XG3 or equivalent will work.
• 2.5 mm Stereo phone plug or other means to bring out the I and Q signals from the I/Q Jack
Procedure:
1. Connect the oscilloscope vertical inputs to the tip and ring of the phone plug and plug into the IQ output
jack
2. Set up signal generator to supply a S9 (-73 dBm ) signal to the KX3 antenna connector.
3. Tune the radio to the signal supplied by the signal generator.
4. In the KX3 menu go to " RX I/Q " Set it to "On"
5. With the Oscilloscope set to 500us per division and 10 mv you should see something similar to the
picture below. Note that one trace is shifted in phase to the other by 90 degrees.
Elecraft • www.elecraft.com • 831-763-4211
Guidelines for using a KX3 I and Q signals with an SDR
• Check configuration of your PC's soundcard for Recording devices and levels. We have
found that many PCs default to Line level inputs rather than Mic level inputs.
• Confirm that the Mic input is set up for Stereo inputs. We see many PC's - laptops mostly -
do not have a 2nd channel for the Microphone input. This will not work, of course, with the
KX3/SDR setup as there are 2 complete channels required for I and Q signals to be
amplified.
• Once set up for Mic input, ensure that all Input Level sliders are set to their mid point.
Many drivers will have a +10/+20dB boost slider. Noise floor will rise with its use but the
signal will appear in an SDR easier. We have seen that some soundcard drivers will also
provide a handy bar graph of VU metering to allow you to see whether there's I/Q coming
into the PC.
• Bring up whatever SDR software program and ensure that it has been configured to select
the appropriate soundcard links for input and output.
• Monitor the levels of the I/Q with that program. Most of the SDR programs have some
form of level indicator for input.
Eliminating Ground Loops with PC soundcards – Hum and Noise
At times, we get reports of noise and hum on the I and Q signals. Typically, we find this is due to
use with a PC with floating soundcard inputs which may create ground loops. In turn, this creates
extra noise on the I and Q signals input into the soundcard. Additionally, noisy brick power
supplies can raise the noise floor further.
Generally, the use of isolation transformers will break the ground loops between a PC soundcard
and the KX3's outputs. These are typically available from Radio Shack, PAC-Audio and
Amazon.com, although some adapting of the connectors may be required for the 2.5mm connector
on the KX3 I/Q port.