[Elecraft] IQ output of KX3 to PX3

[Phil Wheeler]

Odd thing, Mike: I looked for that under KX3 App 
Notes at the website and cannot find it.

Phil W7OX

On 3/17/15 12:34 PM, Mike Lichtman via Elecraft wrote:
> 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.