Sdr Applications - ELECRAFT KX3 Owner's Manual

SDR Applications

The KX3 provides a special receiver output jack,
RX I/Q, for use with software-defined radio (SDR)
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applications running on a computer.
The primary benefit of such applications is
providing a real-time display of a band of
frequencies above and below the signal to which the
KX3 is tuned. You can use this display to quickly
find signals of interest, examine modulation and
noise characteristics, etc. (This capability can also
be found in dedicated instruments called
panadapters or spectrum analyzers.)
Since the KX3's RX I/Q signal is analog, it must be
converted to a digital signal by means of an analog-
to-digital converter (ADC). Most PCs have this in
the form of a built-in stereo sound-card. It is
important that the input to the PC be in stereo.
Most "mic" inputs are mono, and cannot be used.
Most "line inputs" are stereo, and can be used.
If your PC lacks a suitable input, there are third-
party soundcards available for USB, PCI, PCIe,
Firewire and other expansion buses.
There are many SDR applications available as
freeware on various platforms (PC, Mac, iPad, etc.).
The quality of the spectrum display they provide
depends heavily on the quality of the soundcard's
ADC inputs. An excellent reference on potentially
suitable sound cards may be found on the web by
searching for "LP-PAN sound card tests".
A cable with a 2.5 mm (3/32") plug on one end and
a plug that matches your sound card input on the
other end (typically 3.5 mm [1/8"] stereo) is
required.
Once you have a cable and soundcard, you'll need
to download suitable software and load it on your
PC. Follow the directions supplied by the program,
usually in the form of a help file.
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The RX I/Q jack's analog signal is in I and Q format. I
and Q refer to the two parts of a phase-quadrature signal,
which simply means the two signals are 90 degrees apart
in phase. Software applications can use this information
to mathematically reconstruct the signals for display
purposes.
A PC program suitable for most purposes is
HDSDR. It is available as of this writing at
http://www.hdsdr.de
Follow the directions provided by the HDSDR
program for setup and operation.
Tips for setting up SDR Programs with the KX3:
 When you first run the SDR program, the
frequency displayed may be incorrect, but the
spectrum display will still be accurate in terms
of offsets. Setting the center frequency display
to 1.000 or 10.000 MHz, for example, will
make it easy to see how many kHz above or
below the current KX3 frequency that another
signal is.
 Select the correct soundcard input for the
program. If your PC has an internal soundcard
but you are using an add-on card, you must
configure the program to use the correct
soundcard.
 Set the correct sampling rate. Use of 48 kHz
sampling will result in a display of almost 48
kHz: 24 kHz above and 24 kHz below the
frequency to which the KX3 is tuned. 96 kHz
sampling will yield approximately +/- 48 kHz
of spectrum, and 192 kHz sampling will yield a
display of about +/- 96 kHz.
 Enable the RX IQ output from the KX3 by
setting MENU: RX I/Q to
the KX3's receive-mode current drain by about
10-15 mA.)
 The RX I/Q outputs from a receiver are not
"flat" over an infinite frequency range; the
signal-conversion process results in some slope
(decrease in gain) as you move farther from the
center frequency. In the case of the KX3, the
signal will be reduced by about 2.5 dB at +/- 24
kHz, 4 dB at +/- 48 kHz, and 7 dB at +/- 96
kHz. The spectrum amplitude on the display,
including the apparent noise floor of the
receiver, will "roll off" by these amounts.
 If you have an accurate signal level source
(such as an Elecraft XG-series signal source or
a signal generator), you can calibrate the signal
amplitude following directions provided by the
program.
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