Specifications
From January 2005 QST © ARRL
you don’t like the default settings for low
and high frequency cutoff for the SSB
filters, you can change those to suit your
preferences. The RTTY filters also have
adjustable center frequencies and
bandwidths. I won’t describe the entire
process of setting these filter parameters in
this review. Suffice it to say that you will
tap the
DISPLAY button and then press and
hold
RCL a few times to reach the filter-
setting menu. Then the
BAND+ and BAND–
buttons adjust the filter values up and down.
Your new settings will be stored
automatically for that filter selection.
Figures 7 and 8 show some of the filter
response curves measured in the ARRL
lab. One default filter setting for each
mode is
LOW PASS
. Basically this filter
setting leaves the DSP filter wide open.
The low pass filters have a bandwidth of
2.5 kHz in CW, 3 kHz in SSB and 3.5 kHz
in RTTY. With
LOW PASS
selected, the
radio response will follow the selected
crystal IF filter passband shape. Selecting
the narrower DSP filters will increase the
passband skirt selectivity. You can select
any combination of crystal IF filter and
DSP audio filter setting for each mode.
For example, if I really want to eliminate
nearby signals while trying to copy a CW
signal I might select the 200 Hz crystal
filter and the 100 Hz DSP filter. If
necessary, I could even set the DSP filter
to 50 Hz. That is pretty tight! One of the
KDSP2 menu options allows you to put the
filter in bypass mode to save power.
In addition to the band-pass filters, the
KDSP2 provides a very effective noise
reduction filter for CW and SSB modes,
and an automatic notch filter for SSB
mode. To activate the noise reduction, tap
the Display button to see the primary filter
display menu. Figure 1 shows the K2
displaying this menu. That display
indicates that SSB filter 2 is in use. The
decimal points in front of the
nr
and
nt
indicate that both the noise reduction and
the auto notch filters are active. Tap the
BAND+
button to activate the noise
reduction and tap it again to deactivate it.
You can select noise-reduction levels from
1 to 4 by holding the
STORE
button.
Noise reduction level 1 is incredibly
effective. You go from listening to
background noise as you tune across the
band to hearing no background noise at
all. Signals just pop out of the silence as
you tune. It can be a bit unnerving,
because it sounds like the radio is dead
until you tune across a signal. Of course,
if you find some band conditions that
require a more aggressive noise
reduction, select one of the higher levels.
That will also add some distortion as the
filter suppresses some of the speech.
Interestingly, the DSP denoiser actually
Manufacturer’s Specifications
Measured in the ARRL Lab
Table 2
Elecraft KDSP2 DSP Module for K2 Transceivers
Noise reduction: Adaptive, level not specified. 20-30 dB (setting dependent).
Tone reduction: Adaptive, level not specified. 33 dB, 52 dB with noise reduction.
Audio response: Not specified. See Figures 7 and 8.
Power requirements: 13.8 V dc, 0.06 A. Not measured.
Size: 2.1 × 3.1 × 1.2 inches (HWD).
Figure 7—Passband of the KDSP2 filter in
CW mode. Red trace is 100 Hz, blue is
250 Hz and black is 800 Hz bandwidth.
Low pass (no DSP) and “soft” settings
not shown.
Figure 8—Passband of the KDSP2 filter in
SSB mode. Red trace is 1.3 kHz, blue is
1.6 kHz and black is 2.2 kHz bandwidth.
Low pass (no DSP) and “soft” settings not
shown.
Figure 6—The KDSP2
plugged into the K2
Control board. Note the
filter circuit board and
the DSP processor
module plugged into it.
improved the noise floor and the IMD
dynamic range by a few dB.
The automatic notch filter is a “seek and
destroy” filter for carriers that appear on
frequency in the middle of your QSO.
Whether it is someone tuning their tube-
type amplifier or adjusting an antenna tuner,
the auto notch filter will remove
the carrier. It is also quite effective at
eliminating short wave broadcast carriers
on 40 meters. The auto notch also makes it
easier to actually listen to those short wave
broadcast stations on 40 meters. I found
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
–60
–50
–40
–30
–20
–10
0
Audio Frequency: 0 to 2 kHz
Reference Level: 0 dB
0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2 3.6 4.0
–60
–50
–40
–30
–20
–10
0
Audio Frequency: 0 to 4 kHz
Reference Level: 0 dB
the received AM audio to be easier to
understand with the notch turned on and
the radio tuned very slightly off zero beat.
Beginning with release 2.04, all DSP
functions can be turned on or off from the
front panel without use of the menu system.
Part of the beauty of the KDSP2 is that
it allows you to experiment with different
settings for the noise reduction and
autonotch filter parameters.
Another range of adjustments for the
DSP filters involves the filter gain. There
are five gain menus for you to adjust. The