11
Using the Audio Filter
Low-Pass Filter (LPF)
The LPF is always on, providing high-pitch QRM and noise reduction in all modes. This effect will be
most pronounced on quiet bands, with low-noise receiving antennas, with the preamp off, or when using
headphones. When atmospheric noise is strong, the K2's AGC is heavily activated, reducing the gain of the
I.F. amplifier. Under these circumstances, the effect of the LPF may be less noticeable.
After installing the KAF2, you may find that you need the preamp less often. This is due to the improved
audio signal-to-noise ratio provided by the LPF. Having the preamp turned off is most beneficial when the
K2 is used in close proximity to very strong stations, such as during Field Day, or if you live in an area
with high-power HF broadcast stations.
Band-Pass Filter (BPF)
The BPF is useful for QRM and noise reduction in CW mode as well as some data modes. Just hold
A F I L
to select the BPF mode (
O F F
,
A F 1
,
A F 2
). If you have jumper W1 installed,
A F 1
will be skipped. When
you turn on the K2, the band-pass filter will be
O F F
.
You can use the BPF either by itself or in combination with narrow settings of the crystal filter. When used
with the crystal filter, the BPF's response (-3dB @ 80 Hz, -20 dB @ 270 Hz) will provide a significant
improvement in skirt selectivity. This can really help with DXing, QRP contests, and other situations where
much of the activity takes place right at the noise floor. When
A F 2
is selected, overall gain will be around
2 dB, which helps overcome the loss of the K2's narrowest crystal filter settings.
If you use the BPF with a wideband filter setting (such as
O P 1
), you'll need to be aware of the possibility
of
AGC pumping
. This can occur when a strong signal is within the crystal filter passband, even though you
might not hear it through the narrow passband of the audio BPF. The strong signal activates AGC, reducing
the gain of the receiver, which in turn makes the weak signal you're listening to decrease in volume. To
avoid this, you can either use the narrow crystal filter along with the audio BPF, or turn AGC OFF (by
holding
P R E / A T T N
and
A G C
together) and using the manual RF GAIN control.
Data Modes:
The BPF has one interesting advantage over narrow crystal filters when used in data modes.
If you use the BPF in combination with the wide filter (
O P 1
), there will be zero frequency shift between
transmit and receive. If AGC pumping is a problem, try using manual gain control (see above).
AFIL status display
If the BPF is turned on (
A F 1
or
A F 2
), you'll see this reflected in the filter status display. Filter status is
checked by holding
A G C
and
X F I L
together. The audio filter mode is shown immediately following the
crystal filter selection (e.g.,
F L 2 0 . 7 0
followed by
A F 2
).
