... or There's No Such Thing as a Free Lunch.
Until recently, I only had the 13 kHz and 2.7 kHz roofing filters
installed in my K3. I mostly use CW and usually found the DSP
filtering perfectly adequate for my narrow bandwidth needs. But after
having a few QSO's disrupted by nearby strong QRM, I decided to try a
narrower roofing filter. So I ordered the 500 Hz 5-pole filter from
Elecraft.
It arrived, I installed it, and configured it (including the frequency
offset). To set the gain, I used the birdie at 49.380 MHz and
Spectrogram to make sure that the audio signal amplitude with the 500
Hz filter active was the same as with the 2.7 kHz filter active. This
required 6 dB of gain on the 500 Hz filter. However, I noticed that
boosting the filter gain by 6 dB also boosted the noise floor by 6
dB. In other words, the S/N ratio at 500 Hz bandwidth was 6 dB worse
than at 550 Hz bandwidth. After thinking about it for a moment, I
realized it made perfect sense. I checked with Gary at Elecraft and
he said that in general, the narrower roofing filters will have
greater loss than the wider ones, and nothing he said made me think
that 6 dB of loss for a 500 Hz filter was unusual.
I tuned around and found some strong signals and did some A/B
comparisons with the 500 Hz filter on and off. The filter definitely
works as advertised for dealing with strong signals on a nearby
frequency.
I then sought out some weak stations, some of those 'RST 219' guys.
These stations virtually disappeared into the noise at a DSP BW=0.20
with the 500 Hz filter enabled, but were somewhat copyable with the
2.7 kHz roofing filter at the same DSP BW. In the absence of strong
QRM, the wider roofing filter gives me better performance because of
its lower loss.
But does it make any practical difference? I encounter weak signal
conditions (usually due to QSB) more often than I encounter s9+30
signals 500 Hz away. So given the current situation, I'll probably
keep the 500 Hz filter turned off most of the time. I wish there was
an easy way (i.e. one button press) to enable or disable a roofing
filter so that I could engage it only when really needed.
I've heard that the 8-pole filters tend to have a little less loss
than the 5-pole ones, but I'm undecided as to whether to try that
route. Any voices of experience?
73
--
Joe KB8AP
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