Posted by
Ron D'Eau Claire-2 on
May 28, 2005; 3:26pm
URL: http://elecraft.85.s1.nabble.com/K2-XTALS-tp378841p378843.html
Geoff wrote:
Because the same intermediate sized crystal ladder filter is used for both
USB and LSB, increasing the bandwidth could result in problems with unwanted
sideband suppression when the BFO is on the low frequency skirt of the
filter. This assumes that the filter is properly designed and terminated. It
is possible to fiddle things to steepen the LF skirt, but usually at the
cost of increased ripple in the passband.
---------------------------------------
Geoff brings up a number of legitimate concerns when messing with the filter
bandwidth.
Fortunately, one of our most thorough and experienced technical guru's, John
Grebenkemper, KI6WX, took these issues into consideration in designing the
filter mods for increased bandwidth and less passband ripple. That's why the
modifications are "official" Elecraft options and one can order from
Elecraft (SSBCAPKT). The mod kit includes all the capacitors needed for one
of several of different bandwidths.
This mod kit assumes one has the new crystals and, for pre S/N 3000 units,
the 2ND XFIL mod has been installed. The 2ND XFIL mod is required for best
sounding receive SSB. It does not affect transmit audio.
My modified filters are a full 2.6 kHz wide. I made the change as much for
CW than SSB. While I'll pick up the mic on occasion, I work CW 99% of the
time and I enjoy scanning the bands with a very wide bandwidth in which I
can hear several QSO's at once. I only crank down the bandwidth when an
unwanted signal become so loud it's obnoxious.
Still, getting the best quality audio was an interesting project, even if I
don't do a lot of SSB work. The receive audio is really surprisingly good.
AM stations at the upper part of the North American AM broadcast band sound
as good as they do on most regular AM radios when properly tuned in. Of
course, accurate tuning is critical. A 10 or 20 Hz error is obvious and
irritating when listening to music.
The issues with opposite sideband suppression that Geoff raised do not come
up because there is no need or desire to move the lower edge of the audio
bandpass closer to the carrier frequency for transmit. What John's mods did
was to provide a smoother, flatter, wider bandpass while maintaining the
roll-off at the skirts.
Normally, this wider bandwidth will be used to pass the higher-frequency
sibilances that are so important for best speech intelligibility. Passing
more of the low-frequency tone in human voices is usually a waste of power.
That low frequency energy is simply what is called a "drone", or the
fundamental note, produced by the throat. It's rich in audio harmonics, and
it's those audio harmonics spread across the audio spectrum up to several
kc/s that get modulated by one's mouth to make speech.
My voice has been, on occasion, described as sounding a bit like a metal
trash can being dragged along a gravel road. That's my natural, sometimes
rough, low frequency drone. I find the most natural and intelligible setting
for my voice rolls off the low frequencies from about 400 Hz down. Other
voices might sound better passing frequencies down as low as 300 or even 200
Hz. That's where actual listening tests are important. When listening to our
own voice in headphones, much of that low frequency sound comes right
through our bones. Unless that bone conduction is taken into consideration,
it's easy to arrive at a filter setting that sounds great when listening to
oneself in the headphones but which sounds unnatural and tinny to others on
the air. I suggest recording your signal then playing it back to hear what
others actually hear.
Keep in mind that only FL1 is used on transmit! No matter what filter is
selected for receive, the K2 switches to FL1 when the PTT is pressed. So
only FL1 need be set for optimum transmit audio. FL2, FL3 and FL4 can be set
to move the passband up or down to adjust the bass/treble response on other
signals or to help move away from nearby QRM. It works a bit like "passband
tuning" in some rigs. For example, I have one position set to bring the low
frequency rolloff down to about 100 Hz for listening to broadcast stations.
Of course you can also assign some of these filter positions to the CW
filters to provide a more restricted bandpass for listening to weak signals
in difficult band conditions.
Ron AC7AC
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