[Elecraft] Filter Challenged...I'm so confused

Fri Mar 7 12:01:00 2003

You can do anything that you like with filter positions FL2, FL3 and FL4
in the SSB modes. When you go into CAL FIL and change the bandwidth to
OPT1, you are simply selecting the fixed-bandwidth SSB filter on the SSB
board instead of the variable bandwidth CW filter on the R-F board. 

As I recall, OPT2, OPT3 and OPT4 act identically with OPT1 - any of
those selections will choose the fixed-bandwidth SSB filter on the SSB
module. It's a good idea to use OPT1 just in case some future firmware
update changes that. 

Your understanding is right. You MUST select OPT1 for the filter in FL1
position in the USB and LSB modes because the SSB module automatically
uses whatever you selected for FL1 as the "transmit" filter. If you do
not have OPT1 selected in FL1 position, the SSB module won't work in
transmit.

Filter positions FL2, FL3 and FL4 can be ANYTHING you choose: the OPT1
filter or the variable-bandwidth CW filter with any bandwidth setting
you might want. It will have no effect on transmit. So why would one
want to use OPT1 in the FL2, FL3 or FL4 positions? 

Remember that you have three things you can "adjust" in CAL FIL: 1) The
filter you will use - either the fixed bandwidth OPT1 filter or the
variable bandwidth filter. 2) If you choose the variable bandwidth
filter you can set the bandwidth you want. 3) The two frequencies of the
Beat Frequency Oscillator (BFO) associated with that Filter Position.

The BFO frequency determines the frequency of the audio signal that you
hear in the speaker when a signal appears in the filter passband. For
SSB, you want to adjust the BFO so that a signal near the lower edge of
the filter passband will produce a tone of about 400 Hz in the speaker,
and a signal near the upper edge of the passband will produce a tone of
about 2400 Hz in the speaker. That corresponds to the minimum audio
spectrum for good "communications" speech quality. 

You have two BFO frequencies to set for EACH filter position. In SSB
there is one for upper sideband and one for lower sideband. One BFO
frequency is below the filter bandpass so filter will pass one of the
SSB sidebands, and the other BFO frequency is above the filter bandpass
so the filter will pass the other SSB sideband. That's how upper
sideband (USB) and lower sideband (LSB) are selected. You do the same
things in CW mode. That's how CWn (normal) and CWr (reverse) are set up.

One reason for using the OPT1 filter in SSB for FL2 or 3 or 4 is to
produce the equivalent of "passband tuning". You can define different
BFO frequencies in FL2 or 3 or 4 that will "shift" the passband of the
filter to higher or lower frequencies. For example, placing the BFO
closer to the edge of the filter passband will cause the audio you hear
to be lower-pitched, and placing the BFO farther from the edge of the
filter passband will cause the audio to be higher-pitched. As long as
you leave the BFO settings in FL1 alone, these changes will have NO
effect on your SSB signal "on the air". 

Of course you are free to use the variable bandwidth filter  in FL2, 3,
or 4 and adjust the bandwidth as well. The variable bandwidth filter is
designed for narrow CW bandwidths, so it is not as smooth when used out
at "phone" bandwidths near 2 kHz, but it does work well enough. I use it
set narrower for data modes that employ SSB transmit so I can narrow the
receiving bandwidth. 

Bottom line: you did NOT "go wrong". You have simply made some choices
that you can change at any time. 

One final note: the stock K2 filter is NOT 2.4 KHz wide. It's closer to
2 kHz. That's right at the minimums for good communications speech
intelligibility. That was done on purpose by Elecraft since the narrower
the filter, the more the RF is concentrated into a narrower spectrum and
the more "punch" the signal will have in getting through when the signal
is not too strong. The disadvantage is that the BFO frequencies must be
set carefully for this filter to deliver the proper performance. It
needs to pass the range of about 300 to 400 Hz up to 2300 to 2400 Hz to
work at it's best. That's why it is so important to make final
adjustments of your BFO frequencies in FL1 position for the
best-sounding signal on the air and then leave those settings along. If
you don't like the sound of SSB reception with FL1 set for best transmit
quality, adjust the BFO frequency in FL2, 3, or 4 positions for the
receive quality you like best. 

Ron AC7AC
K2 # 1289

-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Leroy Buller
Sent: Friday, March 07, 2003 7:27 AM
To: [email protected]
Subject: [Elecraft] Filter Challenged...I'm so confused

OK, lets go back to Filters 101.....

I am a little confused....

To set up the SSB filters, the FL1 place needs OP1 and I have always
assumed that OP1 will be the 2.4 kHz filter.  For some reason FL2 also
has OP1 in too and it is 2.4 Khz as well.  The others are 1.8 and 1.6
and work OK.  Where did I go wrong?

Lee - K0WA
"Filter challenged - head case"