Keep up the good work Bill!! May I second you request for comprehensive IMD
/ BDR test results. Crystal filters, particularly ladders, do have a habit
of coming back to bite unless their linearity, or lack of, is taken into
account while setting Gain Distribution et al during the design activity.
I sometimes wonder whether the use of wide SSB filters e.g 2.7 kHz in our
ham community is due to poor narrower filter design, and the reulting group
delay characteristic. This in turn can result in a SSB voice signal having a
"raspy" sound. In my opinion based on observation and use, 2.1 kHz is
enough for very good soundimg audio IF things are set up properly (carrier
position, modulator AF drive, ALC, mic, etc, etc), leaving more spectrum
for others to use.
73,
Geoff
GM4ESD
On Friday, May 04, 2007 at 7:59 PM, Bill Tippett wrote:
SSB - 2.7 kHz unless you want ESSB (not sure how high
that goes these days but IMHO we should be trying to
minimize communications bandwidths rather than playing
broadcast disc jockey (my personal opinion!). Those
folks should go to 29 MHz FM if they want to do that.
.
<snip>
A roofing filter's purpose is simply to minimize
spurious artifacts from being created *inside* the RX
by external signals. It does NOT create the ultimate
selectivity (which is determined by the DSP filter at
the 2nd IF). Please read the last sentence again!
Here's a simple summary:
<snip>
...roofing filter prevents very strong signals from
creating spurious (i.e. *NOT REAL*) signals which
will then appear inside the DSP.
<snip>
The number of poles (i.e. shape factor of the
filter) can affect IMD/BDR issues inside the RX. For
this reason hopefully Elecraft will provide actual
IMD/BDR measurements with each option to help answer
the following questions:
1. Is the optional 8-pole 2.8 kHz better than the
stock 5-pole 2.7 kHz (by "better" I mean resulting
in better IMD/BDR performance which is the primary
purpose of a roofing filter).
<snip>
The variable BW filters are interesting but
I would not consider them until we have some actual
results for IMD/BDR. They are NOT providing ultimate
selectivity as they do in the case of the K2, which
seems to be confusing many folks.
73, Bill W4ZV
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