Bill,
All of what you say is true *EXCEPT* for one item - and that is the AGC.
The hardware AGC is developed before any DSP processing, so when there
are strong signals inside the roofing filter passband, they will
activate the AGC and reduce the receiver sensitivity. If you have the
DSP cranked down to a narrow bandwidth, you may not even hear the
offending strong signal because it is outside the DSP bandwidth but
still inside the roofing filter bandwidth.
BTW, this can happen on the K2 (and many other receivers) if a wide
bandwidth is selected and the DSP is used to reduce the signal to only
the desired one.
That is not really tough to deal with if you are already listening to an
S-9 signal even though an S-9 +30 signal comes on - the S-9 signal will
be reduced to the equivalent of an S-4 or S-5 signal and that can easily
be solid copy (because other signals in the DSP passband are similarly
reduced).
Now, if the signal you are trying to copy is an S-1 or S-2 level and an
S-9 signal comes into the roofing filter passband (again it may be out
of the DSP passband and will not be heard), the receiver gain will be
reduced and you will no longer hear the S-1/S-2 signal - just like QSB,
but it is QSB induced in the receiver, not due to propagation effects.
I will have at least one narrow filter available in my K3. Bill, you
were correct that a filter is needed for each major mode, but the
filters that any one person needs will be determined by their operating
habits, and not based on the 'average'. For ragchewing and casual
contacts, even the wide 2.7 kHz filter may be all that is needed, but
for serious CW, QRP, contesting or DX chasing, an array of filters will
be needed - just how many will depend on the operator's desires to
reduce QRM and the amount available in the ham radio budget.
73,
Don W3FPR
Bill Tippett wrote:
W5EWA:
>but now someone says that we DON'T need them all.
Definitely not...unless we have more money than
sense. You need only ONE per major mode, which will
cover the widest bandwidth you ever expect to use in
that mode. For example:
CW - 500 or 400. 250 and 200 are definite overkill
IMHO and not necessary in addition to narrow DSP BWs.
The reason is that IMD/BDR spurious issues at <500 Hz
will be far overridden by other issues such as phase noise,
key clicks, etc that come from the transmitted signals.
There is no point in making a receiver many dB better
than the environment in which it must live! No matter
how good a receiver is it cannot eliminate transmitter
defects (maybe Flex's SDR-X can but that remains to be
proven).
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.
AM - 6 kHz
FM - 15 kHz
In fact you may not need ANY filters beyond
the 2.7 kHz stock filter unless you expect to have lots
of very close spaced S9+30 dB signals on CW (e.g. huge
low-band pileups or in contests). The 2.7k should be
perfectly adequate for nearly any situation on SSB,
although it wouldn't surprise me to see someone offer
1.8 kHz, mainly for contesters.
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:
Antenna > roofing filter > DSP filter > ear.
...roofing filter prevents very strong signals from
creating spurious (i.e. *NOT REAL*) signals which
will then appear inside the DSP.
...DSP filters provide the ultimate selectivity (i.e.
separating the *REAL* signals from each other).
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).
2. Is the 5-pole 500 Hz better than the 8-pole 400 Hz?
3. Is the 8-pole 250 Hz better than the 5-pole 200 Hz?
(academic to me since I don't feel either is necessary
...see comment above re TX key clicks, phase noise, etc).
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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