Hi Geoff. My post was somewhat tongue-in-cheek, and it's mostly
semantics, but I would say that the K3 filter would be a roofing filter
or not depending on your settings. I guess my opinion is that if it's
set for the same BW as the 2nd IF, then its not a roofing filter, and if
it's set wider then it is. At any rate, it's a roofing filter since that
what they decided to call it, and would usually be used that way. The
beauty of the design is that it doesn't have to be.
One other bit of semantics... if the DSP filter is set wider than the
roofing filter (if the firmware even allows this), then the term roofing
filter would definitely not be correct, and the DSP filter would then be
an "image reject filter" or something similar ;-)
I think I will coin another term for Elecraft... FDR, for Firmware
Defined Radio ;-)
73,
Larry N8LP
Geoffrey Mackenzie-Kennedy wrote:
With respect Larry I must disagree about the term "roofing filter"
being misleading. I completely agree that a narrow filter at the
first IF is desireable if not essential, and it could be identified as
a roofing filter in some instances - see below. This approach has been
the norm in the design of certain classes of high performance receiver
for some time, and obviously this filter's bandwidth must be
compatible with the receiver's operating mode.
I think that the confusion in understanding the meaning of the term
"roofing filter" started in the amateur community sometime after
commercial double conversion receivers began to appear on the amateur
market, and appears to be increasing. Because these receivers used
very wide roofing filters, and many still do, the myth arose that
roofing filters were always wide and were only used in double
conversion receivers. In turn this gave birth to other myths about the
poor performance of double conversion receivers vs single conversion
receivers, which often can be traced back to poor design and poor
electro-mechanical construction. The term "roofing filter" was
intended, and has since when correctly used, to identify the first
narrow bandwidth IF filter appearing in a receiver's signal path after
the first signal mixer, but *only* in those cases where additional IF
filtering was introduced further down the IF chain for the purpose of
establishing the overall RF / IF selectivity - as found in many
amateur double conversion receivers and early single conversion ISB
receivers for example. In the ISB receivers with which I was involved
in the 1950s, the typical bandwidth of the roofing filter was slightly
greater than twice the required traffic bandwidth of each of the
following USB and LSB filters i.e.roughly speaking 7 kHz for a basic
two channel at baseband receiver, not tens of kHz. In later years a
variety of roofing filters, some wide some very narrow have crossed my
path. The term does *not* and was *not* intended to imply that that
the receiver's architecture is double conversion nor that the
bandwidth of the roofing filter is by default wide, and is not used to
identify any filter outside of the IF cascade. Although it is tempting
to identify the roofing filter as the 1st IF filter, this could imply
that there were other IFs used elsewhere in the receiver in question
e,g dual conversion or triple conversion, and is usually avoided.
In the case of a straightforward single conversion receiver using a
single set of filters (or variable bandwidth in the case of the K2)
the IF filter should not be identified as a roofing filter.The small
filter prior to a product detector to attenuate unwanted sideband IF
generated noise does not count as a second filter, because according
to the "rules" the same result can be achieved by using an image
reject mixer as a product detector On the other hand if for some
strange reason a single conversion comms receiver did employ a widish
bandwidth IF filter close after the mixer and narrow bandwidth IF
filters further down the IF chain, at the risk of questionable IMD
performance if the cascade between the filters is weak, it would be
correct to identify the first filter as a roofing filter.
In my opinion if a filter is performing the role of a roofing filter
its identity should not be changed from "roofing filter", which is a
well established term both inside and outside of the amateur community.
73,
Geoff
GM4ESD
On Friday, May 04, 2007, at 2:11 AM, Larry Phipps wrote:
I think the term "roofing filter" is misleading. A narrow filter at
the first IF protects a receiver even better than a "roofing filter",
so there is nothing inherently distortion reducing in using a wider
filter at the first IF and then a narrower one later. The ideal
situation for IMD would be a pair of matched narrow filters at both
IFs. The real reason for a "roofing filter" it seems to me, is to
allow passband or slope tuning. This compromises IMD and AGC
performance for the sake of a feature... which may or may not be
valuable to the user.
Therefore, the "roofing filter" should be termed the "passband tuning
enabling filter", or "PBTE" filter ;-)
Thankfully, I think Elecraft has done a brilliant job of giving us
the options we want without compromises. By tying the DSP bandwidths
and PBT functions to the "roofing filters", we have the ability to
have the combination of 1st and 2nd IF BW we want,,, and with the
variable "roofing filters", I think we will be able to almost set the
relative BWs between the two... allowing a window for PBT or not as
we choose. This is an exciting development, and will be copied by
many companies over the next year. Kudos to the design team on this.
73,
Larry N8LP
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