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
Jack Smith wrote:
One more data point on the term "roofing filter."
I have a copy of the June 1981 RSGB Radio Communications magazine
that reviewed Drake's TR7.
It describes the design as follows:
"A low-noise, wide-dynamic range amplifier is used at 48.05 MHz to
precede the 10-KHz wide roofing filter."
Leaving aside the RSGB's odd use of hyphens, it describes the filter
as a "roofing filter" although Drake does not use the term in its
technical manual.
Is it the case that "roofing filter" is UK terminology that has made
its way to the US side of the pond only recently?
Jack
Geoffrey Mackenzie-Kennedy wrote:
Just cannot resist saying this Jack, the term "roofing filter" has
certainly been around for a long time - close to 50 years I believe,
possibly longer. I think that I first ran across the term being used
to identify the first IF filter in an Independent Sideband Receiver
which the company for whom I worked manufactured for HF
Point-to-Point applications in the very late 1950's, along with
companion high power ISB transmitters.
Perhaps some other term might be less confusing and prevent any
misuse of the term , but don't let us change from "roofing filter" now!!
73,
Geoff
GM4ESD
Jack Smith wrote:
The term "roofing filter" has been around a long time, at least 20
years.
Usually used in the context of up-converting receivers, where the
first IF is 40.455 MHz, or 45.000 MHz or even higher. The term means
the first selective filter in the receiver. If the first IF is > 30
MHz, the roofing filter is generally wider than one normally needs
for SSB or even AM, and the receiver's ultimate selectivity is
provided in a later IF stage.
As to why it is called a "roofing" filter, I imagine it derives from
the fact that it puts a "roof" over the receiver's selectivity.
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