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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