Posted by
N8LP on
May 04, 2007; 2:51pm
URL: http://elecraft.85.s1.nabble.com/8-pole-vs-5-pole-I-knew-this-would-happen-tp446796p446814.html
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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