The problem here is that some manufacturers have stretched the definition of 'Roofing Filter' almost to breaking point.
It's always been my understanding that a roofing filter sits above the range of frequencies received, so, if your receiver covers 0-30 MHz and you have a 70 MHz first IF frequency then that filter is up in the 'roof'.
However if your first IF frequency is 4.915 MHz, or 10.7 MHz then you have a very strange construction method.
Something like having a ground floor, then your roof, then a first floor.
An IF filter at these frequencies is not a roofing filter, it is a first IF filter.
Actually, according to an Elecraft engineer, he feels it is; here is his article on that topic, which I'm copying from "http://www.elecraft.com/K3/Roofing_Filters.htm
What "Roofing Filter" means to Elecraft
There's been so much discussion about this topic that I'd thought I'd better try to clarify why we used the term when announcing the K3.
A "Roofing filter" is simply a filter in the radio's first IF through which all signals must pass before they will be "seen" by later receiver stages.
The narrower this filter is, the less exposure later stages will have.
Thus a "narrow" roofing filter is desirable -- but "narrow" is relative, as I'll explain.
The term "roofing filter" has most often been used in relation to triple- or quadruple-conversion receivers.
Such receivers have an IF above the highest RF band covered; it's typically something in the range of 30 to 70 MHz or higher.
But "roofing" as a term should be interpreted as "protective," not "high in frequency."
A roofing filter protects later stages, including amplifiers, mixers, narrower filters, and DSP subsystems, just as the roof on your house keeps rain out of all of the rooms.
But a roofing filter can be equally at home at a low first IF, if that is how the radio is designed.
It still provides the same protective function.
When we released the K2 in 1999, we never described our 1st IF crystal filters as roofing filters.
We had only one IF, so the receiver model was simpler; there were no narrow filters at later stages that required protection.
But in 2007, we find that the term is in widespread use.
Average hams now think of roofing filter bandwidths as the standard of comparison between receivers.
This is why manufacturers have jumped through hoops to try to provide the narrowest possible roofing filters.
Many operators have an understanding (justified) that a roofing filter that is wider than the communications bandwidth will not best protect the receiver's later stages.
So the term now seems appropriate to use even in a radio such as the K2, K3, or Orion, all of which use low-frequency IFs (5 to 9 MHz).
In recent years, the roofing filter has become the centerpiece of receiver redesign:
Suppose that manufacturer "A" initially designed their receiver to use a 15- or 20-kHz roofing filter.
Yes, this allows the receiver to handle NBFM and other wide modulation modes; it may also be selected to constrain the signal bandwidth ahead of a noise blanker or spectrum scope.
But it comes at a price.
If you're using CW mode, you'll have much narrower filters selected at the radio's 2nd and 3rd IFs.
Yet the 1st IF roofing filter allows a broad swath of signals into the earlier stages.
You don't need this energy in your passband.
It can cause trouble.
Manufacturer "A," realizing they have a problem with dynamic range at close spacing, then announces that they've had a breakthrough: they can now offer a 6-kHz, or more recently 3-kHz roofing filter.
This will certainly improve the situation for SSB and AM operation, but it still opens the barn door in CW or DATA modes, because the bandwidth is a factor of 10 wider than needed for communications.
So why don't they offer much narrower roofing filters that can be switched in for CW and data modes, or at times when adjacent-channel SSB QRM is very high?
It's because they can't make filters any narrower at such a high IF.
Enter the "down-conversion" rig (K2, K3, Orion, etc.).
By converting to a low first IF, the designer can easily create narrow filters that are compatible with the required communications bandwidth.
This is why we are offering filters with bandwidths as low as 200 Hz.
And yes, these are still "roofing" filters, because they limit exposure (bandwidth), thus protecting later stages (in the K3 case, the IF amp, 2nd mixer, and DSP).
Wayne N6KR CTO, Elecraft, Inc. Now, all of the above was very interesting reading to me when I started this project; however, my original questions, which deal with the K3's block-diagram and schematic, still remain unanswered.
73 Jerry KM3K