[Elecraft] What "roofing filter" means to the K3's principle designer

[wayne burdick]

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 I.F. 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 I.F. 
*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 I.F. 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 I.F. 
crystal filters as roofing filters. We had only one I.F., 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 re-design:

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 I.F. 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 spacings, 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 I.F.

Enter the "downconversion" rig (K2, K3, Orion, etc.). By converting to 
a low first I.F., 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, as well as 
(in the future), variable-passband crystal filters.

And yes, these are still "roofing" filters, because they limit exposure 
(bandwidth), thus protecting later stages (in the K3 case, the I.F. 
amp, 2nd mixer, and DSP).

73,
Wayne
N6KR

CTO
Elecraft, Inc.

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