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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http://www.elecraft.com

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