Re: What "roofing filter" means to the K3's principledesigner

[wayne burdick]

Good idea. Eric was just suggesting this.

tnx
W

On May 5, 2007, at 10:27 AM, Greg wrote:

> I'm going to put this on it's own page and then link to it from FAQ
> due to
> its length.
>
>
>
> -----Original Message-----
> From: [hidden email]
> [mailto:[hidden email]]On Behalf Of wayne burdick
> Sent: Saturday, May 05, 2007 7:32 AM
> To: Elecraft Reflector
> Subject: [Elecraft] What "roofing filter" means to the K3's
> principledesigner
>
>
> 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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