This is why we are offering filters with bandwidths as low as 200 Hz.Īnd yes, these are still "roofing" filters, because they limit exposure (bandwidth), thus protecting later stages (in the K3S case, the IF amp, 2nd mixer, and DSP). By converting to a low first IF, the designer can easily create narrow filters that are compatible with the required communications bandwidth. 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.Įnter the "down-conversion" rig (K2, K3S, Orion, etc.). 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.
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. You don't need this energy in your passband. Yet the 1st IF roofing filter allows a broad swath of signals into the earlier stages. If you're using CW mode, you'll have much narrower filters selected at the radio's 2nd and 3rd IFs. 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. Suppose that manufacturer "A" initially designed their receiver to use a 15- or 20-kHz roofing filter. In recent years, the roofing filter has become the centerpiece of receiver redesign: So the term now seems appropriate to use even in a radio such as the K2, K3S, or Orion, all of which use low-frequency IFs (5 to 9 MHz). 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. This is why manufacturers have jumped through hoops to try to provide the narrowest possible roofing filters. Average hams now think of roofing filter bandwidths as the standard of comparison between receivers. We had only one IF, so the receiver model was simpler there were no narrow filters at later stages that required protection.īut now, we find that the term is in widespread use.
When we released the K2 in 1999, we never described our 1st IF crystal filters as roofing filters. It still provides the same protective function. But a roofing filter can be equally at home at a low first IF, if that is how the radio is designed. 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. 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. The term "roofing filter" has most often been used in relation to triple- or quadruple-conversion receivers. Thus a "narrow" roofing filter is desirable - but "narrow" is relative, as I'll explain. The narrower this filter is, the less exposure later stages will have. 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 discussing the K3S.Ī "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. Shipping Box Sets (Item return for Servicing).Accessories (Speaker, Mics, Power Supply, Compact Cases, K-Pod).
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