On Mon, 2006-05-29 at 10:42 -1000, Ken Brown wrote:
> In order for us to have an intelligent discussion about roofing filters,
> we need to agree on a definition of roofing filter. I don't recall
> hearing that term prior to a few years ago. By the usage I have seen
> here on this email reflector it seems that "roofing filter" refers to a
> filter in the first IF of a receiver. It is unclear to me whether that
> would mean that the two pole,15 kHz BW monolithic crystal filter in my
> Omni VI is the roofing filter or eight pole 2.4 kHz BW filter is the
> roofing filter. Or perhaps that term does not even apply to the Omni VI
> without modification. In order for a filter to be a "roofing filter"
> does it have to follow the first mixer (be in the first IF) and precede
> any gain stages in the first IF? Does it have to be fairly narrow, like
> on the order of a couple times the bandwidth of the mode being used, or
> less? Or can it be any filter in the first IF? Certainly bandpass
> filters in the RF stages before the first mixer do not qualify as
> roofing filters, yet they do perform a similar function, in that they
> limit the amount of total power that the following stages have to deal
> with, without producing IMD. What IS a roofing filter?
>
> DE N6KB
>
I'd say the roofing filter is the filter that determines the intermod
bandwidth. Typically it is in the first IF, should be very close to the
output of the first mixer, but there may be a very broad band stage in
between to properly load that first mixer for signals not passing
through the crystal, and any serious gain in the first IF should be
after the roofing filter. That first IF broadband terminating stage
should affect blocking the same at 1 MHz spacing as at 1 KHz spacing,
e.g. it needs to be really good. Like a gaggle of JFETs in grounded gate
(as in the Omni VI) or a transformer coupled feed back circuit used in
the Corsair II. Virtually transmitter driver stages but with low noise.
You can't get good performance out of a diode ring or switched four FET
ring mixer without that broadband termination stage after the mixer. Not
terminating the unwanted signals that come out of the mixer increases
mixer loss (most of the time though it can reduce the mixer loss),
increases mixer noise, and kills off the upper end of the dynamic range
(kills third order intermod).
It would be nice to have the roofing filter AT the antenna connection as
a few have been wired, but that makes for difficulty tuning beyond the
lower half of the DX segment on 40 meters.
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
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