[R-390] Why use a Roofing Filter?

Fri Aug 1 01:31:47 EDT 2014

Charles, I noticed in Dallas' article that he puts his filter just before
the first IF...a stage before the mechanical filters.  Would a filter there
offer more improvement?  Your note suggested that the roofing mod that
Dallas describes simply sticks a 6 kc filter where the rest of the mech
filters are now.

I've been toying with ways to output the IF of my 390A for further
processing by SDR software.  It seems to me that something without any of
the mechanical filter influence would be good...let as much of the
filtering be done in software as possible, and with as wide as practicable
slice of spectrum as possible.  So, maybe a nice wide ceramic (?) filter
ahead of the 1st if, or even in place of the 16kc mech filter (which in my
receiver is dead).

         Thoughts?

                Dennis

On Wed, Jul 30, 2014 at 11:42 AM, Charles Steinmetz <csteinmetz at yandex.com>
wrote:

> Roy wrote:
>
>  I suspect that there are some of us who: - are not all that sure what a
>> roofing filter is - wonder why the R-390A needs to be "improved" by adding
>> one - think that maybe the conditions under which we use our radios at our
>> places, does or does not warrant the improvement.  So, a short description
>> of what the thing is, where it goes in the radio, and why it might be an
>> improvement would be welcome.
>>
>
> Basically, it is an IF filter that sets the maximum bandwidth of the
> system.  To be effective, it needs to go as far "upstream" as possible in
> the radio, to keep out-of-band energy out of as many stages as possible.
>
> These days, it is very common to make general coverage radios by
> upconverting received signals to a VHF first IF frequency (70 MHz or
> thereabouts is common).  This gives good image rejection, but exposes the
> radio to out-of-band energy at VHF frequencies.  Most radios do not have
> sharp RF filtering (because it is hard to get the RF filters to track), so
> this is a problem.  Enter the roofing filter -- installed at the output of
> the first mixer, it limits the frequencies that can enter the VHF first IF.
>  Typical BW is 20 kHz -- wider than the widest filter bandwidth in the
> final IF.  (These radios often have no RF amplification, and have "brute
> strength" first mixers and post-mixer amplifiers with 3rd order intercepts
> in the +40 dBm range to handle the strong out-of-band energy they will
> receive.)
>
> Contesters (people who spend their radio lives trying to pick weak signals
> out of pile-ups) frequently install much narrower roofing filters, to
> improve the closer-in overload performance of their radios.  It is not
> uncommon for these folks to install roofing filters that are only a few kHz
> wide.  Again, this filter needs to go as far upstream as you can get it --
> at the output of the first mixer.  (Note, however, that trying to design
> VHF filters that narrow is a losing proposition.  If that sort of
> performance is what one wants, better to start with a single-band,
> downconverting rx architecture instead of a general-coverage upconverting
> rx.  That also allows you to make the RF filters much narrower, too, which
> further improves close-in IMD performance.)
>
> The retrofitted "roofing filters" for boatanchors (and, in particular, for
> the 390/390A) are typically installed much farther downstream for
> convenience, thereby pretty much nullifying most of the benefit by leaving
> all of the preceding IF circuitry unprotected.  In the case of the
> 390/390A, because the VFO feeds the last mixer, the preceding IFs must be
> wide to accommodate a whole band -- so any roofing filter placed where it
> really needs to be to do its job would need to be a tracking filter.  In
> practice, people put them after the 3rd mixer, generally ahead of the
> existing 455 kHz IF filters (the mechanical filters, in the case of the
> 390A).  Placed there, the "roofing filters" can clean up the stop band of
> the narrower mechanical filters, but that's it.  And since the real IMD
> limitations in a 390A are the RF Amp (V201) and the First Mixer (V202), the
> retrofitted "roofing filter" can't do anything to improve the weakest links
> of these radios.
>
> In sum, the overall architecture of a 390A does not accommodate a real
> roofing filter.  People add what they think are roofing filters anyway,
> because they've heard that it is a good idea.
>
> A 6 kHz filter added to a 390A DOES reduce the close in IMD -- but ONLY
> compared to the existing 8 or 16 kHz filters.  The existing 4 kHz filter is
> better than the added 6 kHz filter.  So the improvement is not a matter of
> "roofing," it is simply a reflection of the fact that narrower IF
> bandwidths have better close in IMD performance that wider IF bandwidths --
> it is inherent in the nature of close in IMD measurements.  Replacing the
> existing 8 kHz filter with a 6 kHz filter (or using the 4 kHz filter) would
> do the same thing.
>
> I concur that in today's band conditions, the existing 8 and 16 kHz
> filters have no practical use, and that a 5 to 6 KHz filter is optimal.  If
> I were choosing a filter array from scratch today, I'd probably choose 1.5
> kHz, 2.1 kHz, 3 kHz, and 6 kHz.
>
> Best regards,
>
> Charles
>
>
>
>
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-------------------------
Dennis L. Wade
KG6ZI
Carmichael, CA