[R-390] Why use a Roofing Filter?

Bob Camp kb8tq at n1k.org
Fri Aug 1 08:36:58 EDT 2014 

Hi

Ignoring the “why” of R-390 + SDR there are some right and wrong ways to do it.

————

1) You need a way to drive the AGC on the radio, or you need to pick off ahead of the IF.
2) You need a bandpass shaped dither source if you are going to pick off after filtering. 
3) You need pretty good shielding on your SDR to keep it’s clocks out of the R-390

There are other nice to do things:

4) Compensating for the filter passband is nice if you pick off after a filter
5) Driving the S meter makes tuning a bit easier.
6) Using the audio volume control on the radio makes operation a bit more natural. 
7) Using the bandwidth control switch on the radio ….
8) Using the AM switch on the radio …
9) Using the AGC switch ….

If you go the whole way down the list, that’s a lot of playing with the radio. If you go part way, you have something that’s a bit more of a hassle to operate than it could be. The net result of the list would be mating up a SDR IF up with the RF deck of an R-390.  The resulting radio would look cool. It would have the overload and drift issues of the original RF deck combined with the normal issues of an SDR. It *might* get some benefit from the RF filtering. 

Yes I’ve given this a lot of thought on and off over the years. 

Bob




On Aug 1, 2014, at 1:31 AM, Dennis Wade <sacramento.cyclist at gmail.com> wrote:

> 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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> 
> 
> 
> -- 
> "If they trust you, it is an extraordinary privilege, and you simply can't
> abuse it."
>      - A. Alan Post  1914-2011.  California Legislative Analyst 1949-1977.
> 
> -------------------------
> Dennis L. Wade
> KG6ZI
> Carmichael, CA
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