[R-390] R390A Roofing Filter Kit Update

[Charles Steinmetz]

Perrier  wrote:

>The kit will consist of a Collins 5.7 KHz mechanical filter (P/N 
>526-8695-010), PC mounting board, two ferrite cores, wire and a copy 
>of the Dallas Lankford article covering the coil winding and 
>installation instructions.

As I noted in a previous post (7/30/14), any IF filter added to a 390 
or 390A will not be a real roofing filter and will not provide the 
advantages that a roofing filter can provide to receivers designed 
with other architectures.

The problem of out-of-band IMD was addressed by Collins and, for 
better or worse, the design the engineers settled on was the 
multistage tracking RF that we all know and love.  By its nature, 
this architecture prevents the use of a roofing filter.  The Collins 
engineers COULD (in principle) have added a tracking IF roofing 
filter, but they didn't -- and you can thank them every time you 
align your 390/390A for not doing so.

There is always an improvement in close-in IMD if you use a narrower 
IF filter -- that is inherent in the nature of close-in IMD 
measurements.  The existing 4kHz and narrower filters already have 
lower close-in IMD than an added 5.7kHz or 6kHz filter, and they will 
not be helped by the addition.  The only help an added filter can 
provide is by narrowing the IF compared to the existing 16kHz and 
8kHz filters.

So, if one is going to install a filter narrower than 8kHz, there is 
no benefit (and there are potential detriments) to putting it in-line 
with the existing mechanical filters.  It would be better to replace 
either the existing 16kHz or 8kHz filter with the new filter, thus 
avoiding the insertion loss of the new filter on top of the existing 
4kHz and narrower filters when they are selected as well as any 
unfortunate interactions that might occur between the responses of 
the 4kHz and narrower filters and the response of the new filter.

Or, much simpler, just use the existing 4kHz filter if you find that 
you need better close-in IMD performance than the 8kHz and 16kHz 
filters provide.  As noted above, the existing 4kHz and narrower 
filters already have lower close-in IMD than an added 5.7kHz or 6kHz filter.

 From my previous post, lightly edited:

>A roofing filter 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.
>
>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 instead of the first mixer, the IFs preceding the last 
>mixer must be wide enough 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 what 
>they call "roofing filters" after the 3rd mixer, generally ahead of 
>IF amp V501 and the existing 455 kHz mechanical IF filters.  Placed 
>there, the "roofing filter" 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.  It's really not.
>
>In today's band conditions, the existing 8 and 16 kHz filters have 
>no practical use, and a 5 to 6 KHz filter is optimal for pretty much 
>all AM listening.  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