Re: [TenTec] I Love the "ORION"!!! I just bought one!

[Jerry Volpe <kg6tt@tomorrowsweb.com>]

Jim Davis wrote:

> Question to anyone knowledgeable:
>
> Electrically what is the difference in performance
> between the three types of filters used today, namely
> the "Mechanical", the "Ceramic", and the so-called
> "Roofing"???
>
> I really don't have a clue but I'd like to LEARN!!!
>
> Jim/nn6ee 
>
>
> 		
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>  
I'm going to dabble around on this one. Hopefully, without getting too 
technical, I can give you a broad answer without deriving too many flames!

The term 'roofing' refers not to the actual filter design itself but to 
where a filter is placed within the intermediary frequency (IF) chain of 
the receiver. It basically sets the maximum bandpass for the entire IF 
chain and is usually made narrow enough to maximize 'image rejection' 
characteristics but broad enough to provide the  minimum bandpass needed 
by the reciever's widest operating mode (FM, AM, SSB, CW, etc.).

Until recently receiver designers typically utilized a single 'roofing' 
filter designed to pass the broadest bandwidth of incoming signals into 
the receiver's IF chain and then derived additional, mode specific, IF 
bandwidth selectivity further down the IF chain where there were greater 
choices in filter types and ofter reduced filter costs. The desire to 
produce more 'competition' quality receivers has designers looking more 
carefully at the roofing filter choice and in the case of today's more 
expensive ham transceivers typically offer multiple roofing filter 
options. An operator who mainly uses CW might want to choose 
increasingly selective roofing filters with bandpass characteristics 
specific for CW reception, while a operator you uses SSB but not AM or 
FM might want a roofing filter that tightens in on the essential 
bandpass needed for SSB. An operator you likes to listen to AM or even 
FM needs to be able to choose a very broad roofing filter.

A roofing filter can be comprised of a basic LC filter (typical in 
older, inexpensive receiver designs), a ceramic filter, a mechical 
filter, or a crystal filter. The actual names of the filter types 
describe what the filter uses as resonating internal components to 
'constrain' the flow of RF energy outside the desired filter bandpass.

Generally speaking LC style filters are rarely used today (other than 
for amplifier stage coupling) in IF's ranges above 50 or 100 kHz due to 
the difficulty in generating really high 'Q' filters at higher frequencies.

Ceramic filters are by far the least expensive 'usable' filter to 
manufacture and can be made to operate effectively from medium to very 
high frequency IF center frequencies (455 KHz and above). When compared 
to mechanical or crystal designes, ceramic filters rarely provide very 
steep shape factors and they tend to suffer from poorer band reject 
characteristics. Nevertheless, ceramic filters do tend to be the filter 
of choice for average 'roofing' filter applications and sometimes for 
later IF stages as well.

Mechanical filters (I believe originally developed commercially by 
Collins Radio... or at least made popular in Collins designs of the 
1950s-1970s) can be used quite effectively in IFs of  low to medium 
center frequency design (455 KHz to several MHz)  where they provide 
very steep shape factors (little 'ringing') and excellent band reject 
characteristics for IF's. Quality mechanical filters tend to be somewhat 
expensive to manufacture and are not operationally practical at higher 
IF center frequencies so they would not be used as a 'roofing' filter in 
todays multi-conversion receiver designs.

Crystal filters have been around since the 1930s (in very simple 
designs) and can be manufactured to operate over a very wide range of 
center frequencies and with shape factors and ultimate rejection 
characteristics that rival mechanical filters. Since crystal filters can 
be designed/ manufactured to work in nearly every IF frequency range 
typical in ham receiver designs they are becoming to filter of choice 
for more exacting IF applications including their use as higher quality  
'roofing filters'.

I hope this info is useful.

73,
Jerald, KG6TT


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