[1000mp] Filters

[Jim Smith]

I agree with Hans.  Why?  

The 8.2 filters are before the AGC detection point.  The 455s are after. 
 Why does this matter?  In crowded band (contest) conditions a wide 8.2 
filter combined with a narrow 455 filter will allow a strong signal 
which falls within the 8.2 bandwidth but not within the 455 bandwidth to 
control the AGC.  So, you bought this expensive rig and you can't hear 
the guy you're trying to work because a signal you can hardly hear has 
control over the AGC and is pushing the S meter way over 9.  (Been 
there.  Cursed that.  Finally figured it out.  Didn't buy the T-shirt. 
 Did buy the 8.2)    

Why bother with 455 filters if you have 8.2s?  Strong signals outside 
the passband of the 8.2 will be heard because typical filter rejection 
starts to degrade beyond about 60 dB or so.  This means that a signal 
outside the filter passband which is 60 dB stronger than one in the 
passband will be heard and will be about as loud.  Add in a 455 filter 
with the same bandwidth and now it takes an out-of-passband signal 120 
dB stronger than the one you want to hear to be just as loud.  (That'll 
be me once I get everything working right.)

73 de Jim Smith    VE7FO
 
 

hans@remeeus.nl wrote:

>Hello Bob,
>
>I would suggest that you buy the 8.2 MHz IF filter first. Always put filters
>in the earliest stage you can.
>
>Yes, there is a noticable difference when you put a 455 kHz filter
>afterwards. You get more selectivity!
>
>73,
>Hans Remeeus.
>