[1000mp] Re: filter strategy

Tue, 06 Aug 2002 01:21:48 -0700

IMHO the 2nd IF slots, i.e. the 8 MHz ones, should be filled first .   
 It is very disconcerting (other, stronger, terms come to mind) when 
trying to work a weak station on cw to see your S meter jumping up to S9 
+ on cw as the AGC is being controlled by a signal outside the filter 
passband.  Same of course on SSB.  In other words, with no filter in the 
8 MHz slot and a narrow filter in the 455 kHz slot, a strong signal just 
outside the passband of the 455 filter will "pump" the AGC rendering the 
weak signal somewhere between hard to copy and inaudible as the AGC 
reduces the gain to accomodate the strong (but pretty well inaudible) 
signal.  If, instead, there were a filter in the 8 MHz slot, the strong 
signal would be filtered out before it got to the AGC system.  

You can check this out for yourself on a MkV (don't know anything about 
the other MP rigs) without any additional filters.  
Set the mode to cw.
Set BANDWIDTH to NOR.
Set APF to any of the narrow selections, 240, 120 or 60 Hz.   Suggest 240.
Tune in a strong am broadcast signal, S9 or better, so you hear the beat 
note of the carrier nice and loud.
Tune back and forth.  Note how, when tuning in one direction, the beat 
note becomes inaudible while the S meter doesn't change until you have 
moved a couple of kHz or so.

If you want to kill any interest in contesting that someone might have, 
sit him down at an MP with all the 455 slots filled and none of the 8 
MHz during a busy contest when conditions are good and tell him he's 
doing a single 20 and that he must use the narrow filters.  Unless he's 
an exceptional op, my guess is he wouldn't last 3 hours before giving up 
in frustration.

Hope this helps and 73

de Jim Smith    VE7FO 

Gorsline wrote:

>In several postings about how to filter the MP family, there's been contradictory advice for how to start if you're not able to add matching filters in both IFs.  Should you start with filling the 2nd or 3rd IF slots first?  One posting the other day recommended starting in the 3rd IF, where I had always heard that the closer to the front end, the better.
>So which is right and why?  Has anybody done any practical measurements?  How does DSP use change the conclusions?
>
>tnx  73,  George  VE3YV / K8HI
>
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