[TenTec] IF filter bandwidth and audio power output.

[Ken Brown]

>As I understand, when the bandwidth is reduce by 1/2 then shouldn't the 
>total audio output be reduced by 6 dB?  And thus S/N be improved by an equal 
>amount?
>  
>
The answer to that question is "it depends."

If the audio output is being produced by an RF input (noise or signal) 
that has a uniform power density across the full bandwidth of the wider 
filter, then cutting the bandwidth in half will cut the audio power 
output in half, provided there was no AGC or limiting or other controls 
changing the overall system gain with the different bandwidths. That is 
a reduction of 3 dB.

If the audio output is being produced by an RF input that does not have 
a uniform power density  across the full bandwidth the audio output 
could remain the same, or it could be reduced by most any amount, 
including a lot more than 1/2.

If the RF input is a CW carrier in the center of BOTH the narrow 
bandwidth filter and the wider one, the the audio output power should 
not change when changing bandwidths.

If the RF input is a CW carrier outside the bandwidth of the narrow 
filter (or on it's skirt) but well inside the bandwidth of the wider 
filter, then the reduction in audio power output when switching to the 
narrow filter could be much greater than 3 dB.

So it depends on the input RF and tuning of the receiver.

All of the above is not Orion II specific. I don't have an Orion, so if 
the Orion does something different from the above, I could only guess 
that there are gain changes made when different filter bandwidths are 
selected.

Generally speaking with most receivers, if you have a CW carrier tuned 
just right, the audio power output produced by that signal should not 
change when you go to a narrower filter. If it does, you either didn't 
have it tuned right (so that it is in the center of either the narrow or 
the wide filter), or the filters have different losses (that are not 
compensated for) or do not have the same center frequencies, or the CW 
carrier has significant noise sidebands which fall outside the narrow 
filter bandwidth and inside the wider filter's. Another possibility is a 
very weak CW signal, such that the noise (either from the antenna, or 
the receiver) is the predominant producer of audio power. In that case 
cutting the bandwidth by 1/2 will cut the audio power by 1/2.

DE N6KB