[Elecraft] A comment about Receiving

[Bill W4ZV]

Don Wilhelm-4 wrote:
> 
> The bandwidth can make a difference - the noise is broadband while the 
> signal is narrowband.
> If the bandwidth of the receiver input is wide, then a preselector will 
> make a difference because it decreases the total noise that the receiver 
> is handling.
> 
> Note that many(most) tuners will not produce that pre-selector effect.  
> Only those with a resonant circuit will do that.  Most common T-network 
> of L-network tuners create either a high pass or a low pass filter.
> 

I still don't understand.  If signals pass through a 500 Hz roofing filter
on the front end of the rig, what effect can wideband noise have on the
receiver?  This assumes the noise is not strong enough to trigger the
radio's hardware AGC, but wideband noise is typically far below that level
(about S9+30). 

Unless the tuner has an extremely high-Q (i.e. like a crystal filter) I
don't see how it can help a rig like the K3 with a CW roofing filter.  Most
tuners' bandwidths are in the order of hundreds of kHz...not hundreds of Hz.

73,  Bill  W4ZV


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