[Elecraft] [K3] Unanswered Questions ...

[Joe Subich, W4TV]

OK, that brings me back to the original question ... since the 
standard 2.7 KHz, 5-pole filter is 2900 Hz at -6dB which should 
be wide enough on receive for AM (by shifting the receive passband 
to use one sideband) and for ESSB (except for the ultra fidelity 
group), is there any harm in using the FM filter for AM and ESSB 
transmit?  

If one wants maximum performance on AM/ESSB receive, I understand 
the 6 KHz filter will provide better IF image rejection (now I'm 
really interested in the FM filter curves - including the stopband
and ultimate rejection specification).  However, if the priorities 
are maximum performance on SSB and CW (including protection from 
AGC pumping), a slight (?) reduction image rejection due to the 
wider filter may be an acceptable compromise given the limited 
number of available filer "slots". 

What is the ultimate rejection in the 8-pole filters, the on-line 
curves only go to -80 dB?  Some of the filters (250, 400, 1.8K) 
show signs of "flaring out" at 70 to 75 dB down and for others (6K, 
2.8K) the scale is wide enough that the skirts are "off screen" 
before they reach -80 dB.  

Of course, this may all be academic when variable width filters 
are available.  

73, 

   ... Joe, W4TV 
 

> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net 
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Lyle Johnson
> Sent: Wednesday, October 24, 2007 11:32 PM
> To: Elecraft at mailman.qth.net
> Subject: Re: [Elecraft] [K3] Unanswered Questions ...
> 
> 
> >> In FM mode, capture effect helps suppress the effect of "image" 
> >> leakage.  AM and (E)SSB modes have far more dynamic range and the 
> >> 15 kHz IF image must be suppressed by some 10s of dB more than is 
> >> required for excellent FM operation.
> > 
> > Are you speaking of receive performance or transmit performance?  
> 
> Receive characteristics.
> 
> > ...
> > Is there something in the digital domain that I'm missing - 
> > aliasing, artifacts, etc. that effects this?  
> 
> No, the issue is IF image rejection.
> 
> 73,
> 
> Lyle KK7P