[Elecraft] HaroldZ, Variable bandwidth filter, etc

[Joe Subich, W4TV]

> What is the two tone dynamic range of the K3 when both 
> signals are inside the roofing filter bandwidth?
> 
> Close-in IMD measurements are virtually always done using a 
> CW bandwidth filter. None of the published data (ARRL, 
> Sherwood Engineering) tests IMD inside the DSP process 
> without xtal filter selectivity.

With roofing filters that are matched to the appropriate bandwidth 
for the operating mode dynamic range inside the roofing filter is 
not a consideration.  Once the interfering signal gets inside the 
roofing filter, it's interference and nothing is going to get rid 
of it.  

For example, the point of a 200 Hz filter is not so much to reduce 
IMD (remember the interfering signals need to be 200 Hz and 400 Hz 
(or 300 Hz and 600 Hz, etc. away to cause IMD "on frequency") but 
to prevent close in signals from effecting AGC, etc.  As Rob Sherwood 
has said many times, the problem with close in interference is just 
as likely to be "dirty" signals (phase noise, key clicks, and 
transmitter IMD) as it is to be receiver effects as long as the 
dynamic range is 90 dB or more.  

73, 

   ... Joe, W4TV 
 


> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net 
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of William Carver
> Sent: Thursday, August 06, 2009 9:56 PM
> To: Elecraft at mailman.qth.net
> Subject: [Elecraft] HaroldZ, Variable bandwidth filter, etc
> 
> 
> What is the two tone dynamic range of the K3 when both 
> signals are inside the roofing filter bandwidth?
> 
> Close-in IMD measurements are virtually always done using a 
> CW bandwidth filter. None of the published data (ARRL, 
> Sherwood Engineering) tests IMD inside the DSP process 
> without xtal filter selectivity.
> 
> My experience with a homebrew K3-like topology, except a 
> strong diode second mixer (instead of NE602-type mixer), is 
> the limiting factor INSIDE THE ROOFING FILTER bandwidth can 
> be the DSP performance. If you put a FLEX DSP processor 
> behind a K3 2.8 KHz SSB filter you would have FLEX numbers 
> close in, K3 numbers outside the roofing filter. The 
> advantages of both. 
> 
> So rather than bandaid by buying more filters, or push for a 
> variable filter, if the dynamic range inside the filter is 
> not good enough one should concentrate on (1) strengthening 
> the post-xtal-filter intercept of the receiver circuits and 
> (2) reducing the close-in phase noise of the LO. 
> 
> Bill - W7AAZ
> 
> 
> 
> 
> 
> 
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