[Elecraft] HaroldZ, Variable bandwidth filter, etc

[Steve Ellington]

Joe et. al.
Someone mentioned combining the Flex I/Q mixer with the roofing filters of 
the K3. I've noticed Icom started using the I/Q mixer with the IC-7800, 
IC-7700 and now the IC-7600. This seems to be how they can get by with only 
a broad skirted 3 kHz roofing filter and still have respectable performance. 
I'm wondering what it would be like if Elecraft did use a similar DSP but 
with narrow xtal filters ie. 200 Hz? It does sound like a good mix but I'm 
no expert on DSP.
Steve
N4LQ
N4LQ at carolina.rr.com
----- Original Message ----- 
From: "Joe Subich, W4TV" <lists at subich.com>
To: <Elecraft at mailman.qth.net>
Sent: Thursday, August 06, 2009 11:47 PM
Subject: Re: [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.
>
> 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
>>
>>
>>
>>
>>
>>
>> ______________________________________________________________
>> Elecraft mailing list
>> Home: http://mailman.qth.net/mailman/listinfo/elecraft
>> Help: http://mailman.qth.net/mmfaq.htm
>> Post: mailto:Elecraft at mailman.qth.net
>>
>> This list hosted by: http://www.qsl.net
>> Please help support this email list: http://www.qsl.net/donate.html
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:Elecraft at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html