[Elecraft] Roofing filter

[Joe Subich, W4TV]

> It's not true that once signals get through the xtal filter 
> they cannot be removed.  A DSP filter CAN remove them...after 
> all, the FLEX radio works without ANY roofing filter! 

That's not true.  Even the Flex will not be able to remove a 
-50 dBm signal (S9 +20) and allow you to copy a -135 dBm signal 
(at the noise floor) 50 or 100 Hz away.  That's a blocking issue 
not an IMD issue.  True IMD requires two or more interfering 
signals with a specific relationship and with narrow filters 
one of the two tones is almost never inside the filter.  

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: Friday, August 07, 2009 3:16 AM
> To: Elecraft at mailman.qth.net
> Subject: [Elecraft] Roofing filter
> 
> 
> It's not true that once signals get through the xtal filter 
> they cannot be removed. 
> A DSP filter CAN remove them...after all, the FLEX radio 
> works without ANY roofing filter! 
> But DSP can only do it when fed with undistorted signals 
> within the range of its A to D converter. The circuitry 
> following the filter needs to have very low distortion.
> 
> The K3 has more than 100 dB of dynamic range at the first 
> mixer, say for signals spaced 10 KHz apart. What if two 
> signals are 500 Hz apart, both passing through the 2.8 KHz 
> filter? Can we expect a 50 Hz wide DSP filter to filter those 
> two tones out? I expect it. My Orion-one was fitted with a 1 
> KHz filter and it worked very well on CW. I have a homebrew 
> receiver with a 3.1 KHz wide filter and it worked well on CW. 
> 
> 'Course in both cases, no matter how good without, they'd be 
> better if preceded by a CW-width xtal filter. And at those 
> spacings the LO phase noise has to be lower for the desired 
> weak signal to be heard. Maybe we're talking K4 here.
> 
> Bill - W7AAZ
> 
> P.S. Had a nice chat with a guy in Atlanta tonight, first QSO 
> of newborn K3. (with, I must add, 2.8, 0.4 and 0.25 KHz 8 
> pile filters!)
> 
> ______________________________________________________________
> 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