Roofing filter

[William Carver]

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!)

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[Joe Subich, W4TV-4]

> 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: [hidden email]
> [mailto:[hidden email]] On Behalf Of William Carver
> Sent: Friday, August 07, 2009 3:16 AM
> To: [hidden email]
> 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:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html

______________________________________________________________
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Post: mailto:[hidden email]

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