[ICOM] PRO III 6kHz Roofing Filter

[Bill Tippett]

VA7OJ wrote:

 >Not to enter into a debate and tie up the reflector, but that is precisely
why George (blessed be his memory) and I always used the phrase
"properly-designed".

         OK Adam...I'm sure George will find the perfectly
linear devices to accomplish that where he is now.  Meanwhile,
we are unlikely to see it happen here on Earth anytime soon,
and should not imply such a design exists...except perhaps in
the minds of some overly exuberant advertising writers.  ;-)

                                         73,  Bill  W4ZV

-----Original Message-----
From: <http://mailman.qth.net/mailman/listinfo/icom>icom-bounces at 
mailman.qth.net [mailto:icom-bounces at mailman.qth.net] On
Behalf Of Bill Tippett
Sent: 06 June 2005 04:22
To: <http://mailman.qth.net/mailman/listinfo/icom>icom at mailman.qth.net
Subject: [ICOM] PRO III 6kHz Roofing Filter

VA7OJ wrote:

  >Apparently the difference is not earth-shattering - which bears out the
late George W5YR's (and my) contention that in a properly-designed DSP
receiver, the DSP filters do the "heavy lifting", not the roofing filter.


          Your website results below demonstrate otherwise.
The 47 kHz offset result shows the dynamic range of the DSP in combination
with the relatively narrow 15 kHz roofing filter, since both interfering
signals fall outside the 15 kHz passband.  When the interfering signals are
narrowed to 4.4 kHz spacing, both then fall within the roofing filter, which
demonstrates DSP performance *without* any help from the roofing filter
(since it passes both interfering signals).  The difference in performance
is about 40 dB in IP3 and 30 dB in 3rd order IMD dynamic range.
The idea that DSP can somehow overcome distortion introduced in the IF
stages preceding the DSP stage is simply an advertising myth, which should
not be propagated.

                                                          73,  Bill  W4ZV

<http://www.qsl.net/ab4oj/icom/ic756pro3/i1odp.html>http://www.qsl.net/ab4oj/icom/ic756pro3/i1odp.html


Results for 47 kHz offset:

2f1 - f2: Power level of either of 2 equal test signals to raise IMD
products 3dB over the noise  = -24 dBm

IP3 = + 29.5 dBm

IMD3 Dynamic Range = 107 dB

2f2 - f1: Power level of either of 2 equal test signals to raise IMD
products 3dB over the noise = -22 dBm

IP3 = +32.5 dBm

IMD3 Dynamic Range = 109 dB


Results for 4.4 kHz offset:

2f1 - f2: Power level of either of 2 equal test signals to raise IMD
products 3dB over the noise = -  53 dBm

IP3 = -15.5 dBm

IMD3 Dynamic Range = 78 dB

2f2 - f1: Power level of either of 2 equal test signals to raise IMD
products 3dB over the noise = -50 dBm

IP3 = - 9.5 dBm

IMD3 Dynamic Range= 81 dB