[ICOM] PRO III 6kHz Roofing Filter

[Bill Tippett]

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


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