[Icom] IC756ProII vs FT-1000MP/MkV/Field - Apples and Oranges

[George, W5YR]

Ed, I think that you are overlooking a couple of points re the architecture
of the Icom PRO with the statement 

" There are no narrow filters in the IC756ProII first and second I.F.
stages."

At the first IF of 64.455 MHz, there is a crystal 15 KHz roofing filter at
the input of the IF amplifier. At the second IF of 455 KHz, a 15KHz ceramic
filter follows the second mixer. this filter feeds the Noise Blanker Gate
which in turn feeds a second 15 KHz ceramic filter preceding the third
mixer to the final (DSP) IF of 36 KHz. 

Perhaps these are not what you would call "narrow filters" but they are
adequately narrow to ultimately prevent overload of the DSP A/D
converter(s) and that is all that is required. In a linear system, the
ultimate band-limiting can take place at any point provided that system
operation is linear up to that point. Proper distribution of stage gains
and appropriate AGC loop operation virtually ensure linearity in the PRO up
to the DSP stage.

"The reason that there are no narrow discrete filters in the first two
i.f's of the IC756ProII, is due to the need to allow wide bandwidth i.f. to
pass
to the band scope. This is also why the roofing filter is put into the
'third' i.f. stage, and only there to improve the performance of the Dsp
system further down line. Many other rigs put their roofing filters in the
first i.f. stage."

This statement is in error since it overlooks the other two roofing filters
described above. The PRO, like "many other rigs" does indeed have a crystal
roofing filter at the first IF.

The scope system in the PRO is fed from the first mixer output at an IF of
64.455 KHz via separate IF stages to a mixer which with a 77.8 MHz LO
converts to 13.345 MHz for the scope circuitry. There are two ceramic
bandpass filters in cascade in the mixer output which feeds another mixer
with a 12.89 MHz +/- 100 KHz LO to produce an output which via yet another
ceramic bandpass filter drives the scope IF system.

Note that all of the above circuitry for the scope operation is completely
independent of the circuitry following the first mixer which drives the
actual receiver IF and subsequent circuitry, including the DSP. No
compromises in receiver IF design are required to support scope operation.

Thus, it is incorrect to say that (a) only a single roofing filter is used
in the PRO - there are three in the main receiver channel and three more in
the scope channel; (b) that "the single roofing filter" is located for the
benefit of the scope operation, since the scope operates as a completely
independent receiver; and (c) that the presence of the scope has any
negative effect on the design and architecture of the "main" channel of the
receiver. 

It is also of dubious merit to ascribe performance ills to a lack of narrow
IF filters - I presume this refers to typical 2.5 - 3.0 KHz IF crystal
filters - in the PRO. 
While it is "conventional wisdom" that such narrow filters are all to the
good, despite their phase delay, uneven amplitude response  and other
negative effects, recall that we once had radios built to the "conventional
wisdom" that two r-f amplifier stages are better than one since
"sensitivity" is the principal measure of a good receiver. 

Times change and so does the architecture and design of our radios.
Conventional radios such as the Kenwoods are vastly different in many
respects from the PROs, the Kachina 505DSP, and even the Pegasus and
Jupiter. I don't think that it is any accident that most of the really
high-line commercial and military radios have used extensive digital
filtering and signal processing for the past several years.

Without further belaboring the point and mentioning even more
misstatements, let me close with the quotation

"When comparing radios, it is important the comparer know both rigs very
well . . ."

No offense, Ed - so put the asbestos suit away!   <:}

73/72/oo, George W5YR - the Yellow Rose of Texas         
Fairview, TX 30 mi NE of Dallas in Collin county EM13qe   
Amateur Radio W5YR, in the 56th year and it just keeps getting better!
QRP-L 1373 NETXQRP 6 SOC 262 COG 8 FPQRP 404 TEN-X 11771 I-LINK 11735
Icom IC-756PRO #02121  Kachina 505 DSP  #91900556  Icom IC-765 #02437

 

WD8ARZ wrote:
> 
> I resisted, I really did resist responding to this topic ...... however
> ..... I know opinions and emotions tend to rule, but technical aspects are
> more important to performance.