[CQ-Contest] Fwd: [FCG] riding the RF gain...

[Clive Whelan]

 Tree N6TR wrote:
> Typically, they show 3rd order products down about 30 or
> 35 db from the two signals.  With reduced RF gain, it is 
> more like 50 or 55 db.  This might improve the ability to
> hear a weak signal in noise, or lots of QRM.
>



Hmm don't really understand those absolute figures. Have I 
understood correctly. I take this to mean the following:

Signal A on ( say) 21025
Signal B on ( say) 21028
say at equal strength


IMD products audible on 21022 and 21031 30 or 35 dB down?


If so this would be horrendous and the band would be full of 
fragments of broken CW when Station A and B were key down 
together but not just one or the other. This was the original 
problem with the TS870S ( see Radcom and QST reviews ca. 1996), 
and the IMD products were *only* 56dB down, as opposed to 90dB 
or more for signals spaced >15kHz. This was said to be due to 
the inherent problems of DSP filtering and poor grade roofing 
filters, but I suspect was rather inadequate gain distribution, 
i.e. too much RF and not enough IF. I believe this was addressed 
by Kenwood sometime in 1996. However the 870 is still not 
perfect and I experienced the above problem on 21Mhz in the 
recent CQWW CW, finding it necessary to introduce 6dB of 
attenuation, in addition to operating in the AIP mode ( minus 
one of the RF amps I think).

I have *never* experienced this with the FT1000MP nor e.g the 
TS930, and suspect the same would apply to the TS850. N.B.  that 
this does not mean that the FT1000MP is a better ( contest) rig 
than the TS870S, in fact I believe the opposite to be the case, 
and I have recently retired my FT1000MP/TS930 combo in favour of 
a pair of TS870Ss, the reasons for which are beyond the scope of 
this note. Of course everyone has their personal preferences 
-caveat ymmv!-, and my only real point is that surely IMD 
products only 30-35dB down  can't be right?

Of course I may have misunderstood in which case I am happy to 
be disabused.


73


Clive
GW3NJW
gw3njw at gw7x.org
Contest Cambria-http://www.gw7x.org


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