Carl has written:
> Chip,
>
> The AGC in the RX340 has a 120 db range. There is 80 db
> for analog AGC and 40 db for the DSP AGC. As long as
> there is not more than 40 db difference in the signals passing
> through the16 khz roofing filter, the DSP AGC controls
> everything and the DSP filters will work with no AGC pumping
> whatsoever.
> But when you get a difference in signals through the 16 khz
> roofing filter of more than 40 db, the analog AGC has to
> come into play to keep the DSP from being overloaded.
> So this is a design limitation of the DSP system only having
> a 40 DB range. I will say that the analog system employed in
> the RX340 is excellent and makes this limitation hardly
> noticable except in the extreme signal strange range you noted.
Well, out here in KH6 land we just do not rcv signals that
are ever much more than 20 to 30 dB over S9; we are just
too far away from the Big Gun stations. There are some
kW+ stations out here: KH7R on Oahu; KH6CC on the Big
Island of Hawaii, and one or two others. However, they
are seldom heard at all entering only occasional contests
these days (KH6CC usually only on 160, and his CW
key clicks are heard over all the islands, hi - he uses
TS-830's and a BIG home brew amp). Ken, KH7R now
lives in Las Vegas, and his station has not been used
multi-multi in some time now; believe he plans to put
the station up for sale this spring, if any one is interested
in 6 towers, with monobanders over monobanders, etc.)
But this post is about the relation between signal to noise
ratio and the IF gain (Manual Gain control of the RX-340).
Late in the afternoon yesterday KH7T down on the Big Island
and I ran some checks on this. Found that I could crank in
some 30 to 40 dB of IF gain cut, dropping the noise level
greatly, while the KH7T signal remained "loud". He ran
100, 50, 10, and less than 1 watt output. Yes the 340
S meter tracked the dB reductions almost exactly, give
or take a bit of NVI radiation QSB on 40 meters in the
late afternoon, but the AGC action of the 340 caused
the SSB and CW signals to remain, audibly, the same
"loudness". The path between KH7T, John, and KH7M,
Jim is about 320 miles, and largely over the Pacific.
It appears the S meter derives its' signal strength somewhere
ahead of everything already discussed: ahead of all agc,
preamp, attenuator and IF gain adjutments. The S meter
reads independently from any of these! A unique property
of the RX-340, so far as I know.(Maybe the K2 also?).
What I learned was that much of the accompanying noise
along with weak signals as recvd out here, can be reduced
by lowering the IF gain level without loosing the strength of
the desired signal; that is, the signal to noise ratio is
improved!
Must experiment with this some more. Ten Tec has built a
very interesting radio in the 340! Wonder how much of
this will be reflected into the coming Orion (note the OmniVI+
has been discontinued now by Ten Tec, see their web site).
73, Jim KH7M
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