--- On Wed, 2/25/09, Jim Brown <[email protected]> wrote:
>
> Someone just posted that they had trouble in pileups during
> the contest
> last weekend, but the radio worked better when they turned
> on the
> attenuator and turned down the RF gain. DUH! I learned
> that in 1957 --
> but younger operators never learned it, since modern radios
> depend so
> heavily on AGC.
Nothing personal Jim, but your post is a handy one to add to.
I've been licensed for over 50 years so I've been through all of the turn off
the AVC (dating myself) and ride the rf gain control business.
But I must say that having to turn off the AGC and ride the rf gain control in
a modern whiz-bang super-duper multi-kilobuck radio seems oxymoronic to me.
Why should I have to ride the gain? I thought all of this DSP stuff was
supposed to be smarter than I am. Shouldn’t the radio be smart enough to know
when a signal is so strong that the attenuator needs to be inserted to
"protect" the DSP or the second mixer, for example?
Besides the K3 AGC appears to never be "off" despite what the display reports.
For example while listening to a strong AM broadcast station, if I reduce the
RF Gain* with the AGC on so that the S-meter stops peaking# and adjust the
audio gain for comfortable listening and then turn the ACG off, the signal
practically disappears. If the AGC was truly off then there shouldn't be any
change. Adjusting the RF Gain higher to bring back the signal level will also
result in the S-meter following the signal peaks.
Running this experiment also seems to demonstrate a problem with the gain
distribution in this radio. If I reduce the RF Gain to the point that the
S-meter is just beginning to be affected, there is a distinct reduction in the
SNR. It's quite disconcerting to have a S9+50 dB, interference-free signal
that sounds noisy.
Comparing to my TS870 with the same signal, I can decrease the gain (increase
the S–meter reading) by at least 20 dB without noticing any reduction in SNR.
* RF Gain seems to be a misnomer; unless I'm mistaken (always a possibility)
there isn't a gain-controlled RF amplifier in the radio. The only hardware
that is obviously gain-controlled is the second stage in the first i-f
amplifier.
# I'm seeing 5 to 6 dB of increase in meter reading between no modulation and
modulation peaks. I guess I'm old fashioned but I thought that the AM carrier
strength should set the AGC level. This peak-reading AGC may also be a
contributor to the distorted audio that has been widely reported.
Wes Stewart, N7WS
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