Chen,
> Imagine that there are three parameters, the current two, plus a
> "upper threshold," after which the slope becomes "15" (i.e. flat at
> S0 to THR, then constant "SLOPE" until "upper threshold" and then flat
> from there until the front end starts to smoke),.
The AGC is already a three piece curve. Below the AGC threshold (-117
to -99 dBm depending on THR) the audio:RF response is 1:1. Above the
threshold the audio:RF ratio varies between 0.4 dB/dB to 0.05 dB/dB
(according to K8OZA's measurements) until the HAGC activates between -43
and -48 dBm at which time the output becomes flat (no further increase).
What this discussion appears to be asking for is the option to raise
the level of first inflection point (AGC threshold) from approximately
-99 dBm with THR=08 to somewhere in the mid -70 dBm range without using
the attenuator and losing the weak signals. That is, nobody wants to
give up the ability to receive a -128 dBm signal by using a 10 or 20 dB
of attenuation when the receiver is easily capable of detecting a -130
dBm signal without use of a preamplifier but they do want reasonable
"level cues" to use in separating signals close to the same frequency.
What is needed is to broaden out the steps in THR from as little as 1
dB per step to something like a uniform 6 dB per step or adding more
steps (e.g., 16 choices at 3 dB/step so that the threshold can be set
over a -117 to -70 dBm range rather than the current limited -117 to -99
dBm range (K8OZA measurements).
Since HAGC does not activate until the RF input reaches -48 to -43 dBm,
one must assume that the IF amplifier, 2nd Mixer and ADC are capable
of functioning properly well beyond the levels generated at -70 dBm
input and raising the threshold even if it means using a flatter slope
would not cause serious performance issues.
73,
... Joe, W4TV
On 12/9/2010 2:05 PM, Kok Chen wrote:
> On Dec 9, 2010, at 12/9 9:17 AM, k...@aol.com wrote:
>
>> But, since AGC works on every signal in the passband, if there are
>> two signals - one S9 and the other S3 appearing at the same time,
>> the S3 signal will disappear because the AGC is knocking down the
>> S9 signal to an S3 level and thereby also reducing the S3 signal to
>> the noise.
>
> My impression from reading what has transpired is that some people are
> more concerned that when the S9 signal is *not* there, that the S3
> appears to be too loud?
>
> We know that the S9+ signals have to be kept from saturating the A/D
> converter (or even kept in check so the large signal doesn't do
> naughty things to the IF amplifier and the SA612 second mixer -- I
> don't know which of these three is the wimpiest in the chain). So
> some AGC is neccessary.
>
> Perhaps people will be happier if the AGC response is not as linear
> (in the log-log scale) as the one in the K3 once you have past the AGC
> threshold. I.e., perhaps people would care less if an S7 signal sound
> as loud as an S9 signal, but they do want the S3 signal to sound less
> loud as the S7 signal. A non-linear transfer function like that is
> certainly "easy" to implement (as long as I don't have to be the one
> to implement it :-), but will this make more people happy? Who knows?
>
> Basically, this is what I am simple-mindedly visualizing: currently,
> there are only two parameters to control a piecewise linear curve.
>
> Imagine that there are three parameters, the current two, plus a
> "upper threshold," after which the slope becomes "15" (i.e. flat at
> S0 to THR, then constant "SLOPE" until "upper threshold" and then flat
> from there until the front end starts to smoke),. You then smooth out
> the piecewise linear curve using a pair of homotopy between those
> three partitions. You will end up with (1) a smooth curve instead of
> the piecewise linear one currently, and (2) a way to say "I care more
> about distinguishing an S3 from an S9 signal than I do an S7 and S9
> signal."
>
> Notice that if a simple homotopy is applied to smooth the AGC transfer
> function, and even if "upper threshold" is set to S7, you will still
> hear a difference between S7 and S9 -- just much less than between S3
> and S9. Also, if the AGC curve is no longer piecewise linear you will
> still hear the S3 signal even if the lower threshold is set to an S3.
>
> I still think the only way to satisfy "everybody" is if everyone would
> homebrew their own rig, or sell an "open" commercial rig where you are
> allowed to make any mods -- hardware, firmware, or software. See
> first option :-) :-).
>
> 73
> Chen, W7AY
>
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