[Elecraft] RX Mush, why the fuss?

[Guy Olinger K2AV]

One problem here is that a lot of the discussion has been in analog terms.

We have to remember that in the K3, the digital AGC controlled by the AGC
parms is just that, a digital algorithm. What it does has nothing to do
with diodes. It can do anything weird and completely non analog resembling,
it only cares about the program code.

The graphs I have seen are entirely based on steady or very slow moving
signal states, not a photo of an audio rate amplitude varying signal
traversing the AGC knee. They seem intended, well-enough done, just to
convey the rudimentary function variants.

You need to have the program code to estimate exactly what is happening to
the AGC at audio rates. Good luck with that.

The only control you have over the non parameterized hardware AGC is to
reduce the gain in front of it so it isn't engaged. You can't turn it off,
it's always potential if the signal coming through the roofing filter is
getting up to around 20 over 9.

If you have a pile-up of 20 over signals, it's time to turn off PRE, or
turn on ATT, or back off the RF gain. Otherwise you are engaging the
hardware AGC, not sophisticated, which is only there to properly range
input to the ADC chip.

This ain't your grand-daddy's analog radio.

73, Guy

On Thu, Mar 2, 2017 at 11:16 PM, Al Lorona <alorona at sbcglobal.net> wrote:

> Oh no! I fear this is going to get bogged down in definitions. From
> Wikipedia:
>
>
> "Limiting can refer to non-linear clipping, in which a signal is passed
> through normally but 'sheared off' when it would normally exceed a certain
> threshold. It can also refer to a type of variable-gain audio level
> compression, in which the gain of an amplifier is changed very quickly to
> prevent the signal from going over a certain amplitude.
> * Hard limiting ("clipping") is a limiting action in which there is
>     * (a) over the permitted dynamic range, negligible variation in the
> expected characteristic of the output signal, and
>     * (b) a steady-state signal, at the maximum permitted level, for the
> duration of each period when the output would otherwise be required to
> exceed the permitted dynamic range in order to correspond to the transfer
> function of the device."
>
>
>
> AGC of the type we're discussing falls under this definition, and not the
> non-linear "back-to-back diode" clipping you might be thinking of. Look at
> the first graph under the section titled "Adjusting AGC SLP" and observe
> that for Slope=15, above about -104 dBm, the output follows a horizontal
> line. Moreover, the very first table that follows that graph shows that
> with Slope=15 if the input increases by 10 dB the output increase is
> virtually zero. That transfer characteristic is what audio engineers call
> "hard limiting".
>
> I also fear we may be talking past each other when we say 'linear' and
> 'nonlinear'. The AGC curve is 'nonlinear' in the sense that if we pour more
> RF input into the receiver, the output doesn't get any bigger. It's like
> that by design. All I was saying is that it doesn't sound good to me. I'm
> very happy there is Slope = 5.
>
> Call it whatever you want; the curve speaks for itself.
>
> Al  W6LX
>
>
>
>
> >>> There is a huge difference between AGC action (which is
>
> >>> simply a reduction in gain with linearity retained) and hard limiting.
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