[Elecraft] RX Mush, why the fuss?

[Al Lorona]

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.