[Elecraft] Price Comparison and Comment

[Bill Tippett]

>--- Don Wilhelm <w3fpr at earthlink.net> wrote:
>
> > Bill,
> >
> > All of what you say is true *EXCEPT* for one item -
> > and that is the AGC.
> >
> >
>[...]
>
> > Now, if the signal you are trying to copy is an S-1
> > or S-2 level and an
> > S-9 signal comes into the roofing filter passband
> > (again it may be out
> > of the DSP passband and will not be heard), the
> > receiver gain will be
> > reduced and you will no longer hear the S-1/S-2
> > signal - just like QSB,
> > but it is QSB induced in the receiver, not due to
> > propagation effects.

VA7W:
>Eeeeek! time to kill the agc and back down on the RF
>gain and up the audio gain?

         To some degree but you cannot actually kill AGC
in a rig like Orion (or perhaps K3 which apparently uses
similar analog plus digital AGC).  The reason is that the
analog AGC (actually more of a peak limiter) ahead of
the DSP must *always* be active to protect the DSP
from being over-driven.

         Your "Eeeeek!" sound is a good description
of the sound of a DSP that is over-driven.  In Orion
digital AGC in the DSP is also never fully turned off.
"Off" is simulated by setting AGC Decay to a high
rate (1000 dB/s) but it is never actually off even when
"Off" is selected on the front panel.  I've discovered I
can make it a little more "Off" by setting Decay to
2000 dB/s in the Programmable AGC mode, but that
is also not truly "Off" (which has an infinitely fast
decay rate).

         We can basically throw away some of our
ideas about AGC based on analog rigs like the
K2.  DSP rigs simply don't work the same way.
I'll repeat again...I've never heard classical AGC
pumping in Orion.  If you are using a 500 Hz filter
(+/- 250 Hz BW) and a S9+30 station is 500 Hz
away (then attenuated ~6 dB by the filter skirt),
you will hear keyclicks, phase noise, etc long
before you hear AGC problems.  Also "pumping"
is not a good description of a signal overdriving a
DSP.  To me It sounds more like a machine gun
suddenly going off next to your head.  It's more
of a digital (i.e. on/off) staccato effect...not the
classical analog gain pumping effect.

                         73,  Bill  W4ZV