Posted by
Guy, K2AV on
Nov 19, 2010; 3:44pm
URL: http://elecraft.85.s1.nabble.com/K3-Wide-AGC-threshold-tp5755595p5755818.html
That's not the "wide" AGC. It's the defensive hardware AGC that is in
the actual IF string after the roofing filter before the analog to
digital converter (ADC). It's function is to limit the level
presented to the ADC as you surmise. It is also primitive, easily
seen in the schematics. Handling strong signals without any AGC
pumping, using an SSB roofer ahead of a CW DSP bandwidth, I doubt was
a design goal of the RX. What you experience is common for those
without a CW roofer.
There was some variation in how the K3 handled this which was up in
beta firmware very early on, and various settings were evaluated by
the community. I'm sure you can find the blow-by-blow back in the
archives.
73, Guy.
On Fri, Nov 19, 2010 at 10:17 AM, Bob Henderson <
[hidden email]> wrote:
> As a new K3 owner, it is entirely possible I am treading ground already well
> trodden. My apologies should this be so.
>
> While waiting for delivery of an Inrad 500Hz roofing filter, I have been
> accumulating experience with S/N 4904. I am enjoying my new rig greatly,
> although I confess to finding an odd thing or two puzzling. One of these
> relates to the level at which wide AGC has been set.
>
> My primary shack transceiver is a home built, G3XJP designed Picastar, which
> has a similar architecture to the K3 but is based upon the much older Analog
> Devices ADSP2181 and AD1885 AC'97 codec. Unlike the K3 with its multiple
> choice roofing filter bandwidths, my Picastar uses a single 4kHz wide roofer
> in all modes. The thing puzzling me is that I find de-sensing by strong
> signals within roofer b/w but outside DSP b/w far more noticeable on the K3
> than on Picastar. I expected the opposite to be true given the Picastar
> codec has only a 16 bit ADC whereas the K3 has 24 bit.
>
> A quick measurement reveals K3 wide AGC threshold around 70dB above MDS
> whereas on Picastar it is closer to 80dB. With Star's 16 bit ADC this
> equates to around 10dB headroom whereas the K3's 24 bit ADC implies headroom
> in excess of 30dB. On the face of things it would appear some 20dB of extra
> defence against strong signals in the no man's land between roofer b/w and
> dsp b/w has been sacrificed. I am wondering why? Perhaps I am missing
> something?
>
> Thanks.
>
> Bob, 5B4AGN
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