[Elecraft] Re: AGC Design of the K3

[wayne burdick]

Shane White wrote:

> I noticed a remedy for the following problem on the K3 enhancement 
> list -
>
> Some users prefer tuning the band with a wide filter bandwidth 
> selected (for
> example, using a 2.7 or 2.8 kHz SSB filter in CW mode rather than a 
> 500 Hz
> or narrower filter). Strong off-frequency signals (above about S9+5) 
> that
> are inside the crystal filter passband can then activate the hardware 
> AGC
> circuit, reducing the level of desired signals.
>
> Shouldn't a radio's AGC only use signals within the selected final IF
> passband width to control gain?

Shane,

The K3 has two kinds of AGC: DSP AGC, which is effective up to about 
S9+20 or +30, and hardware AGC, which only kicks in above this level to 
protect the A-to-D converter. This is very typical for DSP-based 
radios. Ideally, the operator selects the narrowest crystal filter they 
can, even though the DSP can be further narrowed within that passband. 
The K3 does this for you as you adjust WIDTH and other DSP controls.

The majority of radios on the market today provide only a wide 
"roofing" filter at the first I.F. These are rarely any narrower than 6 
kHz -- sometimes 3 kHz. This doesn't help protect the front end if you 
wanted a narrower passband for CW, narrow SSB, or data modes.

The K3 helps solve this problem in three ways:

1. It uses a very low first I.F. (8.215 MHz), where crystal filters as 
narrow as 200 Hz are easily fabricated. (Most other rigs have a first 
I.F. in the low VHF range.) The K3 has a very high-dynamic-range front 
end to take advantage of this design strategy. As a result, it's #1 on 
the Sherwood Engineering receiver test site, and is the only rig for 
which the ARRL has measured over 100 dB IMDDR3 at close spacings.

2. It provides slots for up to five first-I.F. crystal filters for the 
main receiver and another five for the subreceiver. (You could call 
these roofing filters, if you like this term -- I don't, since they're 
far higher performance than the usual filters so designated.) Thus you 
can put in a variety of close-spaced filters to optimize for a given 
mode. As I mentioned, the WIDTH and other controls then select the 
optimal filter as you adjust them.

3. Later this year we'll be offering variable-passband crystal filters 
that each cover a significant bandwidth range, e.g. 200-400, 400-800, 
etc. (These are not finalized.) Each of these filters will have 8 
discrete bandwidths. As you adjust the DSP controls, the K3 will again 
optimize the passband of the selected filter.

73,
Wayne
N6KR

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