[Elecraft] Analysis of FT-1000 APF

[Bill W4ZV]

Al Lorona wrote:
> 
>  
> I assumed that the front panel control was a 1k potentiometer. Probably a
> reasonable guess.
>  
> As the pot is tuned from max to min resistance, the audio peak moves from
> 340 Hz to about 1000 Hz (as I reported earlier). The gain at the peak is
> like this:
>  
> 340 Hz :  7.5 dB ,   Q = 11
> 485 Hz :  8.1 dB
> 600 Hz :  8.6 dB
> 740 Hz :  9.1 dB
> 850 Hz :  9.6 dB
> 1000 Hz : 10.5 dB  ,  Q = 35
>  
> So it has the typical characteristic of an active filter as it's center
> frequency is tuned higher (it gets 'peakier'). Maybe this is more info
> than you all care about, but there it is. 
>  
> Should the front panel control be larger than 1k, then all it means is
> that it will tune lower in frequency than I have shown here.
>  
> 'APF' is just a marketing term for a narrow audio band pass filter, at
> least in the FT-1000.
>  
> I don't have any place I can post a picture of these plots, so my word
> description will have to do.
> 

Bless your heart for working through that Al!  I previously posted the
FT-1000 data to the K3 Field Test list a few weeks back hoping someone might
"bite" and do a circuit simulation, but alas nobody took the hook!  I guess
the heavy hitters at Elecraft were too busy with the P3.

If I'm interpreting Q correctly, it appears the filter is ~30 Hz BW at all
pitch settings (which would definitely be near "ring" territory).  I've
previously suggested to Lyle that DUAL PB use 50 Hz for the focus instead of
150 Hz, but I believe there was some problem with that (which I've since
forgotten).  If there were a way to do 50 Hz with the same -20 dB pedestal
BW (600 Hz minimum), I think this could be a nice alternative to traditional
APF.  Of course the present 50 Hz BW is actually something like 80 Hz, so it
would be nice if that could be made closer to 50 Hz or maybe even 30.

Thanks again for doing the simulation!  Maybe Wayne or Lyle will have a
pleasant surprise for us one of these days.

73,  Bill  W4ZV

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