[Elecraft] Analysis of FT-1000 APF

[Al Lorona]

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.
 
Al  W6LX
 
 
 
 
 
 
 
 


--- On Sun, 8/16/09, Al Lorona <alorona at sbcglobal.net> wrote:


From: Al Lorona <alorona at sbcglobal.net>
Subject: Re: [Elecraft] APF vs 10 Hz DSP, why they don't sound the same
To: elecraft at mailman.qth.net, "Paul Christensen" <w9ac at arrl.net>
Date: Sunday, August 16, 2009, 4:44 PM







Yes, I ran the analysis on the circuit as shown, without the tuning control. If anyone wants to pore through the schematics and tell me the value of the control as well as where to put it, I'll be happy to run the analysis at the two ends of the range of the control.
 
What I had wanted to do with my information was remove any mystery surrounding the topology of the circuit in the FT-1000. Now that we know what it is, it may help someone to try and duplicate it with hardware, software, or whatever.
 
Al  W6LX

--- On Sun, 8/16/09, Paul Christensen <w9ac at arrl.net> wrote:


From: Paul Christensen <w9ac at arrl.net>
Subject: Re: [Elecraft] APF vs 10 Hz DSP, why they don't sound the same
To: elecraft at mailman.qth.net
Date: Sunday, August 16, 2009, 4:39 PM


> "The circuit surrounding op amp 3016-2 in the FT-1000 schematic is nothing 
> more than an active bandpass filter, center frequency of about 1025 Hz, Q 
> = 35, with a gain of about 10.5 dB."

The APF filter is active on the Main Rx, CW mode only.  However, Fc is not 
confined to 1025 Hz; with the front panel "APF" control, it's adjustable 
roughly over the range of the CW BFO offset when the filter switch is 
engaged.

Paul, W9AC







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