[Elecraft] K3: honest question re Audio Peak Filter

[Merv Schweigert]

Bill W4ZV wrote:
> Steve Ellington wrote:
>   
>> After filtering the cw signal down to 50hz and you kick in an audio filter 
>> that has 10db or gain, your jaw drops, your eyes glaze over and you 
>> immediately start slobbering about how signals POP OUT OF THE NOISE.
>>
>>     
>
> I've never had an FT-1000D but I know some who do swear by the APF.  We had
> a previous thread about this which resulted in a circuit simulation analysis
> by Al W6LX.  This is using the **EXACT** circuit in the FT-1000D which I
> found sent to Al.  Al's analysis is below (in PowerPoint):
>
> http://sites.google.com/site/ft1000apf/w6lxanalysisoftheapfcircuitintheft-1000
>
> You can see the APF has a 6 dB BW of ~50 Hz and ~8.5 dB gain.  So how do I
> emulate that in the K3?  I have both the 500 Hz 8-pole and 200 Hz 5-pole
> filters in my main RX.  Normally I set the filter gain to 1 dB for the 500
> and 2 dB for the 200, based on Wayne's recommendations.  However, you can
> emulate the APF by setting the gain for the 200 Hz to 8 dB.  Now my rig
> transitions from 1 dB to 8 dB gain as I go to 200 Hz BW or lower, and I have
> ~7 dB gain over the 500 Hz filter at DSP BW = 50 Hz, which is very close to
> the FT-1000D APF's characteristics.  
>
> By pure coincidence I had my rig set to 1822.5 as I was doing this.  Lo and
> behold, at 2:45 PM here I just heard G3FPQ very weak here at 2.5 hours
> before sunset.  Indeed this emulated APF did seem to help, but I'm going to
> do much more listening before I draw any conclusions.  If you have a 200 Hz
> filter, you might want to give this a try.
>
> 73,  Bill
>   
There are reasons to not narrow the bandwidth down when using APF,  if you
use wider bandwidths you "spread" the noise level out across more 
spectrum and
hearing is much easier,  narrowing the bandwidth down that radical 
concentrates the
noise into a small amount of audio,  many times I find that there is not 
QRM present
and with filter width of 2.1KHZ and using the APF signals really do jump 
out of the
noise,   narrowing the bandwidth creates more ringing also in the APF 
due to the
concentration of noise at the same freq as the APF.  One of those things 
you have
to hear to put a handle on.  K3 cannot in any way I have found duplicate 
this, and
the way suggested by Elecraft is to Narrow to .05 with IIR and use the 
shift to
center the signal perfectly in the bandpass,  it boosts the signal a 
small amount
but it is not APF and of course cannot be used at wider bandwidths.
Merv KH7C
>
>