The APF does as you say, builds a narrow filter around an audio signal, and it does this
But what really makes it so great (and distinguishes it from just an audio filter) is that it adds another dimension. In addition It peaks or boosts the
That is why we users say that it pulls the signal up out of the mud. The FT-1000's APF
and my TS-930S (I wish I still had her, sniff, sniff) did this very effectively.
The CW signal can be right down at the noise level, you know it is in there but cannot copy. By adjusting the
APF you can pull the signal up out of the mud and copy the signal.
> I have never used a receiver with a button labeled APF, but I have
> used my K2 with an audio filter. It seems to me the only other
> characteristic of APF is that the center frequency can be changed.
> On the K3, we have the Dual Passband that may work in a similar manner.
> Set the width to the lowest value 50 Hz and turn on Dual PB and you have
> a wide filter with a peak at your sidetone pitch. You can change the
> Pitch if you wish to peak a different audio frequency, but for the life
> of me, I cannot imagine why anyone would want to peak any pitch other
> than his preferred sidetone pitch.
>
> If you want a little more peaking, turn on NR and set it to an
> aggressive setting. The way NR works is that it builds a narrow filter
> around a signal that it thinks is a valid signal - so you have a filter
> at the audio pitch where you have set the VFO.
>
> Yes, the K3 is different than those receiver that have APF, but in my
> opinion, it accomplishes the same thing, but differently.
>
> 73,
> Don W3FPR
>
> On 10/24/2010 2:47 PM, Paul Christensen wrote:
>>> I found the APF in my Icom 7800 worthless.
>> Same here. Icom's implementation of APF in the '7700/'7800 is really an
>> "Audio Passband Filter" with low Q. It's really no different than narrowing
>> a DSP passband filter.
>>
>> What's unique about the FT-1000D's APF is that the Q is very high, although
>> its affected with APF tuning. The high Q of the circuit forms a very narrow
>> filter with fixed gain, and variable peaking frequency.
>>
>> Paul, W9AC
>>
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