[Elecraft] APF (audio peaking filter) on the K3/K3S/KX3

[Bob McGraw K4TAX]

Lightning has dramatic effect on the AGC system of many to most 
receivers.   Rob Sherwood has had a lot to say about this.  Elecraft has 
taken steps to minimize the effect.

Yes different types of noise clearly have different characteristics and 
thus the reason for suggesting reducing the RF Gain, it reduces the 
noise.  Fortunately this time of year in our location in TN the noise is 
quite low, unless a line of thunder storms is located within 500 or so 
miles.

Hear this morning no thunder storms close by, the neighborhood is 
reasonably quiet, and the noise on 40M is about -103 dBm.  With the APF 
a signal at -113 dBm can be easily copied, yet without the APF nothing 
but noise is heard.

My technique is to observe the S meter, no signal, RF Gain at max for 
the given band and antenna.  Then with the antenna disconnected, 
actually by a switch, I adjust the RF Gain until the S meter indicates 
the same value as the noise coming in on the antenna. Then switch the 
antenna back to the receiver.  Also the AUTO SPOT feature is a big aid 
to optimum tuning.  The result is signal present and no noise.

73
Bob, K4TAX



On 2/5/2016 10:49 AM, dave wrote:
>
>> one must also discover the
>> artful use of the RF Gain control as well
>
> Adjustment of the RF gain control is not mentioned in Wayne's note on 
> using APF.
>
> When I tried it here, the RF gain might have made some small 
> difference but it still did not allow un-copyable signals to be 
> copied. When testing APF I have made all adjustments the I thought 
> might affect signal quality. IIRC correctly I also tinkered with the 
> AGC parameters. But the APF never produced results any better than 
> simply narrowing the filters. Usually the regular narrow filters 
> worked better. I know I had not switched APF on for months. I did 
> switch it on to recheck before sending my note. But the noise is not 
> currently high enough to give it a good test.
>
> I have found, in general, that careful adjustment of the RF gain 
> control can be useful. This applies to all receivers I have tried it on.
>
> I'm wondering if the difference is in the type of noise. I have mostly 
> tried APF at times of high lightning activity. I think we in the SE 
> get more of that than other parts of the country. The higher noise 
> level means more weak signals. You need all the help you can get. But 
> that noise differs from the usual band noise.
>
> And it may depend on each individual's hearing.
>
> I have always puzzled over why some rave about how great APF is while 
> for myself, and obviously others, it is of no help at all.
>
> 73 de dave
> ab9ca/4
>