[Elecraft] APF (audio peaking filter) on the K3/K3S/KX3
Barry N1EU
n1eu.barry at gmail.com
Fri Feb 5 12:50:00 EST 2016
I suspect that when the K3 APF was first engineered, the Yaesu FT-1000D APF
(original hi-Q version) was used as the model because many prominent
dx'ers/topbanders always raved about it.
73, Barry N1EU
On Fri, Feb 5, 2016 at 12:37 PM, Scott via Elecraft <
elecraft at mailman.qth.net> wrote:
>
> In the last month of listening to really weak signals here in 9 land I've
> found it helpful but not radical.
>
> Just for fun I tried tuning some of the same VP8SGI signals on a Drake
> 2B/2BQ and found adding the 2BQ in the mix more user friendly and making a
> bigger difference, but hey, the 2B was starting from a worse spot.
>
> Still it would be nice if the APF was user adjustable like the 2BQ or the
> old outbaord audio filters, peak magnitude, and maybe even frequency,
> within
> a selected passband.
>
> Not sure why Wayne picked only 3 db - I'd like it to be user adustable to
> try a bit more peak if conditions permit.
>
> If APF is user adjustable in that way and I missed it somebody please
> point out the page number....or if the experiment's been done and it's not
> workable would like to know that too.
>
> 73 Scott ka9p
>
>
>
> In a message dated 2/5/2016 11:08:53 A.M. Central Standard Time,
> ho13dave at gmail.com writes:
>
>
> > 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
>
>
>
> On 2/5/16 10:13 AM, Bob McGraw K4TAX wrote:
> > While I find the APF works really nice, one must also discover the
> > artful use of the RF Gain control as well. Most hams seem to use
> > excessive RF Gain which will force noise through the filter and all of
> > those undesirable artifacts will be heard. If in doubt, reduce the RF
> > Gain.
> >
> > Using this technique, I can tune the band with the APF engaged,
> > adjusting the RF Gain a bit, and actually hear signals with this
> > configuration which without the APC no signal could be heard. I view
> > this gives one the ability to actually "dig signals out of the noise".
> >
> > 73
> > Bob, K4TAX
> >
> > On 2/5/2016 9:56 AM, dave wrote:
> >>
> >> I'd agree. My experience with APF is that it will indeed pop a
> >> signal that is already copyable, if a bit weak, out of the noise.
> >> Makes for easier copy. However the ringing is so bad on a really
> >> weak signal that it is of no help. I guess it depends on what you
> >> view as 'weak' and your noise level. I was hoping it would make
> >> previously un-copyable signals copyable, but that does not happen.
> >> Instead it improves the copyability of weak but already copyable
> >> signals.
> >>
> >> I find that simply narrowing the filters to 50, 100, or 150 Hz will
> >> give essentially identical results as using the APF and is much
> >> easier to do.
> >>
> >> As for the super critical tuning, I do not find it to be all that
> >> critical. The signal can be improved somewhat by going to the 'fine'
> >> tuning, but it makes only a small difference. 10 Hz steps are OK and
> >> much quicker. The APF has a BW of about 30 Hz so tuning cannot be
> >> all that critical. +/- 5 Hz out of 30 is not that big of a deal.
> >>
> >> This has been one of my biggest disappointments with the K3. After
> >> reading all the hype about APF I was hoping for a real improvement.
> >> But does not happen. OTOH, the narrow filters of the K3 are
> >> outstanding.
> >>
> >> 73 de dave
> >> ab9ca/4
> >>
> >>
> >>
> >> On 2/5/16 8:53 AM, David Kuechenmeister wrote:
> >>> I tried this the other day with no success. That is, the weak
> >>> signal that was barely detectable, became so buried in ringing that
> >>> I couldn't detect it at all. I think I followed the procedure
> >>> outlined in Wayne's email below... Noise was around S5 on 40m and
> >>> the K3 was configured with APF active. I had my 500 Hz filter
> >>> selected and varied the DSP WIDTH from 500 to 300 without
> >>> substantial improvement. FINE tuning selected. What am I
> >>> forgetting? vy 73,Dave N4KD
> >>>
> >>> On Tuesday, February 2, 2016 9:36 AM, Wayne Burdick
> >>> <n6kr at elecraft.com> wrote:
> >>>
> >>>
> >>> Hi all,
> >>>
> >>> This morning I was hearing lots of very weak signals in the
> >>> 40-meter CW band, right at today's noise floor of about S-5. (Among
> >>> others, I copied BG4GOV in Shanghai.)
> >>>
> >>> Many of the signals were not copyable until I turned on APF (audio
> >>> peaking filter). This filter's center peak is very narrow (around
> >>> 30 Hz), but the response broadens out quickly below the 3-dB points
> >>> in the curve. This brings up the desired signal without allowing
> >>> noise to cause ringing--something often heard with narrow filters
> >>> of the "brick-wall" variety. On a noisy band, the effect is just
> >>> short of magic.
> >>>
> >>> APF is very narrow, so you'll need to tune in 1-Hz increments when
> >>> it is turned on. Also, APF works best when used with a DSP/crystal
> >>> filter WIDTH setting of 300-500 Hz.
> >>>
> >>> Use of APF differs for the three transceivers.
> >>>
> >>> KX3: 1-Hz tuning is selected automatically on the when the APF
> >>> switch is pressed.
> >>>
> >>> K3/K3S: APF is accessed via a hold of the XFIL switch. This
> >>> function is labeled "APF" on a K3S, or "DUAL PB" on the K3). Use
> >>> CONFIG:DUAL PB to make sure the selected filter function is APF.
> >>> See the K3/K3S owner's manual for a description of the alternate
> >>> setting. 1-Hz tuning can be selected by tapping FINE.
> >>>
> >>> You might want to give APF a try if you haven't already.
> >>>
> >>> 73, Wayne N6KR
> >>>
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