APF vs 10 Hz DSP, why they don't sound the same

[William Carver]

The "DUAL PASSBAND" makes a pass at providing an APF type of function
where you make the DSP passband as narrow as you choose, but there's a
wider passband that's 20 dB down. Undoubtedly it isn't the same shape as
APF amplitude v.s. frequency, but conceptually "a signal peak floating
on a sea of weaker undesired signals" is similar.

G3XJP implementated a DUAL PASSBAND function (that terminology hadn't
been invented then) in his STAR transceiver, but there the amplitude of
the wider passband can be dialed to be -20, -30, -40, .... -60 dB. It
probably isn't too big a deal to make the "skirt attenuation" number a
user-adjustable variable but I enjoyed being able to dial it up and down
as QRM changed....which would ite up another programmable functio
button.

For those who suggested kicking the gain compensation up when narrower
roofing filters are switched in, that's exactly what I did. I'm well
aware that   IF   the roofing filter is setting the IMD capability of
the receiver adding pre-filter gain reduces the intercept/increases IMD
WHEN  that IMD happens to occur, and   IF  it happens to fall into the
passband with the desired signal. I intentionally emphasized the string
of IF..WHEN..IF.. because depending upon you penchant for living in
pileups, or turning the rig off on big contest weeks, a 4 dB IMD
increase  IF all of those things happened to occur may, or may not, be
that big a deal.

A post-roofing filter gain increment would not affect intercept or IMD
and could be a smoothly varying increase that could conceivably be
matched to a users memory of APF or Heath QF-1 function.

Bill


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[Eric Swartz - WA6HHQ]

The filter gain compensation is -post- crystal  filter. Its done inside
the DSP.

If this is set well above what is really needed, the DSP may generate
some distortion on really strong signals. We usually recommend +4 dB max.

73, Eric   WA6HHQ

William Carver wrote:

> The "DUAL PASSBAND" makes a pass at providing an APF type of function
> where you make the DSP passband as narrow as you choose, but there's a
> wider passband that's 20 dB down. Undoubtedly it isn't the same shape as
> APF amplitude v.s. frequency, but conceptually "a signal peak floating
> on a sea of weaker undesired signals" is similar.
>
> G3XJP implementated a DUAL PASSBAND function (that terminology hadn't
> been invented then) in his STAR transceiver, but there the amplitude of
> the wider passband can be dialed to be -20, -30, -40, .... -60 dB. It
> probably isn't too big a deal to make the "skirt attenuation" number a
> user-adjustable variable but I enjoyed being able to dial it up and down
> as QRM changed....which would ite up another programmable functio
> button.
>
> For those who suggested kicking the gain compensation up when narrower
> roofing filters are switched in, that's exactly what I did. I'm well
> aware that   IF   the roofing filter is setting the IMD capability of
> the receiver adding pre-filter gain reduces the intercept/increases IMD
> WHEN  that IMD happens to occur, and   IF  it happens to fall into the
> passband with the desired signal. I intentionally emphasized the string
> of IF..WHEN..IF.. because depending upon you penchant for living in
> pileups, or turning the rig off on big contest weeks, a 4 dB IMD
> increase  IF all of those things happened to occur may, or may not, be
> that big a deal.
>
> A post-roofing filter gain increment would not affect intercept or IMD
> and could be a smoothly varying increase that could conceivably be
> matched to a users memory of APF or Heath QF-1 function.
>
> Bill
>  

_..._

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[David Woolley (E.L)]

Eric Swartz -WA6HHQ, Elecraft wrote:
>
> If this is set well above what is really needed, the DSP may generate
> some distortion on really strong signals. We usually recommend +4 dB max.

I wonder if distorting the audio is really what APF is about. With a
narrow enough filter, the distortion products will only be based on the
signal.  Alternatively, and more worrying, it may be about defeating the
ear's defences against load sounds, by making the spectrum too narrow,
in which case there may be a risk of burning out the hair cells for the
chosen frequency.

More generally, the APF proponents seem to have the problem that they
treat it almost as magic.  If they could characterise what it does in
terms of filtering, AF gain adjustment, distortion etc. it should be
possible to simulate it in DSP, subject only to having enough processing
power.  In particular, peaking just seems to me to mean turning up the
AF gain in parallel with reducing the bandwidth. If that is all that it
is doing, and if the K3 has soft AF gain controls, a plugin box could
issue both commands in parallel.

PS the relevance of AGC here is whether or not it is applied pre- or
post- the ultimate selectivity.  From what the APF people say, it is
normally applied pre-.
--
David Woolley
"we do not overly restrict the subject matter on the list, and we
encourage postings on a wide range of amateur radio related topics"
List Guidelines <http://www.elecraft.com/elecraft_list_guidelines.htm>
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[Martin-2]

I once owned a FT-107, it had APF, i sure miss that feature.
If you are looking for a good analog Audio-filter, go to

http://www.eurofrequence.de/filtereing/

I own the predecessor with less features for more than 15 years.
I am a satisfied customer, no other relationship.

Martin

--

73, DM4iM
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[Bill W4ZV]

 > More generally, the APF proponents seem to have the problem that they
treat it almost as magic.  If they could characterise what it does in
terms of filtering, AF gain adjustment, distortion etc. it should be
possible to simulate it in DSP, subject only to having enough processing
power.  In particular, peaking just seems to me to mean turning up the
AF gain in parallel with reducing the bandwidth. If that is all that it
is doing, and if the K3 has soft AF gain controls, a plugin box could
issue both commands in parallel.

         The circuit in question is on page 54 of the FT1000 schematic below:

<http://www.radioamateur.eu/schemi/FT1000_user.pdf>http://www.radioamateur.eu/schemi/FT1000_user.pdf

The APF is around Q3016-2 (including Rs 3062, 3065, 3067 per the
"un-modification" sheet on N1EU's website below).  The circuit
schematic above has *not* been modified to the (incorrect) values
shown in the Yaesu service bulletin.

http://n1eu.com/Mods/apf.pdf

The FT-1000's APF is generally regarded by many as the best APF ever
done.  It should be possible to plug this into a circuit simulator
and generate a frequency response plot that Lyle can emulate in the
DSP.  I have a feeling that this is a very narrow and high-Q response
that is on the verge of ringing, but it should be possible to
duplicate the circuit in the DSP.

                                         73,  Bill  W4ZV

         

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[P.B. Christensen]

> DSP.  I have a feeling that this is a very narrow and high-Q response
> that is on the verge of ringing, but it should be possible to
> duplicate the circuit in the DSP.

And only subject to the remaining DSP processing power within the K3.  A
traditional, single parametric EQ channel with variable Q, amplitude, and
frequency range (e.g., 300 Hz - 1200 Hz), would work very well.  A quick
measurement of the FT-1000's APF peaking gain and Q could be used to
establish initial parameters.

Paul, W9AC

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[alsopb]

Unlike the phone guys, most CW guys could care less about audio
distortion or quality for that weak one.  They just want to be able to
copy it.  The information is in the off's and on's, the rest doesn't
matter.

The old QF-1 Q-muliplier had an advertised Q of 4000 @ 455KHz.  That's a
bandwidth of about 100Hz.  There was some skill requiring adjusting
several knobs to make it work for you.  Of course, any K3 implementation
would have to be knobless.

73 de Brian/K3KO

Bill Tippett wrote:

>  > More generally, the APF proponents seem to have the problem that they
> treat it almost as magic.  If they could characterise what it does in
> terms of filtering, AF gain adjustment, distortion etc. it should be
> possible to simulate it in DSP, subject only to having enough processing
> power.  In particular, peaking just seems to me to mean turning up the
> AF gain in parallel with reducing the bandwidth. If that is all that it
> is doing, and if the K3 has soft AF gain controls, a plugin box could
> issue both commands in parallel.
>
>          The circuit in question is on page 54 of the FT1000 schematic below:
>
> <http://www.radioamateur.eu/schemi/FT1000_user.pdf>http://www.radioamateur.eu/schemi/FT1000_user.pdf
>
> The APF is around Q3016-2 (including Rs 3062, 3065, 3067 per the
> "un-modification" sheet on N1EU's website below).  The circuit
> schematic above has *not* been modified to the (incorrect) values
> shown in the Yaesu service bulletin.
>
> http://n1eu.com/Mods/apf.pdf
>
> The FT-1000's APF is generally regarded by many as the best APF ever
> done.  It should be possible to plug this into a circuit simulator
> and generate a frequency response plot that Lyle can emulate in the
> DSP.  I have a feeling that this is a very narrow and high-Q response
> that is on the verge of ringing, but it should be possible to
> duplicate the circuit in the DSP.
>
>                                          73,  Bill  W4ZV
>
>

No virus found in this outgoing message.
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[w4nhj]

If you have an SDR type  panadaptor using the IF out signal, you can try a
peaking filter by downloading and running WinRad software.  WinRad has a
peaking filter that works great.

Frank - W4NHJ

----- Original Message -----
From: "Bill Tippett" <[hidden email]>
To: <[hidden email]>
Sent: Sunday, August 16, 2009 7:38 AM
Subject: Re: [Elecraft] APF vs 10 Hz DSP, why they don't sound the same

> > More generally, the APF proponents seem to have the problem that they
> treat it almost as magic.  If they could characterise what it does in
> terms of filtering, AF gain adjustment, distortion etc. it should be
> possible to simulate it in DSP, subject only to having enough processing
> power.  In particular, peaking just seems to me to mean turning up the
> AF gain in parallel with reducing the bandwidth. If that is all that it
> is doing, and if the K3 has soft AF gain controls, a plugin box could
> issue both commands in parallel.
>
>         The circuit in question is on page 54 of the FT1000 schematic
> below:
>
> <http://www.radioamateur.eu/schemi/FT1000_user.pdf>http://www.radioamateur.eu/schemi/FT1000_user.pdf
>
> The APF is around Q3016-2 (including Rs 3062, 3065, 3067 per the
> "un-modification" sheet on N1EU's website below).  The circuit
> schematic above has *not* been modified to the (incorrect) values
> shown in the Yaesu service bulletin.
>
> http://n1eu.com/Mods/apf.pdf
>
> The FT-1000's APF is generally regarded by many as the best APF ever
> done.  It should be possible to plug this into a circuit simulator
> and generate a frequency response plot that Lyle can emulate in the
> DSP.  I have a feeling that this is a very narrow and high-Q response
> that is on the verge of ringing, but it should be possible to
> duplicate the circuit in the DSP.
>
>                                         73,  Bill  W4ZV
>
>
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html 

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[drewko]

I have one of those old QF-1 filters sitting in a drawer. I bought it
new years ago. It was certainly a lot of fun notching a signal then
switching it to peak and narrowing it down. And a very effective
accessory for my rig at the time, a Century-21 which had no such
refinements.

I should give it a try with the K3. It is kind of a pain because there
is no bypass switch in the unit: it has to be powered on to hear
anything. Also a noticeable AC hum which was apparently a standard
feature of these units.

73,
Drew
AF2Z


On Sun, 16 Aug 2009 13:03:37 +0000, Brian/K3KO wrote:

>Unlike the phone guys, most CW guys could care less about audio
>distortion or quality for that weak one.  They just want to be able to
>copy it.  The information is in the off's and on's, the rest doesn't
>matter.
>
>The old QF-1 Q-muliplier had an advertised Q of 4000 @ 455KHz.  That's a
>bandwidth of about 100Hz.  There was some skill requiring adjusting
>several knobs to make it work for you.  Of course, any K3 implementation
>would have to be knobless.
>
>73 de Brian/K3KO
>

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[Merv Schweigert]

David Woolley (E.L) wrote:

> Eric Swartz -WA6HHQ, Elecraft wrote:
>  
>> If this is set well above what is really needed, the DSP may generate
>> some distortion on really strong signals. We usually recommend +4 dB max.
>>    
>
> I wonder if distorting the audio is really what APF is about. With a
> narrow enough filter, the distortion products will only be based on the
> signal.  Alternatively, and more worrying, it may be about defeating the
> ear's defences against load sounds, by making the spectrum too narrow,
> in which case there may be a risk of burning out the hair cells for the
> chosen frequency.
>  

APF does not distort the audio at all,  its extremely simple,  all it
does is peak
an audio freq by X amount.  In older radios its done with a analog
circuit of
one or two chips and resistor capacitor sets to determine the freq of
the peak.
I assume there is some feedback loop to increase the audio gain at the
peaked
freq.  
> More generally, the APF proponents seem to have the problem that they
> treat it almost as magic.  If they could characterise what it does in
> terms of filtering, AF gain adjustment, distortion etc. it should be
> possible to simulate it in DSP, subject only to having enough processing
> power.  In particular, peaking just seems to me to mean turning up the
> AF gain in parallel with reducing the bandwidth. If that is all that it
> is doing, and if the K3 has soft AF gain controls, a plugin box could
> issue both commands in parallel.
>  
It does nothing as far as filtering,  years ago it was used in cheap
receivers as a CW filter
but that is not the purpose in a radio that has xtal filters or DSp filters.
Yes it is like turning up the AF gain some amount, but at one particular
freq,  usually the
freq you have the pitch set at on CW.  but you do not want to reduce the
bandwidth,
many time you want to use the APF at wider bandwidth settings due to the
different
sound of the background noise at the wider bandwidth makes copy easier
than with
the narrow sound of narrow filters.
> PS the relevance of AGC here is whether or not it is applied pre- or
> post- the ultimate selectivity.  From what the APF people say, it is
> normally applied pre-.
>  
Since APF is totally an audio function the gain and AGC is not effected
by its use
at all,  unless you have a radio that has audio derived AGC.
As I stated before APF is for very weak signal detection,  one would not
usually
use it on signals that are copyable with normal filters etc.   It is
used for copying
signals you normally tune across because they are too weak to copy or you
perhaps dont even hear them.  
It will take a signal that is at the noise level,  and peak it to the
point of being
able to copy.  
Some ops like it so much they use it for louder signals as well.
The closest thing is to outboard a audio filter like the Datong,  I have
tried the
Autek and its not up to par, the Datong is fair but again not the same
as the
circuit in the 1000D radios.  If there was a comparable outboard that worked
I would put one in the P3 empty space and go away laughing.
73 Merv KH7C

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[Merv Schweigert]

Bill,  thanks a million,  I have difficulty explaining what is in the
brain cells,
this helps a lot,  simple circuit,  and agree that running this through a
simulator would give the answers.  
I know some would not like the feature, but one does not have to use it.
Its the difference for me  of making the contact or not,  so a really
big deal.
For causal operators they may never use it.
73 Merv KH7C

>  > More generally, the APF proponents seem to have the problem that they
> treat it almost as magic.  If they could characterise what it does in
> terms of filtering, AF gain adjustment, distortion etc. it should be
> possible to simulate it in DSP, subject only to having enough processing
> power.  In particular, peaking just seems to me to mean turning up the
> AF gain in parallel with reducing the bandwidth. If that is all that it
> is doing, and if the K3 has soft AF gain controls, a plugin box could
> issue both commands in parallel.
>
>          The circuit in question is on page 54 of the FT1000 schematic below:
>
> <http://www.radioamateur.eu/schemi/FT1000_user.pdf>http://www.radioamateur.eu/schemi/FT1000_user.pdf
>
> The APF is around Q3016-2 (including Rs 3062, 3065, 3067 per the
> "un-modification" sheet on N1EU's website below).  The circuit
> schematic above has *not* been modified to the (incorrect) values
> shown in the Yaesu service bulletin.
>
> http://n1eu.com/Mods/apf.pdf
>
> The FT-1000's APF is generally regarded by many as the best APF ever
> done.  It should be possible to plug this into a circuit simulator
> and generate a frequency response plot that Lyle can emulate in the
> DSP.  I have a feeling that this is a very narrow and high-Q response
> that is on the verge of ringing, but it should be possible to
> duplicate the circuit in the DSP.
>
>                                          73,  Bill  W4ZV
>
>          
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
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> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>
>  

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[David Woolley (E.L)]

Merv Schweigert wrote:

> APF does not distort the audio at all,  its extremely simple,  all it
> does is peak
> an audio freq by X amount.  In older radios its done with a analog
> circuit of

So how does that differ from the existing filters in the K3.  What
people seem to be saying is that it actually boosts the audio, but that
is just equivalent to turning up the AF gain, so I was speculating that
the gain is actually turned up to the point where later stages overload,
and that is why people think it differs from a simple filter.

> one or two chips and resistor capacitor sets to determine the freq of
> the peak.
> I assume there is some feedback loop to increase the audio gain at the
> peaked
> freq.

I.E. is a band bass FILTER!

>>  
> It does nothing as far as filtering,  years ago it was used in cheap

If it boost one frequency with respect to another, it is, by definition,
  filter.  This is what I mean by people claiming magical properties.
It is just a linear filter (although possibly followed by non-linear
output stages).

> receivers as a CW filter

> Since APF is totally an audio function the gain and AGC is not effected
> by its use

The gain is influenced.  The audio gain is part of the overall gain of
the receiver.  People seem to be claiming that the "peak" part of the
name means that there is excess audio gain at the filter centre, over
that without the filter.

The reason people introduced AGC was to point out that, being post AGC,
the subjective effect of tuning through a signal was a much greater
change in amplitude than pre-AGC, where the AGC would keep the amplitude
more or less constant, although the signal to noise ratio would peak as
you tuned through the signal.

> at all,  unless you have a radio that has audio derived AGC.
> As I stated before APF is for very weak signal detection,  one would not
> usually
> use it on signals that are copyable with normal filters etc.   It is
> used for copying
> signals you normally tune across because they are too weak to copy or you
> perhaps dont even hear them.  

It's just a normal filter and if it is better than the existing filters,
that is a matter of working out what it is about the filter shape
(considering the overall effect of all filters in the receiver), in
phase and frequency, that makes it subjectively better, and adjust the
DSP filter to reproduce that characteristic.  If one understands exactly
why it seems better, the flexibility of a DSP filter may mean that by
designing a first principles solution, one can do even better.

> It will take a signal that is at the noise level,  and peak it to the
> point of being
> able to copy.  

As does any narrow band pass filter.

The only difference between filtering at audio and at IF, is that
distortion products will remain in band, and it is more likely that it
will be done post-AGC.



--
David Woolley
"we do not overly restrict the subject matter on the list, and we
encourage postings on a wide range of amateur radio related topics"
List Guidelines <http://www.elecraft.com/elecraft_list_guidelines.htm>
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[Steve Ellington]

The DSP in the original Omni 6 was a low pass filter intended to eliminate
white noise from the IF stage. Later, the Omni 6 Plus used the DSP as a CW
Peaking filter. It really sounded great without the typical ringing so
common in narrow analog audio filters.
Steve
N4LQ
[hidden email]
----- Original Message -----
From: "David Woolley (E.L)" <[hidden email]>
To: <[hidden email]>
Sent: Sunday, August 16, 2009 1:43 PM
Subject: Re: [Elecraft] APF vs 10 Hz DSP, why they don't sound the same

> Merv Schweigert wrote:
>
>> APF does not distort the audio at all,  its extremely simple,  all it
>> does is peak
>> an audio freq by X amount.  In older radios its done with a analog
>> circuit of
>
> So how does that differ from the existing filters in the K3.  What
> people seem to be saying is that it actually boosts the audio, but that
> is just equivalent to turning up the AF gain, so I was speculating that
> the gain is actually turned up to the point where later stages overload,
> and that is why people think it differs from a simple filter.
>
>> one or two chips and resistor capacitor sets to determine the freq of
>> the peak.
>> I assume there is some feedback loop to increase the audio gain at the
>> peaked
>> freq.
>
> I.E. is a band bass FILTER!
>
>>>
>> It does nothing as far as filtering,  years ago it was used in cheap
>
> If it boost one frequency with respect to another, it is, by definition,
>  filter.  This is what I mean by people claiming magical properties.
> It is just a linear filter (although possibly followed by non-linear
> output stages).
>
>> receivers as a CW filter
>
>> Since APF is totally an audio function the gain and AGC is not effected
>> by its use
>
> The gain is influenced.  The audio gain is part of the overall gain of
> the receiver.  People seem to be claiming that the "peak" part of the
> name means that there is excess audio gain at the filter centre, over
> that without the filter.
>
> The reason people introduced AGC was to point out that, being post AGC,
> the subjective effect of tuning through a signal was a much greater
> change in amplitude than pre-AGC, where the AGC would keep the amplitude
> more or less constant, although the signal to noise ratio would peak as
> you tuned through the signal.
>
>> at all,  unless you have a radio that has audio derived AGC.
>> As I stated before APF is for very weak signal detection,  one would not
>> usually
>> use it on signals that are copyable with normal filters etc.   It is
>> used for copying
>> signals you normally tune across because they are too weak to copy or you
>> perhaps dont even hear them.
>
> It's just a normal filter and if it is better than the existing filters,
> that is a matter of working out what it is about the filter shape
> (considering the overall effect of all filters in the receiver), in
> phase and frequency, that makes it subjectively better, and adjust the
> DSP filter to reproduce that characteristic.  If one understands exactly
> why it seems better, the flexibility of a DSP filter may mean that by
> designing a first principles solution, one can do even better.
>
>> It will take a signal that is at the noise level,  and peak it to the
>> point of being
>> able to copy.
>
> As does any narrow band pass filter.
>
> The only difference between filtering at audio and at IF, is that
> distortion products will remain in band, and it is more likely that it
> will be done post-AGC.
>
>
>
> --
> David Woolley
> "we do not overly restrict the subject matter on the list, and we
> encourage postings on a wide range of amateur radio related topics"
> List Guidelines <http://www.elecraft.com/elecraft_list_guidelines.htm>
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[alorona]

 
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.

 
Al  W6LX
 
 
 
 

--- On Sun, 8/16/09, Bill Tippett <[hidden email]> wrote:


From: Bill Tippett <[hidden email]>
Subject: Re: [Elecraft] APF vs 10 Hz DSP, why they don't sound the same
To: [hidden email]
Date: Sunday, August 16, 2009, 4:38 AM


> More generally, the APF proponents seem to have the problem that they
treat it almost as magic.  If they could characterise what it does in
terms of filtering, AF gain adjustment, distortion etc. it should be
possible to simulate it in DSP, subject only to having enough processing
power.  In particular, peaking just seems to me to mean turning up the
AF gain in parallel with reducing the bandwidth. If that is all that it
is doing, and if the K3 has soft AF gain controls, a plugin box could
issue both commands in parallel.

         The circuit in question is on page 54 of the FT1000 schematic below:

<http://www.radioamateur.eu/schemi/FT1000_user.pdf>http://www.radioamateur.eu/schemi/FT1000_user.pdf

The APF is around Q3016-2 (including Rs 3062, 3065, 3067 per the
"un-modification" sheet on N1EU's website below).  The circuit
schematic above has *not* been modified to the (incorrect) values
shown in the Yaesu service bulletin.

http://n1eu.com/Mods/apf.pdf

The FT-1000's APF is generally regarded by many as the best APF ever
done.  It should be possible to plug this into a circuit simulator
and generate a frequency response plot that Lyle can emulate in the
DSP.  I have a feeling that this is a very narrow and high-Q response
that is on the verge of ringing, but it should be possible to
duplicate the circuit in the DSP.

                                         73,  Bill  W4ZV

         

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[P.B. Christensen]

> "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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[alorona]

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 <[hidden email]> wrote:


From: Paul Christensen <[hidden email]>
Subject: Re: [Elecraft] APF vs 10 Hz DSP, why they don't sound the same
To: [hidden email]
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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