[Elecraft] Shortwave filters for K3
Bill Maddock
Guys,
Just got done assembling my second K3 and this one has a 13 Khz
filter where #1 had 6 Khz - I like to do shortwave listening - thought
the wider filter would give better fidelity, but DSP only goes
5Khz wide is there a mode for this? I assumed with the 13khz roofer
it would be opened up to 10 khz or more?
Thanks and 73,
Bill N4ZI K3 #1059 & K3 #2914
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Don Wilhelm
The 5 kHz width indicated for AM is the *audio* bandwidth, not the IF.
The IF bandwidth must be more than 2 times the audio bandwidth for an AM
signal - so your 12 kHz filter is good.
Remember that the 6 kHz filter must be used for AM transmit.
AFAIK, most broadcast stations limit their transmitted audio to 5 kHz,
so you are not likely to obtain much better fidelity than the K3 DSP
currently allows.
73,
Don W3FPR
Steve Ellington
This doesn't quiet make sense to me. I have the 6khz filter in my K3. The BW
indicated on the K3 is 5Khz. If I add the 12 KHZ filter and the K3 still
says 5KHZ and that 5KHZ is the "audio" bandwidth, what good have I done?
Most any old SW receiver sounds a bit muffled at only 6khz bandwidth.
Opening them out to 10 or even 15 KHZ always makes them sound better. Yes I
know the BCB stations are limited to 5 Khz but just compare the K3 to a
wideband rx and note the difference.
Steve Ellington
N4...@carolina.rr.com
Don Wilhelm
The DSP audio bandwidth *can* be moved out to 5 kHz for AM - that is
true, and it does not depend on the filter you have installed.
With the 6kHz filter at the IF, the bandwidth is constrained by the 6
kHz roofing filter, so the realizable audio that can be demodulated is 3
kHz - it makes no difference if the DSP opens the audio width up to 5
kHz, the 6 kHz filter still restricts the audio to 3 kHz.
Now, install the 12 kHz roofing filter and the full 5 kHz audio width
allowed by the DSP can be realized.
Study AM modulation for a bit and you should realize that the RF (and
IF) must pass both sidebands, but the demodulated audio will be only
half the width of the RF waveform. The available audio high frequencies
can be constrained by either restricting the RF (or IF) passband or by
restricting the audio bandwidth - either one produces the same result.
With your "old shortwave receiver" controlling the IF at 6 kHz (3 kHz
of audio) when it begins to sound muffled? I would think so because
most of the filtering is done at the IF frequency. Equate that IF width
to the roofing filter width and the DSP bandwidth to the audio bandpass
and you will be correct.
73,
Don W3FPR
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Steve Ellington
I understand the demodulation theory just fine but the concept of having the
DSP act as an audio filter instead of an IF filter causes some confusion
since we've been hyping up "IF-DSP" filtering for several years now and
suddenly we talk about AF-DSP in only one mode, AM.
Maybe if the BW display showed 10 Khz instead of 5 Khz it would limit the
confusion since that seems to be what most of us are used to. We understand
that we only hear one of the two sidebands anyway. Oh well,,, I guess the
question will keep coming up.
Don Wilhelm
Please do not be confused. The DSP *is* at the IF frequency, *and it
does* process AM at the full bandwidth for all modes as required.
The AM bandwidth *used to be* displayed as the IF bandwidth, but many
were confused because it was double the bandwidth that could be heard -
so it was changed to *display* the audio bandwidth.
It is a matter of display only, and not the actual operating IF width.
The display of the audio bandwidth in AM is consistent with the
bandwidth displayed for other modes - the audio bandwidth is what is
displayed. (FM might be different, I don't have the FM filter to try).
In any case, now the DSP displayed bandwidth is equal to the audio
bandwidth, no matter what the actual IF processing width may be.
73,
Don W3FPR
Steve Ellington wrote:
> Don
> I understand the demodulation theory just fine but the concept of having the
> DSP act as an audio filter instead of an IF filter causes some confusion
> since we've been hyping up "IF-DSP" filtering for several years now and
> suddenly we talk about AF-DSP in only one mode, AM.
> Maybe if the BW display showed 10 Khz instead of 5 Khz it would limit the
> confusion since that seems to be what most of us are used to. We understand
> that we only hear one of the two sidebands anyway. Oh well,,, I guess the
> question will keep coming up.
> Steve Ellington
> N4...@carolina.rr.com
>
>
Steve Ellington
Please believe me. I'm not confused. It just needs to be made clear to
people who purchase the 13 Khz filter will only see 5 Khz on the display.
Someone else asked the original question because HE was confused and I can
understand why. I don't recall this to ever be a point of confusion on any
other rig.
Steve Ellington
N4...@carolina.rr.com
----- Original Message -----
From: "Don Wilhelm" <w3...@embarqmail.com>
To: "Steve Ellington" <n4...@carolina.rr.com>
Cc: <elec...@mailman.qth.net>
Sent: Sunday, May 03, 2009 5:03 PM
Subject: Re: [Elecraft] Shortwave filters for K3
Joe Subich, W4TV
> Please do not be confused. The DSP *is* at the IF frequency, *and it
> does* process AM at the full bandwidth for all modes as required.
That is only true to the extent allowed by the roofing filter. Thus
if one has the 6 KHz AM filter and is tuned EXACTLY to the carrier
of the AM station, the audio bandwidth is something around 3300 -
3500 Hz because the Roofing filter will be down 6 dB at +/- 3 KHz.
One can get a full 5 KHz (really 4.2 KHz since that's the maximum
audio frequency supported by the DSP) by tuning slightly to one
side of the carrier to receive one full sideband.
> (FM might be different, I don't have the FM filter to try).
FM displays N/A ... the DSP is "fixed" at the 15 KHz IF/4.2KHz AF.
> > Maybe if the BW display showed 10 Khz instead of 5 Khz it would
> > limit the confusion since that seems to be what most of us are used
> > to.
When the DSP showed the IF bandwidth in AM and the audio bandwidth
in other modes if caused far more confusion than it does now. In
addition, it broke all of the software width controls because of
the inconsistency when changing modes.
> Please believe me. I'm not confused. It just needs to be made clear to
> people who purchase the 13 Khz filter will only see 5 Khz on the
> display.
5 KHz is fiction anyway since the DSP has a high cut of 4.2 KHz
(2.8 KHz in CW). BW may say 5.00 but the real bandwidth is only
4100 Hz (100 to 4200 Hz). Audio response is down 60 dB or more
at 4.5 KHz even when BW says 5.00.
73,
... Joe, W4TV
> -----Original Message-----
> From: elecraft...@mailman.qth.net
> [mailto:elecraft...@mailman.qth.net] On Behalf Of Don Wilhelm
> Sent: Sunday, May 03, 2009 5:03 PM
> To: Steve Ellington
> Cc: elec...@mailman.qth.net
> Subject: Re: [Elecraft] Shortwave filters for K3
>
>
Frank Ross W4NHJ
I thought I read some where that you can use the 13 kHz filter to transmit
AM by using the transmit DSP to limit modulation bandwidth.
Frank
----- Original Message -----
From: "Don Wilhelm" <w3...@embarqmail.com>
To: <n4z...@yahoo.com>
Cc: <elec...@mailman.qth.net>
Sent: Sunday, May 03, 2009 2:01 PM
Subject: Re: [Elecraft] Shortwave filters for K3
Frank Ross W4NHJ
I thought I read some where that you can use the 13 kHz filter to transmit
AM by using the transmit DSP to limit modulation bandwidth.
Frank
----- Original Message -----
From: "Don Wilhelm" <w3...@embarqmail.com>
To: <n4z...@yahoo.com>
Cc: <elec...@mailman.qth.net>
Sent: Sunday, May 03, 2009 2:01 PM
Subject: Re: [Elecraft] Shortwave filters for K3
Don Wilhelm
13 kHz comes quite close to the 15 kHz DSP IF, so using the 13 kHz
roofing filter for AM transmit exposes the possibility of transmission
at an IF image as well as the desired frequency.
As I recall, that was a wish expressed by someone, but I also recall the
Elecraft designers saying that it has a high probability of creating
spurs, but would be further evaluated sometime in the future.
If it has already been evaluated, then the answer is that it really does
create spurs and cannot be used. If it has not yet been evaluated, then
it is still on the list for future consideration.
73,
Don W3FPR
Frank Ross W4NHJ wrote:
> Don,
>
> I thought I read some where that you can use the 13 kHz filter to transmit
> AM by using the transmit DSP to limit modulation bandwidth.
>
> Frank
>
>
Joe Subich, W4TV
> 13 kHz comes quite close to the 15 kHz DSP IF, so using the 13 kHz
> roofing filter for AM transmit exposes the possibility of
> transmission at an IF image as well as the desired frequency.
The IF Image is 30 KHz away (twice the IF), not 15 KHz. Although
neither INRAD nor Elecraft will provide specifications (shape factor)
for the FM filter, one should be able to assume it is considerably
less than 2:1. For comparison, the INRAD 6 pole 7007.5 MHz filter
has a 1:60 dB shape factor of 2.1 and the 7045.0/7065.0 MHz 20 KHz
filters have 1:60 dB shape factors of 2.2. Converting to 6:60 dB
would make the shape factors about 1.9 and adding two more poles
would further sharpen the skirts.
Still, even with a shape factor of 2 by the time one was out 20
KHz (40 KHz bandwidth point) the filter response should be down
more than 100 dB ... somewhere about the level of stray coupling
(feed through) in the IF. 20 KHz leaves plenty of "guard band"
from even the highest sidebands with a 3.5 KHz maximum modulating
frequency. If that's not enough, the filter could even be offset
another 4 KHz "away" from the image during transmit without any
impact on the transmit bandwidth (3500 Hz maximum modulation
frequency).
If I have time between now and the time I leave for Dayton, I'll
set up one K3 to transmit AM (and ESSB) through the FM filter and
make some spectrum analyzer captures (using the SQR-IQ, SpectraVue
and the other K3). If there's a problem, it will be easy enough
to see.
73,
... Joe, W4TV
> -----Original Message-----
> From: elecraft...@mailman.qth.net
> [mailto:elecraft...@mailman.qth.net] On Behalf Of Don Wilhelm
> Sent: Sunday, May 03, 2009 9:16 PM
> To: Frank Ross W4NHJ
> Cc: elec...@mailman.qth.net
> Subject: Re: [Elecraft] Shortwave filters for K3
>
>
David Windisch
Bill, in AM, try tuning 1-2 KHz higher or lower than carrier frequency.
Works
for me (more highs) cuz I have only a 2.8 KHz filter for other-than-cw
modes.
73 Dave W8FGX
Guys,
Just got done assembling my second K3 and this one has a 13 Khz
SNIP
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