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[K3] Unanswered Questions ...

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Joe Subich, W4TV

[K3] Unanswered Questions ...

I have not been able to find answers to these two questions
in the archives, in the K3 FAQ or on the Elecraft web site.

1) What is the bandwidth of the KFL3B-FM filter at -6 dB
and -60 dB?

2) If the KFL3B-FM is installed in a K3 (when available) is
a separate AM filter required for AM and/or ESSB transmit?
In other words, is the requirement simply that one roofing
filter be of sufficient bandwidth to pass the AM/ESSB or
is the "proper" bandwidth required to clean up artifacts
from the DSP modulation process?

73,

... Joe, W4TV

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Greg - AB7R

Re: [K3] Unanswered Questions ...

Joe,

Those measurements are not yet available for the FM filter. I believe a plot will
be posted on the website when the testing is done. If not, I'll put the data on
the FAQ.

Elecraft does not recommend using the FM filter in transmit for anything other than
FM. For ESSB, you will need to use the AM filter. There will also be a config
menu item for ESSB to select the final TX BW when the AM filter is used.

-------------------------
73,
Greg - AB7R
Whidbey Island WA
NA-065
K3#0009

On Wed Oct 24 16:21 , "Joe Subich, W4TV" sent:

>
>I have not been able to find answers to these two questions
>in the archives, in the K3 FAQ or on the Elecraft web site.
>
>1) What is the bandwidth of the KFL3B-FM filter at -6 dB
> and -60 dB?
>
>2) If the KFL3B-FM is installed in a K3 (when available) is
> a separate AM filter required for AM and/or ESSB transmit?
> In other words, is the requirement simply that one roofing
> filter be of sufficient bandwidth to pass the AM/ESSB or
> is the "proper" bandwidth required to clean up artifacts
> from the DSP modulation process?
>
>73,
>
> ... Joe, W4TV
>
>
>_______________________________________________
>Elecraft mailing list
>Post to: [hidden email]
>You must be a subscriber to post to the list.
>Subscriber Info (Addr. Change, sub, unsub etc.):
> http://mailman.qth.net/mailman/listinfo/elecraft
>
>Help: http://mailman.qth.net/subscribers.htm
>Elecraft web page: http://www.elecraft.com

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Joe Subich, W4TV

RE: [K3] Unanswered Questions ...

> Elecraft does not recommend using the FM filter in transmit
> for anything other than FM. For ESSB, you will need to use
> the AM filter.

Why is that, does the DSP modulation process require additional
filtering to make it acceptable on transmit? If so, how "bad"
is the unfiltered signal?

Since the KFL3B-FM is not likely to be any wider than the
typical four pole "roofing filter" in most of the "modern"
transceivers, one would think that AM receive performance
would be good to excellent with just the DSP filtering. In
that case it would seem to be silly to "waste" one filter
filter slot if one wanted to configure a K3 for both FM and
AM (or ESSB) transmission.

If both the KFL3B-FM and KFL3A-6K are required, it means the
user must make a choice between equipping the receiver with
normal and narrow roofing (first IF) filters for both SSB and
CW or "all mode" (FM, AM, and ESSB) operation.

73,

... Joe, W4TV

> -----Original Message-----
> From: Greg - AB7R [mailto:[hidden email]]
> Sent: Wednesday, October 24, 2007 4:53 PM
> To: [hidden email]; 'Joe; Subich; W4TV'
> Subject: Re: [Elecraft] [K3] Unanswered Questions ...
>
>
> Joe, Those measurements are not yet available for the FM
> filter. I believe a plot will be posted on the website when
> the testing is done. If not, I'll put the data on the FAQ.
> Elecraft does not recommend using the FM filter in transmit
> for anything other than FM. For ESSB, you will need to use
> the AM filter. There will also be a config menu item for
> ESSB to select the final TX BW when the AM filter is used.
> ------------------------- 73, Greg - AB7R Whidbey Island WA
> NA-065 K3#0009 On Wed Oct 24 16:21 , "Joe Subich, W4TV"
> sent: > >I have not been able to find answers to these two
> questions >in the archives, in the K3 FAQ or on the Elecraft
> web site. > >1) What is the bandwidth of the KFL3B-FM filter
> at -6 dB > and -60 dB? > >2) If the KFL3B-FM is
> installed in a K3 (when available) is > a separate AM
> filter required for AM and/or ESSB transmit? > In other
> words, is the requirement simply that one roofing > filter
> be of sufficient bandwidth to pass the AM/ESSB or > is the
> "proper" bandwidth required to clean up artifacts > from
> the DSP modulation process? > >73, > > ... Joe, W4TV

KK7P

Re: [K3] Unanswered Questions ...

Hello Joe!

Unlike most DSP-based radios, the K3 converts from the main IF (8.215
MHz) to the 15 kHz DSP IF without an intervening 455 kHz IF. This helps
give the K3 a cleaner sound and better dynamic range since there are
fewer conversions.

However, this also means the IF crystal filter must have exceptional
skirt selectivity to prevent the 15 kHz "image" from getting through.

In FM mode, capture effect helps suppress the effect of "image" leakage.
AM and (E)SSB modes have far more dynamic range and the 15 kHz IF
image must be suppressed by some 10s of dB more than is required for
excellent FM operation.

It is for this reason that the FM filter is recommended only for FM use.
It may also be used for AM receive, but is not recommended for AM
transmit. The 6 kHz filter has the necessary 15 kHz IF image rejection
to be suitable for use with either AM or (E)SSB.

73,

Lyle KK7P

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Joe Subich, W4TV

RE: [K3] Unanswered Questions ...

Lyle,

Thanks for the information ...

> In FM mode, capture effect helps suppress the effect of "image"
> leakage. AM and (E)SSB modes have far more dynamic range and the
> 15 kHz IF image must be suppressed by some 10s of dB more than is
> required for excellent FM operation.

Are you speaking of receive performance or transmit performance?

The conversion "image" either transmit or receive is separated
from the desired frequency by twice the IF (in this case 15 KHz).
In this case the "image" would be 30 KHz away - in this case at
8.245 MHz (or 8.185 MHz depending whether the second LO was above
or below the IF).

Since the specifications for the KFL3B-FM are not available, the
closest filter I can find is the 15 KHz wide, 7.0075 MHz, 6-pole
INRAD "front end" filter. That filter shows a estimated shape
factor of 2.1 (1/60 dB). Assuming the desired signal is centered
in the first IF (roofing) filter passband and we were dealing with
a 6 KHz wide ESSB signal, the closest "image" frequency would be
27 KHz from the center of the "roofing" filter. At 27 KHz from
center, the response of that 15 KHz/6-pole filter is down 80 to
100 dB depending on the filter's ultimate rejection - remember,
the filter is 30 KHz wide at the -60 dB point which is 15 KHz
from the center of the passband.

If we're looking at an AM signal with 5 KHz audio response, the
nearest component is 25 KHz from the center of the filter - again
down far more than 80 dB (and another 6 dB down compared to the
PEP of the transmitted signal).

It would seem to me that 70 to 80 dB suppression of the transmit
image is far more than required by Part 97.307(d) ... and that
something in the .01 mW PEP range for 1500 W PEP of "desired"
signal would be more than acceptable.

Is there something in the digital domain that I'm missing -
aliasing, artifacts, etc. that effects this?

Now, on receive 80 or 90 dB may not be sufficient rejection, it
depends on conditions but still at nearly two times the -60 dB
point on the filter skirt it would seem that the image response
would be determined by be more one of circuit leakage and the
filter's ultimate (stopband rejection) rather than skirt
selectivity.

Has the IF image response been checked with an engineering sample
or prototype of the KFL3B-FM in either receive or transmit?

73,

... Joe, W4TV

> -----Original Message-----
> From: [hidden email]
> [mailto:[hidden email]] On Behalf Of Lyle Johnson
> Sent: Wednesday, October 24, 2007 8:01 PM
> To: [hidden email]
> Subject: Re: [Elecraft] [K3] Unanswered Questions ...
>
>
> Hello Joe!
>
> Unlike most DSP-based radios, the K3 converts from the main IF (8.215
> MHz) to the 15 kHz DSP IF without an intervening 455 kHz IF.
> This helps
> give the K3 a cleaner sound and better dynamic range since there are
> fewer conversions.
>
> However, this also means the IF crystal filter must have exceptional
> skirt selectivity to prevent the 15 kHz "image" from getting through.
>
> In FM mode, capture effect helps suppress the effect of
> "image" leakage.
> AM and (E)SSB modes have far more dynamic range and the 15 kHz IF
> image must be suppressed by some 10s of dB more than is required for
> excellent FM operation.
>
> It is for this reason that the FM filter is recommended only
> for FM use.
> It may also be used for AM receive, but is not recommended for AM
> transmit. The 6 kHz filter has the necessary 15 kHz IF image
> rejection
> to be suitable for use with either AM or (E)SSB.
>
> 73,
>
> Lyle KK7P
>
> _______________________________________________
> Elecraft mailing list
> Post to: [hidden email]
> You must be a subscriber to post to the list.
> Subscriber Info (Addr. Change, sub, unsub etc.):
> http://mailman.qth.net/mailman/listinfo/elecraft
>
> Help: http://mailman.qth.net/subscribers.htm
> Elecraft web page: http://www.elecraft.com
>
>

KK7P

Re: [K3] Unanswered Questions ...

>> In FM mode, capture effect helps suppress the effect of "image"
>> leakage. AM and (E)SSB modes have far more dynamic range and the
>> 15 kHz IF image must be suppressed by some 10s of dB more than is
>> required for excellent FM operation.
>
> Are you speaking of receive performance or transmit performance?

Receive characteristics.

> ...
> Is there something in the digital domain that I'm missing -
> aliasing, artifacts, etc. that effects this?

No, the issue is IF image rejection.

73,

Lyle KK7P

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Joe Subich, W4TV

RE: [K3] Unanswered Questions ...

OK, that brings me back to the original question ... since the
standard 2.7 KHz, 5-pole filter is 2900 Hz at -6dB which should
be wide enough on receive for AM (by shifting the receive passband
to use one sideband) and for ESSB (except for the ultra fidelity
group), is there any harm in using the FM filter for AM and ESSB
transmit?

If one wants maximum performance on AM/ESSB receive, I understand
the 6 KHz filter will provide better IF image rejection (now I'm
really interested in the FM filter curves - including the stopband
and ultimate rejection specification). However, if the priorities
are maximum performance on SSB and CW (including protection from
AGC pumping), a slight (?) reduction image rejection due to the
wider filter may be an acceptable compromise given the limited
number of available filer "slots".

What is the ultimate rejection in the 8-pole filters, the on-line
curves only go to -80 dB? Some of the filters (250, 400, 1.8K)
show signs of "flaring out" at 70 to 75 dB down and for others (6K,
2.8K) the scale is wide enough that the skirts are "off screen"
before they reach -80 dB.

Of course, this may all be academic when variable width filters
are available.

73,

... Joe, W4TV

> -----Original Message-----
> From: [hidden email]
> [mailto:[hidden email]] On Behalf Of Lyle Johnson
> Sent: Wednesday, October 24, 2007 11:32 PM
> To: [hidden email]
> Subject: Re: [Elecraft] [K3] Unanswered Questions ...
>
>
> >> In FM mode, capture effect helps suppress the effect of "image"
> >> leakage. AM and (E)SSB modes have far more dynamic range and the
> >> 15 kHz IF image must be suppressed by some 10s of dB more than is
> >> required for excellent FM operation.
> >
> > Are you speaking of receive performance or transmit performance?
>
> Receive characteristics.
>
> > ...
> > Is there something in the digital domain that I'm missing -
> > aliasing, artifacts, etc. that effects this?
>
> No, the issue is IF image rejection.
>
> 73,
>
> Lyle KK7P