[K3] Perfect for Field Day S&P ... Limited Production 700 Hz 8-Pole Filters

[Chuck Guenther]

I've also found the 700 Hz filter useful in VFO B for tuning through pileups, or just scanning a band.

I think I can take the blame (or credit) for suggesting to Gary that his 700 Hz filters would be useful for Field Day operations. 

My son (AA0BP) and I have operated a number of years with the K9YA group (1A from Northern Illinois). When we donated the TS-850S for this effort, the first thing we did was to take out the narrow (270 Hz and 125 Hz) crystal filters, and replace them with 500 Hz filters.  K9YA has a great QTH and good signal, so 99% of our QSO's are obtained by CQ'ing.  FD operators are not always good at zero beating, and we've found the wider filters indispensable for hearing the off-frequency callers.

I've never used my K3 for Field Day; but I would sure use the 700 Hz filter.



73,
Chuck  NI0C
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[Jim Brown-10]

On 3/3/2014 7:18 AM, Charles Guenther wrote:
> FD operators are not always good at zero beating, and we've found the wider filters indispensable for hearing the off-frequency callers.
>
> I've never used my K3 for Field Day; but I would sure use the 700 Hz filter.

I simply do not understand the desire for a 700 Hz ROOFING filter with a
K3. Remember that the switched crystal filters that we all used in older
rigs are replaced by DSP filtering in the K3 and KX3. All it takes to
get 700 Hz IF bandwidth is to turn the bandwidth knob!

The plug-in filters for the K3 are ROOFING filters -- that is, they go
in the first IF to protect the DSP from overload by VERY strong signals.
Roofing filters have the additional benefit of providing cascaded
filtering when the DSP IF is set to the same bandwidth as the roofing
filter. "Cascading" means that the filter slopes of both filters
additive. If, for example, the DSP IF is down by 10 dB at 400 Hz from
center and the roofing filter is down by 8 dB at 400 Hz, the combined
rejection is 18 dB.

I do a LOT of contesting, most of it on CW, and I almost never use IF
bandwidth greater than 400 Hz, and most often 250 Hz. Based on advice
from designers of RTTY decoding software, I've gone to 400 Hz bandwidth
for RTTY, and no longer use the dual-peak filter. With my first K3 I
bought 400 Hz and 1.8 kHz filters. A year or so later I added 250 Hz. I
have the K3 set to switch to the 250 Hz filter at 320 Hz DSP bandwidth,
and most of the time it is engaged. I've also found that I almost never
set the IF narrower than 2.2 kHz for SSB, and that the 1.8 kHz roofing
filter is too narrow.

Bottom line -- a 700 Hz roofing filter for a K3 is a solution in search
of a problem. If you think you need one, you're either doing something
quite unusual, or you probably don't understand how the K3 works.

73, Jim K9YC
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[Will Ravenel]

Jim,

It is obvious that you understand how the K3 works and that the use of a 700 Hz filter doesn't match your operating style. It is just as obvious to me that some other operators, also with very good understandings of the K3, find it useful.

Can't we just chalk it up to personal preference without insulting those who like to listen with a broader bandwidth than you but still would like the protection a roofing filter provides against strong signals outside that bandwidth?

Will, AI4VE

Sent from my iPhone
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[Jim Brown-10]

On 3/3/2014 9:08 AM, Will Ravenel wrote:
> Can't we just chalk it up to personal preference without insulting those who like to listen with a broader bandwidth than you but still would like the protection a roofing filter provides against strong signals outside that bandwidth?

Hi Will,

I'm not insulting anyone, but rather trying to educate those who have
insufficient understanding of the radio. My hope is that folks can spend
their hard earned cash on things that really do something useful rather
than those that don't.

73, Jim K9YC
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[Jim Brown-10]

On 3/3/2014 9:24 AM, Walter Underwood wrote:
> It has been suggested that a using a 400Hz filter for RTTY (or other digital modes) puts the edges of the signal in the part of the passband with the worst phase distortion. A 700Hz filter puts the signal in a better-behaved part of the passband. The DSP filters have no phase distortion, of course.

The recommendation I've seen was for 400-450 Hz bandwidth. And yes, the
phase distortion created by the filter slopes is the reason. But this
not a case of "if a little is good, more is better." As you open up the
IF bandwidth, you also let in more noise and QRM, which is basis of the
400 Hz recommendation. Good engineering (and life in general) is about
compromise between multiple factors, and the guys who know the most
about decoders made the 400-450 Hz recommendation.

Further, it is quite easy to set the K3 to switch in the roofing filters
at any frequency you want. You do this in the setup menu for each filter
by setting the filter frequency. You could, for example, set the 400 Hz
filter to switch at 350 Hz, so that at 400 Hz DSP IF you would have only
the DSP filter. Or you could set it to switch at 450 Hz, so that at 500
Hz DSP filter setting you would have a 500 Hz DSP IF cascading with the
400 Hz filter.

73, Jim K9YC
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[Joe Subich, W4TV-4]

On 3/3/2014 12:24 PM, Walter Underwood wrote:
 > It has been suggested that a using a 400Hz filter for RTTY (or other
 > digital modes) puts the edges of the signal in the part of the
 > passband with the worst phase distortion. A 700Hz filter puts the
 > signal in a better-behaved part of the passband.

Whoever suggested that does not know enough about RTTY to be credible
or you have severely misinterpreted what was said.  The 170 Hz shift,
45.45 baud RTTY signal has a true bandwidth of about 370 Hz.  The "400
Hz" filters are 400 Hz at -3dB and about 440 Hz at -6 dB.  Their -1 dB
bandwidth (area of generally flat phase response/minimum group delay)
is a very close match to the required 370 Hz bandwidth which provides
the nearly optimum trade off of AGC protection vs. bandwidth.

A 700 Hz or 1000 Hz roofing filter will allow a strong adjacent channel
signal that would otherwise provide no interference to capture and pump
the hardware AGC (or DSP AGC) in the K3 resulting in potentially severe
blocking effects.  Using a 700 Hz or 1000 Hz roofing filter for RTTY is
no better in terms of blocking than using an 1800 or 2700 Hz filter -
the most damaging interference is always the closest in frequency.

73,

    ... Joe, W4TV


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[Jim Brown-10]

On 3/3/2014 11:05 AM, Wes (N7WS) wrote:
> On 3/3/2014 9:36 AM, Jim Brown wrote:
>>  they go in the first IF to protect the DSP from overload by VERY
>> strong signals.
>
> Change that to "2nd mixer" and we are in agreement.

Thanks for the correction, Wes.

73, Jim K9YC
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