250 Hz and 400 Hz Filter Measurements

[Jim Brown-10]

Crew,

I finally got around to dragging out my precision audio spectrum
analzyer to check out the 250 Hz filters. I have two K3s, one
with a single RX and one with two RS. I have 250 Hz and 400 Hz
filters in all three RX.

First, I tweaked the centering of each filter (that is, the
offset), then I measured bandwidth at -6dB, -30dB, and -60dB with
IF bandwidths of 1kHz and 250 Hz.  

Results: There was some variation from one filter to another, but
trends are quite consistent. The only significant difference
between the 250 and 400 Hz filters is at their -6dB points with a
1kHz IF, where the average bandwidths were 311 and 412 Hz
respectively. This is essentially the bandwidth of the roofing
filters themselves.

Once you crank the IF down to 250Hz, there is no significant
difference between the two filters. The average -6dB bandwidths
were 193 and 200 Hz; at -30 dB, both filters averaged 300 Hz; at
-60dB, they averaged 381 and 397 Hz respectively.

There IS one repeatable difference between the 250Hz and 400Hz
filters -- their insertion loss, which is 3dB. That is, the 250Hz
filters have 3dB more insertion loss.

After doing these measurements, I firmly agree with W4ZV, W0YK,
and others, who have noted that there is no good reason for
having both of these filters in a radio. Indeed, there is no good
reason for the EXISTENCE of this particular 250 Hz filter,
primarily because it is NOT a 250Hz filter by any reasonable
measure.

It is long past time for Inrad to lean on their filter supplier
and get them to ship 250 Hz filters. Failing that, I want my
money back for three filters. BTW -- all of these were purchased
from Inrad, not Elecraft.

73, Jim Brown K9YC


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[Craig Smith]

Good Morning Jim ...

Thank you for the detailed measurements.  That helps substantiate some of
the word-of-mouth on the forum.

I have done no measurements, but do have a 400 Hz filter both in my main RX
and in the sub.  My main interest is CW contesting and I have found them
more than adequate for everything except for conditions sometimes
experienced in the 160 meter contests.  On 160 the 200 Hz 5 pole filter in
my main RX occasionally improves the situation with a very strong local
station.  But I agree with you that there is no need for the 250 Hz 8 pole
option.

73   Craig  AC0DS

Craig D. Smith
PowerSmith Consulting
1009 Alder Way
Longmont, CO  80503
phone:  [phone]      email:  [hidden email]
web:  www.PowerSmithConsulting.com
 

<> -----Original Message-----
<> From: [hidden email]
[mailto:[hidden email]]
<> On Behalf Of Jim Brown
<> Sent: Friday, July 16, 2010 12:48 AM
<> To: Elecraft List
<> Subject: [Elecraft] 250 Hz and 400 Hz Filter Measurements
<>
<> Crew,
<>
<> I finally got around to dragging out my precision audio spectrum
<> analzyer to check out the 250 Hz filters. I have two K3s, one
<> with a single RX and one with two RS. I have 250 Hz and 400 Hz
<> filters in all three RX.
<>
<> First, I tweaked the centering of each filter (that is, the
<> offset), then I measured bandwidth at -6dB, -30dB, and -60dB with
<> IF bandwidths of 1kHz and 250 Hz.
<>
<> Results: There was some variation from one filter to another, but
<> trends are quite consistent. The only significant difference
<> between the 250 and 400 Hz filters is at their -6dB points with a
<> 1kHz IF, where the average bandwidths were 311 and 412 Hz
<> respectively. This is essentially the bandwidth of the roofing
<> filters themselves.
<>
<> Once you crank the IF down to 250Hz, there is no significant
<> difference between the two filters. The average -6dB bandwidths
<> were 193 and 200 Hz; at -30 dB, both filters averaged 300 Hz; at
<> -60dB, they averaged 381 and 397 Hz respectively.
<>
<> There IS one repeatable difference between the 250Hz and 400Hz
<> filters -- their insertion loss, which is 3dB. That is, the 250Hz
<> filters have 3dB more insertion loss.
<>
<> After doing these measurements, I firmly agree with W4ZV, W0YK,
<> and others, who have noted that there is no good reason for
<> having both of these filters in a radio. Indeed, there is no good
<> reason for the EXISTENCE of this particular 250 Hz filter,
<> primarily because it is NOT a 250Hz filter by any reasonable
<> measure.
<>
<> It is long past time for Inrad to lean on their filter supplier
<> and get them to ship 250 Hz filters. Failing that, I want my
<> money back for three filters. BTW -- all of these were purchased
<> from Inrad, not Elecraft.
<>
<> 73, Jim Brown K9YC
<>
<>
<> ______________________________________________________________
<> 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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[Bob Naumann W5OV]

FWIW - I have the 5-pole 500 and the 5-pole 200 in my K3 and I find that
they cover all contest conditions thus far.

I have not felt that I needed any additional filtering at all under any
circumstances and I rarely use the 200.

When things are light - I switch to the 1kHz or even the 2.1 sometimes on CW
when conditions allow.

73,

Bob W5OV
 
-----Original Message-----
From: [hidden email]
[mailto:[hidden email]] On Behalf Of Craig D. Smith
Sent: Friday, July 16, 2010 7:36 AM
To: 'Jim Brown'; [hidden email]
Subject: Re: [Elecraft] 250 Hz and 400 Hz Filter Measurements

Good Morning Jim ...

Thank you for the detailed measurements.  That helps substantiate some of
the word-of-mouth on the forum.

I have done no measurements, but do have a 400 Hz filter both in my main RX
and in the sub.  My main interest is CW contesting and I have found them
more than adequate for everything except for conditions sometimes
experienced in the 160 meter contests.  On 160 the 200 Hz 5 pole filter in
my main RX occasionally improves the situation with a very strong local
station.  But I agree with you that there is no need for the 250 Hz 8 pole
option.

73   Craig  AC0DS

Craig D. Smith
PowerSmith Consulting
1009 Alder Way
Longmont, CO  80503
phone:  [phone]      email:  [hidden email]
web:  www.PowerSmithConsulting.com
 

<> -----Original Message-----
<> From: [hidden email]
[mailto:[hidden email]]
<> On Behalf Of Jim Brown
<> Sent: Friday, July 16, 2010 12:48 AM
<> To: Elecraft List
<> Subject: [Elecraft] 250 Hz and 400 Hz Filter Measurements
<>
<> Crew,
<>
<> I finally got around to dragging out my precision audio spectrum
<> analzyer to check out the 250 Hz filters. I have two K3s, one
<> with a single RX and one with two RS. I have 250 Hz and 400 Hz
<> filters in all three RX.
<>
<> First, I tweaked the centering of each filter (that is, the
<> offset), then I measured bandwidth at -6dB, -30dB, and -60dB with
<> IF bandwidths of 1kHz and 250 Hz.
<>
<> Results: There was some variation from one filter to another, but
<> trends are quite consistent. The only significant difference
<> between the 250 and 400 Hz filters is at their -6dB points with a
<> 1kHz IF, where the average bandwidths were 311 and 412 Hz
<> respectively. This is essentially the bandwidth of the roofing
<> filters themselves.
<>
<> Once you crank the IF down to 250Hz, there is no significant
<> difference between the two filters. The average -6dB bandwidths
<> were 193 and 200 Hz; at -30 dB, both filters averaged 300 Hz; at
<> -60dB, they averaged 381 and 397 Hz respectively.
<>
<> There IS one repeatable difference between the 250Hz and 400Hz
<> filters -- their insertion loss, which is 3dB. That is, the 250Hz
<> filters have 3dB more insertion loss.
<>
<> After doing these measurements, I firmly agree with W4ZV, W0YK,
<> and others, who have noted that there is no good reason for
<> having both of these filters in a radio. Indeed, there is no good
<> reason for the EXISTENCE of this particular 250 Hz filter,
<> primarily because it is NOT a 250Hz filter by any reasonable
<> measure.
<>
<> It is long past time for Inrad to lean on their filter supplier
<> and get them to ship 250 Hz filters. Failing that, I want my
<> money back for three filters. BTW -- all of these were purchased
<> from Inrad, not Elecraft.
<>
<> 73, Jim Brown K9YC
<>
<>
<> ______________________________________________________________
<> 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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[Richard Ferch]

As someone who mainly operates during CW and RTTY contests, I draw a
slightly different conclusion from Craig's and Jim's. In the absence of
a true 270 Hz roofing filter, the existing "250 Hz" filter appears to be
the best roofing filter option on offer for RTTY contest use. Therefore,
while I agree with Ed's conclusion about these two filters, of the two
the one that is unnecessary for me is the 400 Hz filter.

 From amongst the existing choices, a 500/250 combination seems most
effective for my situation. If I were more heavily into 160 CW as
compared with RTTY, I might opt for a 400/200 combination instead. For a
single roofing filter for CW/RTTY use, you might find a 400 to be the
best single-filter compromise. Different solutions for different situations.

73,
Rich VE3KI


Craig AC0DS wrote:

> I agree with you that there is no need for the 250 Hz 8 pole
> option.

Jim Brown K9YC wrote:

> Indeed, there is no good
> reason for the EXISTENCE of this particular 250 Hz filter,
> primarily because it is NOT a 250Hz filter by any reasonable
> measure.

and Ed Muns W0YK wrote:

> Accordingly, in their application as
> roofing filters for the K3, I see no reason to have both the "250" and the
> "400" in the same receiver.

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[Stan Gibbs, KR7C]

Does anyone have a frequency response plot for the 200 Hz 5-pole filter, similar to the 8-pole plots on the Elecraft web site?

[Ed Muns, W0YK]

VE3KI wrote:
> As someone who mainly operates during CW and RTTY contests, I
> draw a slightly different conclusion from Craig's and Jim's.
> In the absence of a true 270 Hz roofing filter, the existing
> "250 Hz" filter appears to be the best roofing filter option
> on offer for RTTY contest use. Therefore, while I agree with
> Ed's conclusion about these two filters, of the two the one
> that is unnecessary for me is the 400 Hz filter.

Same here.  For the record, I use 8-pole 500 Hz INRADs for CW.  I use the
8-pole 370 Hz (labeled 250 Hz) for RTTY with the DSP Dual-Tone Filter.  The
5-pole 200 Hz works for RTTY, but rolls off the outer edges of the tones
(especially in conjunction with the Dual-Tone Filter) and I prefer the
steeper skirts on the 8-pole 370 Hz filter.  It is enough wider than the DSP
filtering such that little cascade roll-off occurs.

>  From amongst the existing choices, a 500/250 combination
> seems most effective for my situation. If I were more heavily
> into 160 CW as compared with RTTY, I might opt for a 400/200
> combination instead. For a single roofing filter for CW/RTTY
> use, you might find a 400 to be the best single-filter
> compromise. Different solutions for different situations.

Exactly.  There isn't a single solution for all scenarios.  That's why there
are filter sockets and a range of alternative filters.  The key is to
understand how the K3 filters work (both DSP and crystal) so that thoughtful
choices can be made by each individual.

73,
Ed
--------------
Ed Muns - W0YK
www.w0yk.com

> Craig AC0DS wrote:
>
> > I agree with you that there is no need for the 250 Hz 8 pole
> > option.
>
> Jim Brown K9YC wrote:
>
> > Indeed, there is no good
> > reason for the EXISTENCE of this particular 250 Hz filter,
> > primarily because it is NOT a 250Hz filter by any reasonable
> > measure.
>
> and Ed Muns W0YK wrote:
>
> > Accordingly, in their application as
> > roofing filters for the K3, I see no reason to have both
> the "250" and the
> > "400" in the same receiver.

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[Guy, K2AV]

In my radio I have the 400 set as 450 Hz, and I have the 250 set at
350 Hz, well aware of how close they are.  I use the 350 setting for
running when the stuff above or below starts to infringe and the
difference between the two. Pulling in the roofing filter against a
9+30 signal just above or below is very noticeable and extremely
useful.

Each to his own, just don't start a ban the 250 campaign.  I know a
lot of contest operators using the 400/250 8 pole combo in the manner
I described.

73, Guy.

On Fri, Jul 16, 2010 at 2:47 AM, Jim Brown <[hidden email]> wrote:

> Crew,
>
> I finally got around to dragging out my precision audio spectrum
> analzyer to check out the 250 Hz filters. I have two K3s, one
> with a single RX and one with two RS. I have 250 Hz and 400 Hz
> filters in all three RX.
>
> First, I tweaked the centering of each filter (that is, the
> offset), then I measured bandwidth at -6dB, -30dB, and -60dB with
> IF bandwidths of 1kHz and 250 Hz.
>
> Results: There was some variation from one filter to another, but
> trends are quite consistent. The only significant difference
> between the 250 and 400 Hz filters is at their -6dB points with a
> 1kHz IF, where the average bandwidths were 311 and 412 Hz
> respectively. This is essentially the bandwidth of the roofing
> filters themselves.
>
> Once you crank the IF down to 250Hz, there is no significant
> difference between the two filters. The average -6dB bandwidths
> were 193 and 200 Hz; at -30 dB, both filters averaged 300 Hz; at
> -60dB, they averaged 381 and 397 Hz respectively.
>
> There IS one repeatable difference between the 250Hz and 400Hz
> filters -- their insertion loss, which is 3dB. That is, the 250Hz
> filters have 3dB more insertion loss.
>
> After doing these measurements, I firmly agree with W4ZV, W0YK,
> and others, who have noted that there is no good reason for
> having both of these filters in a radio. Indeed, there is no good
> reason for the EXISTENCE of this particular 250 Hz filter,
> primarily because it is NOT a 250Hz filter by any reasonable
> measure.
>
> It is long past time for Inrad to lean on their filter supplier
> and get them to ship 250 Hz filters. Failing that, I want my
> money back for three filters. BTW -- all of these were purchased
> from Inrad, not Elecraft.
>
> 73, Jim Brown K9YC
>
>
> ______________________________________________________________
> 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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[David Gilbert]

Why not just tighten the DSP on the 450 Hz filter instead of kicking in
the 250 Hz filter?  It's just as easy to do for when that occasional
S9+30 signal pops up nearby, and at both 30db down and 60 db down you'd
have exactly the same performance.

I happen to have the 250 Hz filter, but after seeing K9YC's data I'm
convinced I would have been just as well off with the 400 Hz fllter (I
don't see the need for both) and I would have saved myself 3db insertion
loss to boot.

73,
Dave   AB7E



On 7/16/2010 11:38 AM, Guy Olinger K2AV wrote:

> In my radio I have the 400 set as 450 Hz, and I have the 250 set at
> 350 Hz, well aware of how close they are.  I use the 350 setting for
> running when the stuff above or below starts to infringe and the
> difference between the two. Pulling in the roofing filter against a
> 9+30 signal just above or below is very noticeable and extremely
> useful.
>
> Each to his own, just don't start a ban the 250 campaign.  I know a
> lot of contest operators using the 400/250 8 pole combo in the manner
> I described.
>
> 73, Guy.
>
> On Fri, Jul 16, 2010 at 2:47 AM, Jim Brown<[hidden email]>  wrote:
>    
>> Crew,
>>
>> I finally got around to dragging out my precision audio spectrum
>> analzyer to check out the 250 Hz filters. I have two K3s, one
>> with a single RX and one with two RS. I have 250 Hz and 400 Hz
>> filters in all three RX.
>>
>> First, I tweaked the centering of each filter (that is, the
>> offset), then I measured bandwidth at -6dB, -30dB, and -60dB with
>> IF bandwidths of 1kHz and 250 Hz.
>>
>> Results: There was some variation from one filter to another, but
>> trends are quite consistent. The only significant difference
>> between the 250 and 400 Hz filters is at their -6dB points with a
>> 1kHz IF, where the average bandwidths were 311 and 412 Hz
>> respectively. This is essentially the bandwidth of the roofing
>> filters themselves.
>>
>> Once you crank the IF down to 250Hz, there is no significant
>> difference between the two filters. The average -6dB bandwidths
>> were 193 and 200 Hz; at -30 dB, both filters averaged 300 Hz; at
>> -60dB, they averaged 381 and 397 Hz respectively.
>>
>> There IS one repeatable difference between the 250Hz and 400Hz
>> filters -- their insertion loss, which is 3dB. That is, the 250Hz
>> filters have 3dB more insertion loss.
>>
>> After doing these measurements, I firmly agree with W4ZV, W0YK,
>> and others, who have noted that there is no good reason for
>> having both of these filters in a radio. Indeed, there is no good
>> reason for the EXISTENCE of this particular 250 Hz filter,
>> primarily because it is NOT a 250Hz filter by any reasonable
>> measure.
>>
>> It is long past time for Inrad to lean on their filter supplier
>> and get them to ship 250 Hz filters. Failing that, I want my
>> money back for three filters. BTW -- all of these were purchased
>> from Inrad, not Elecraft.
>>
>> 73, Jim Brown K9YC
>>
>>
>> ______________________________________________________________
>> 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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>
>    

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[Jim Brown-10]

On Fri, 16 Jul 2010 14:38:24 -0400, Guy Olinger K2AV wrote:

>Each to his own, just don't start a ban the 250 campaign.  

Both you and Ed raise good points. No intent there -- I agree that
the so-called 250 Hz filter is slightly more useful than the 400
Hz. My real objective is to get Inrad to clean up their act!  BUT
-- take a look at the data in this link. Lots of food for thought.
There's something different about the K3 Sub-RX, and the Roofing
filters seem to fall apart below about -25dB. Makes a guy wonder
what's going on!

http://audiosystemsgroup.com/K3FilterStudy-250HzRoof.pdf 

73, Jim K9YC


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[Joe Subich, W4TV-4]

Guy,

 > Each to his own, just don't start a ban the 250 campaign.  I know a
 > lot of contest operators using the 400/250 8 pole combo in the
 > manner I described.

I don't see anyone starting a "band the 250 Hz" campaign.  I do see
a campaign for INRAD to be HONEST in their marketing and label the
filter properly.  I also see an unmet need for a REAL 250 - 275 Hz
8 pole filter.

73,

    ... Joe, W4TV

On 7/16/2010 2:38 PM, Guy Olinger K2AV wrote:

> In my radio I have the 400 set as 450 Hz, and I have the 250 set at
> 350 Hz, well aware of how close they are.  I use the 350 setting for
> running when the stuff above or below starts to infringe and the
> difference between the two. Pulling in the roofing filter against a
> 9+30 signal just above or below is very noticeable and extremely
> useful.
>
> Each to his own, just don't start a ban the 250 campaign.  I know a
> lot of contest operators using the 400/250 8 pole combo in the
> manner I described.
>
> 73, Guy.
>

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[Joe Subich, W4TV-4]

Jim,

 > There's something different about the K3 Sub-RX, and the Roofing
 > filters seem to fall apart below about -25dB. Makes a guy wonder
 > what's going on!

I would bet the subreceiver has excess gain in the IF and you're
seeing amplified IF noise (particularly since the bandwidth is
about 1 KHz).  I don't see any way for the K3 Utility to calibrate
main/receiver gain separately (Dick?) but I'd suggest checking the
gain calibration of the subreceiver or measuring all of the filters
only in the main receiver.

73,

    ... Joe, W4TV

On 7/16/2010 3:28 PM, Jim Brown wrote:

> On Fri, 16 Jul 2010 14:38:24 -0400, Guy Olinger K2AV wrote:
>
>> Each to his own, just don't start a ban the 250 campaign.
>
> Both you and Ed raise good points. No intent there -- I agree that
> the so-called 250 Hz filter is slightly more useful than the 400
> Hz. My real objective is to get Inrad to clean up their act!  BUT
> -- take a look at the data in this link. Lots of food for thought.
> There's something different about the K3 Sub-RX, and the Roofing
> filters seem to fall apart below about -25dB. Makes a guy wonder
> what's going on!
>
> http://audiosystemsgroup.com/K3FilterStudy-250HzRoof.pdf
>
> 73, Jim K9YC
>

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[wayne burdick]

I agree that a narrower 8-pole filter would be very useful.

Meanwhile, those who are interested in a very narrow CW/DATA crystal  
filter might want to consider our 5-pole model. This is one of the  
filters Sherwood (an independent reviewer) used during his K3 receiver  
testing. See the second to last column in his performance comparison  
table:

     http://www.sherweng.com/table.html

While it's true that the 5-pole 200-Hz filter is a bit narrower than  
optimum for 170-Hz shift RTTY, many operators (including me) use it  
for this all the time. The response shape is gaussian at the top, and  
in practice the slight additional loss at the edges has little impact.

73,
Wayne
N6KR


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[Dick Dievendorff]

RF Gain calibration has separate calibrations for main and sub. It's a DSP computation, and the K3 Utility has code that invokes these DSP computations in main and then sub  The wizard has more pages if you have a sub installed

Dick, K6KR


Sent from my iPhone

On Jul 16, 2010, at 1:07 PM, "Joe Subich, W4TV" <[hidden email]> wrote:

>
> Jim,
>
>> There's something different about the K3 Sub-RX, and the Roofing
>> filters seem to fall apart below about -25dB. Makes a guy wonder
>> what's going on!
>
> I would bet the subreceiver has excess gain in the IF and you're
> seeing amplified IF noise (particularly since the bandwidth is
> about 1 KHz).  I don't see any way for the K3 Utility to calibrate
> main/receiver gain separately (Dick?) but I'd suggest checking the
> gain calibration of the subreceiver or measuring all of the filters
> only in the main receiver.
>
> 73,
>
>    ... Joe, W4TV
>
> On 7/16/2010 3:28 PM, Jim Brown wrote:
>> On Fri, 16 Jul 2010 14:38:24 -0400, Guy Olinger K2AV wrote:
>>
>>> Each to his own, just don't start a ban the 250 campaign.
>>
>> Both you and Ed raise good points. No intent there -- I agree that
>> the so-called 250 Hz filter is slightly more useful than the 400
>> Hz. My real objective is to get Inrad to clean up their act!  BUT
>> -- take a look at the data in this link. Lots of food for thought.
>> There's something different about the K3 Sub-RX, and the Roofing
>> filters seem to fall apart below about -25dB. Makes a guy wonder
>> what's going on!
>>
>> http://audiosystemsgroup.com/K3FilterStudy-250HzRoof.pdf
>>
>> 73, Jim K9YC
>>
>
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[John, KI6WX]

Beware of the conclusions of the so-called K3 Filter Study.  I don't know
who wrote it since there is no name on it, but the 250 Hz filter does not
fall apart at -25 dB.  The results shown are exactly what you would expect
for a well-designed receiver with no signal at the input.  The author of
this study doesn't state the conditions of the K3 such as band, preamp
status and antenna connection, but I will assume no antenna connected with
the preamp on.

If you look at the first plot on page 3 labeled Study of Inrad 400 and 250
Hz Filters, you will see a broad shoulder at about 30 dB down that the
author is concerned about.  This shoulder drops off at 100 Hz and 1100 Hz
due to the DSP filtering.  So let's look at what is causing this.

There is no signal into the K3, so all of the noise is internally generated.
At 600 Hz, the noise is coming from the preamp and is then filtered by the
250 Hz and 400 Hz roofing filters.  The shoulder is caused by the internally
generated noise after the roofing filter.  The noise from the preamp is
dominate, so that is sets the noise figure.  However, if we made it even
more dominate, so that the shoulder was lower, we would reduce the dynamic
range of the K3.  Receiver design is a tradeoff so that you create both
minimum noise and maximum dynamic range.

If the author had coupled a noise diode to the input of this K3, he would
have found that the shoulder would disappear because he now had a noise
signal into the K3 much stronger than the preamp.  My advice would be to
disregard this report because the measurements were not made in such a way
as to accurately determine the response of the K3 filters.

-John
 KI6WX

>
> Both you and Ed raise good points. No intent there -- I agree that
> the so-called 250 Hz filter is slightly more useful than the 400
> Hz. My real objective is to get Inrad to clean up their act!  BUT
> -- take a look at the data in this link. Lots of food for thought.
> There's something different about the K3 Sub-RX, and the Roofing
> filters seem to fall apart below about -25dB. Makes a guy wonder
> what's going on!
>
> http://audiosystemsgroup.com/K3FilterStudy-250HzRoof.pdf
>
> 73, Jim K9YC
>
>
> ______________________________________________________________

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[Doug Turnbull]

I love this reflector and the discussion which takes place on it.   Thank
you all for this discourse.  Good radio, good users and good company and we
are all learning.  
          73 Doug EI2CN

-----Original Message-----
From: [hidden email]
[mailto:[hidden email]] On Behalf Of John, KI6Wx
Sent: 17 July 2010 18:49
To: 'Elecraft List'
Subject: Re: [Elecraft] 250 Hz and 400 Hz Filter Measurements

Beware of the conclusions of the so-called K3 Filter Study.  I don't know
who wrote it since there is no name on it, but the 250 Hz filter does not
fall apart at -25 dB.  The results shown are exactly what you would expect
for a well-designed receiver with no signal at the input.  The author of
this study doesn't state the conditions of the K3 such as band, preamp
status and antenna connection, but I will assume no antenna connected with
the preamp on.

If you look at the first plot on page 3 labeled Study of Inrad 400 and 250
Hz Filters, you will see a broad shoulder at about 30 dB down that the
author is concerned about.  This shoulder drops off at 100 Hz and 1100 Hz
due to the DSP filtering.  So let's look at what is causing this.

There is no signal into the K3, so all of the noise is internally generated.
At 600 Hz, the noise is coming from the preamp and is then filtered by the
250 Hz and 400 Hz roofing filters.  The shoulder is caused by the internally
generated noise after the roofing filter.  The noise from the preamp is
dominate, so that is sets the noise figure.  However, if we made it even
more dominate, so that the shoulder was lower, we would reduce the dynamic
range of the K3.  Receiver design is a tradeoff so that you create both
minimum noise and maximum dynamic range.

If the author had coupled a noise diode to the input of this K3, he would
have found that the shoulder would disappear because he now had a noise
signal into the K3 much stronger than the preamp.  My advice would be to
disregard this report because the measurements were not made in such a way
as to accurately determine the response of the K3 filters.

-John
 KI6WX

>
> Both you and Ed raise good points. No intent there -- I agree that
> the so-called 250 Hz filter is slightly more useful than the 400
> Hz. My real objective is to get Inrad to clean up their act!  BUT
> -- take a look at the data in this link. Lots of food for thought.
> There's something different about the K3 Sub-RX, and the Roofing
> filters seem to fall apart below about -25dB. Makes a guy wonder
> what's going on!
>
> http://audiosystemsgroup.com/K3FilterStudy-250HzRoof.pdf
>
> 73, Jim K9YC
>
>
> ______________________________________________________________

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[Jim Brown-10]

On Sat, 17 Jul 2010 10:48:36 -0700, John, KI6Wx wrote:

>My advice would be to
>disregard this report because the measurements were not made in such a way
>as to accurately determine the response of the K3 filters

John,

You are badly mistaken. The K3 was excited by broadband noise, and there was
enough of it for none of the noise to be internally generated. I am the
author of the study, and if you had read the email that pointed to it you
would have seen that.

These measurements are QUITE valid, and represent the response of the radio
from antenna input to audio output. The wide and narrow measurements on the
same page are the same data, plotted to different scale -- the narrow plots
to show filter bandwidth, the wide plots to show behavior well down the
slope of the filter curve. The dynamic range of the measurement is at least
75dB.

73, Jim Brown K9YC


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[W8JI]

> These measurements are QUITE valid, and represent the response of the
> radio
> from antenna input to audio output. The wide and narrow measurements on
> the
> same page are the same data, plotted to different scale -- the narrow
> plots
> to show filter bandwidth, the wide plots to show behavior well down the
> slope of the filter curve. The dynamic range of the measurement is at
> least
> 75dB.

I'm still mulling this over but........the curves of the "filter response"
are very similar to the output distribution I've seen when I was using a
multiple carriers to measure IMD in CATV amplifiers. The problem is that was
over 30 years ago, so my recall might be flawed.

I'm having a little trouble understanding how the equivalent of an infinite
number of carriers in a passband can be used to measure filter shape when
the distortion in every stage after the filter can cause mixing and produce
intermodulation products outside the filter passband.

If I wanted to know the deeper attenuation response of a filter or system
with a filter, I would use an input signal that would not fill the system
after the filter with multiple frequencies (that are unattenuated since they
are inside filter passband) that could mix and display intermodulation
products. I think there is significant risk using broadband noise through
the filter might display intermodulation distribution more than filter skirt
response.

My inclination would be to turn off the AGC and sweep with a single tone.
I'm not so sure a measurement like this isn't just a measurement of
odd-order IMD in stages following the filter.

73 Tom

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[Kok Chen]

I just did a quick experiment.

First, I lied to my K3 that my 400 Hz filter is really a 2.8 kHz filter.  I then turned AGC off and set the K3 WIDTH setting to 2.4 kHz.  

This way, I will be able to see the response of both the 2.4 kHz DSP filter and the 400 Hz roofing filter, as Dr. Grebenkemper KI6WX had described.

Sure enough, with band noise as input to the K3, I see a similar spectrum as Jim showed here

http://audiosystemsgroup.com/K3FilterStudy-250HzRoof.pdf

In my case, it is a 400 Hz wide hump sitting on a wider 2.4 kHz noise pedestal.

I switched to antennas of various gains and directions, and as expected, the peak of the crystal filter hump rises and falls, while the wider DSP filtered noise floor remains constant (lots of spectral averaging of my FFT output :-).  Remember that I have turned AGC off.

I then changed to using the Elecraft N-gen as the noise source.  The noise became stronger now than using band noise -- the peak of the 400 Hz hump is now about 30 dB over the DSP noise pedestal.  But the wideband noise pedestal (presumably the internal K3 noise, filtered by the DSP filter) remained at about the same magnitude.

When I changed the antenna input a dummy load, the 400 Hz hump disappears into the wider noise pedestal. But again, the 2.4 kHz noise pedestal did not change width nor amplitude. In my case, the DSP pedestal is a little over 20 dB higher than the noise floor outside of the DSP pedestal.

Not finding a louder noise source, I resorted to the CW signal from the Elecraft XG2, set to 50 µV output.  As I tune across the carrier I can see it rise up to 70 dB above the DSP noise pedestal and then falling back to the DSP noise pedestal.  The shape looked very reasonable for a crystal filter.

So I can definitely see at least 70 dB worth of decent 400 Hz filtering coming from the roofing filter.

The lower amplitude noise humps (from 0 to 30 dB over the DSP noise pedestal, depending upon the strength of the noise source), like the plots by Jim, are in my case, the result of the band noise (and N-gen noise) not being strong enough for me to see the full dynamic range of the roofing filter.  The XG2 showed that the filter floor of the roofing filter is at least over 70 dB below the peak 50 µV signal.

So, I would like to suggest that Jim try using a stronger noise source than band noise.  Even an N-gen (with K3 preamp on) was only giving me 30 dB over the DSP noise floor.  Perhaps a receiving preamp would be useful (no, I don't have one to try, otherwise I would :-).  

I think Jim will see what John was talking about, i.e., the plot with 25 dB to 30 dB hump over a wider pedestal is simply an artifact that the noise source to ping the roofing filter is only 25 dB to 30 dB louder than the internal noise of the K3 between the crystal filter and of the DSP filter.  Perhaps, the variation of the hump over the DSP pedestal as you vary the input noise might be persuading enough without resorting to using extremely strong noise sources.

73
Chen, W7AY

P.S., now I need to go undo my filter settings, or I might wake up tomorrow thinking that my K3 had been bricked to only seeing a 400 Hz passband! :-)



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[Bill W4ZV]

Kok Chen wrote

So, I would like to suggest that Jim try using a stronger noise source than band noise.

Yes, think about this using common sense.  Assuming typical band noise might typically be -120 to -130 dBm, and the internal noise floor of the radio is -137 dBm, you cannot expect to measure 60-70 dB down from such a low level noise source without running into internal noise floor problems.

KS7D's swept measurement program does a much better job and requires only a signal source such as the XG1 or XG1.  

http://www.ks7d.com/Downloads.htm

It also removes typical mistakes such as:

1.  Not remembering to turn AGC Off.
2.  Not remembering to lock the DSP filter much higher than the filter being measured so that there is no cascading effect.
3.  Not adjusting Pitch to a frequency such that the low end is not truncated at 200 Hz by the MCU shifting the lower cutoff at low Pitch settings.
4.  Not remembering the effect Mode (CW vs Data A) can have on measurements

73,  Bill

[W8JI]

> Yes, think about this using common sense.  Assuming typical band noise
> might
> typically be -120 to -130 dBm, and the internal noise floor of the radio
> is
> -137 dBm, you cannot expect to measure 60-70 dB down from such a low level
> noise source without running into internal noise floor problems.

I'm not sure how much this problem manifests itself in the K3 noise
measurement system, but you guys might not be seeing noise floor in the
receiver at all.

One way to measure intermodulation is to use a broadband noise source and
notch the signal from one area prior to injecting the signal into amplifier
stages. The noise, from intermodulation products of the noise mixing with
noise, fills the "hole" notched on the broadband noise. I recall the
passband displayed for the filter bandwidth noise test when the notch is
turned onto a passband of noise as being about like the display of typical
IMD.

I think that measurement is also measuring the total IMD of every stage (at
least in part) from the roofing filter to the soundcard, not just bandwidth
or background noise.

I just can't imagine measuring a filter followed by multiple stages with
limited IM performance, especially including a DSP system and audio
amplifier, that way in an effort to determine bandwidth.

Is anyone else here familiar with notched noise IMD measurements?? I only
used them a few times in the 80's, so my recall is limited.

73 Tom

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