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start without ANY extra roofing filters....

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Bill W4ZV

start without ANY extra roofing filters....

I wrote:

> Bottom line:

1. Narrower is not always better (Ten-Tec experience)
2. 8-poles is not always better than 5-poles (per Inrad)
3. Let IMD and BDR measurements be your guide

More evidence below to support waiting for IMD/BDR
measurements before ordering any roofing filters.

73, Bill W4ZV

http://mailman.qth.net/pipermail/1000mp/2007-April/020755.html

There is a fascinating article describing IMD tests on the IC-7800 by DC4KU
in CQ-DL, August 2005 (in German). In these tests, IP3 at 2 kHz offset
degrades by an astounding 16 dB when switching from the 15 kHz to the 6 kHz
roofing filter. This degradation is due to passive IMD in the filter, and
possibly also to IMD in the filter driver amplifiers caused by mismatch when
the filter is excited outside its passband. I can send you an
English-language summary of the relevant part privately, if you wish.

It is highly significant that professional receivers manufactured by the
likes of R&S, Rockwell-Collins, Racal and Harris have a single roofing
filter. This filter is typically 12 to 16 kHz wide, to pass multi-channel
ISB, VFT (multiplexed teletype) and high-speed crypto, all of which have
extremely stringent in-band IMD requirements. To quote a British engineer
who used to design shipboard HF receivers for the Royal Navy:

The up-converting architecture, with a roofing filter at a first IF above
the highest RF frequency, allows the designer to limit the bandwidth
presented to the first IF chain and second mixer. The bandwidth of this
filter is a trade-off. Its 3 dB BW must be sufficient to pass the widest
emission the receiver is required to handle, but not so narrow that IMD and
temperature-drift effects in the filter become a concern.

Cheers for now, 73,
Adam VA7OJ/AB4OJ

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N8LP

Re: start without ANY extra roofing filters....

Well done, Bill. This again shows the importance of real life
measurements... and why I didn't order any filters until more is known
about them.

Anybody interested in the subject of filter design must read the article
by list member Jack, K8ZOA in the current QEX. It gives a lot of
valuable xtal filter design insight, and has a page of excellent
references at the end for those who wish to read more on the subject.
This is a complex subject, but as Jack points out, proper
characterization of the crystals and rigorous attention to detail can
produce accurate models and repeatable designs. Jack touched on drive
level dependency in his article. Perhaps he can focus in on the effects
of xtal nonlinearity as it affects IMD for a future piece (not trying to
create work for you Jack ;-)

This is a subject which seems to be gaining in importance as receiver
designs surrounding the xtal filter seem to be improving to the point
where the filters are becoming the limiting factor in IMD performance.

73,
Larry N8LP

Bill Tippett wrote:

>
>
> I wrote:
>
> > Bottom line:
>
> 1. Narrower is not always better (Ten-Tec experience)
> 2. 8-poles is not always better than 5-poles (per Inrad)
> 3. Let IMD and BDR measurements be your guide
>
> More evidence below to support waiting for IMD/BDR
> measurements before ordering any roofing filters.
>
> 73, Bill W4ZV
>
> http://mailman.qth.net/pipermail/1000mp/2007-April/020755.html
>
> There is a fascinating article describing IMD tests on the IC-7800 by
> DC4KU
> in CQ-DL, August 2005 (in German). In these tests, IP3 at 2 kHz offset
> degrades by an astounding 16 dB when switching from the 15 kHz to the
> 6 kHz
> roofing filter. This degradation is due to passive IMD in the filter, and
> possibly also to IMD in the filter driver amplifiers caused by
> mismatch when
> the filter is excited outside its passband. I can send you an
> English-language summary of the relevant part privately, if you wish.
>
> It is highly significant that professional receivers manufactured by the
> likes of R&S, Rockwell-Collins, Racal and Harris have a single roofing
> filter. This filter is typically 12 to 16 kHz wide, to pass multi-channel
> ISB, VFT (multiplexed teletype) and high-speed crypto, all of which have
> extremely stringent in-band IMD requirements. To quote a British engineer
> who used to design shipboard HF receivers for the Royal Navy:
>
> The up-converting architecture, with a roofing filter at a first IF above
> the highest RF frequency, allows the designer to limit the bandwidth
> presented to the first IF chain and second mixer. The bandwidth of this
> filter is a trade-off. Its 3 dB BW must be sufficient to pass the widest
> emission the receiver is required to handle, but not so narrow that
> IMD and
> temperature-drift effects in the filter become a concern.
>
> Cheers for now, 73,
> Adam VA7OJ/AB4OJ
>
>
>
> _______________________________________________
> Elecraft mailing list
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> Elecraft web page: http://www.elecraft.com
>
>

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Jan Erik Holm

Re: start without ANY extra roofing filters....

In reply to this post by Bill W4ZV

This I have noticed during my own IP3 measurements on my
FT-1000D, not as much as 16dB but around 3 dB.
This was with the INRAD roofing filter and 2 kHz offset.
in any case this was enough for me not to use the filter.

I´m eagerly waiting for measurement figures on the K3.
Beats me why they can´t be presented, that I don´t understand
at all, after all it´s no rocket sience. This will also help
people select the roofing filter.
Maybe I just have to by one and measure myself, then atleast
I know it´s done right.

/SM2EKM
-------------------------------------------------------------------
Bill Tippett wrote:

>
>
> I wrote:
>
> > Bottom line:
>
> 1. Narrower is not always better (Ten-Tec experience)
> 2. 8-poles is not always better than 5-poles (per Inrad)
> 3. Let IMD and BDR measurements be your guide
>
> More evidence below to support waiting for IMD/BDR measurements
> before ordering any roofing filters.
>
> 73, Bill W4ZV
>
> http://mailman.qth.net/pipermail/1000mp/2007-April/020755.html
>
> There is a fascinating article describing IMD tests on the IC-7800 by DC4KU
> in CQ-DL, August 2005 (in German). In these tests, IP3 at 2 kHz offset
> degrades by an astounding 16 dB when switching from the 15 kHz to the 6 kHz
> roofing filter. This degradation is due to passive IMD in the filter, and
> possibly also to IMD in the filter driver amplifiers caused by mismatch
> when
> the filter is excited outside its passband. I can send you an
> English-language summary of the relevant part privately, if you wish.
>
> It is highly significant that professional receivers manufactured by the
> likes of R&S, Rockwell-Collins, Racal and Harris have a single roofing
> filter. This filter is typically 12 to 16 kHz wide, to pass multi-channel
> ISB, VFT (multiplexed teletype) and high-speed crypto, all of which have
> extremely stringent in-band IMD requirements. To quote a British engineer
> who used to design shipboard HF receivers for the Royal Navy:
>
> The up-converting architecture, with a roofing filter at a first IF above
> the highest RF frequency, allows the designer to limit the bandwidth
> presented to the first IF chain and second mixer. The bandwidth of this
> filter is a trade-off. Its 3 dB BW must be sufficient to pass the widest
> emission the receiver is required to handle, but not so narrow that IMD and
> temperature-drift effects in the filter become a concern.
>
> Cheers for now, 73,
> Adam VA7OJ/AB4OJ
>

Jack Smith-6

Re: start without ANY extra roofing filters....

In reply to this post by N8LP

Hat tip to Larry ... now all I need is a commission on every QEX sold ...

Crystals are non-linear and their motional parameters are, to some
degree or other, a function of drive voltage. Since a filter's loss is a
function of its motional parameters, the corollary to that is
intermodulation can and will be caused by what we think of as purely
passive elements such as crystals. (This phenomenon is also found in
ferrite cores and powdered iron core inductors, as they have a
non-linear B versus H curve.)

There are examples of receivers that have as the limiting IP3 crystal
filter intermodulation. See Experimental Methods in RF Design for a
discussion of Wes Hayward's observation of crystal filter IMD when
building a receiver featured in that book. It's devilishly hard to
measure crystal filter IMD, however, for a variety of reasons.

This is why a filter with fewer elements (poles) can, in some
circumstances, yield a better IP3 than a filter with more poles, as
counter-intuitive as that might seem. Whilst the filter with more poles
will keep more trash out of later receiver stages, small changes to the
motional parameters of the crystals that make up the filter with more
poles will have a greater effect on the filter's transfer function than
for a filter with the same crystals but fewer poles. Thus, although
later stages are better protected from undesired signals, that very
protection itself causes intermodulation interference.

That's why a high performance receiver must be designed in a holistic
fashion.

Jack K8ZOA

Larry Phipps wrote:

> Well done, Bill. This again shows the importance of real life
> measurements... and why I didn't order any filters until more is known
> about them.
>
> Anybody interested in the subject of filter design must read the
> article by list member Jack, K8ZOA in the current QEX. It gives a lot
> of valuable xtal filter design insight, and has a page of excellent
> references at the end for those who wish to read more on the subject.
> This is a complex subject, but as Jack points out, proper
> characterization of the crystals and rigorous attention to detail can
> produce accurate models and repeatable designs. Jack touched on drive
> level dependency in his article. Perhaps he can focus in on the
> effects of xtal nonlinearity as it affects IMD for a future piece (not
> trying to create work for you Jack ;-)
>
> This is a subject which seems to be gaining in importance as receiver
> designs surrounding the xtal filter seem to be improving to the point
> where the filters are becoming the limiting factor in IMD performance.
>
> 73,
> Larry N8LP
>
>
>
> Bill Tippett wrote:
>>
>>
>> I wrote:
>>
>> > Bottom line:
>>
>> 1. Narrower is not always better (Ten-Tec experience)
>> 2. 8-poles is not always better than 5-poles (per Inrad)
>> 3. Let IMD and BDR measurements be your guide
>>
>> More evidence below to support waiting for IMD/BDR
>> measurements before ordering any roofing filters.
>>
>> 73, Bill W4ZV
>>
>> http://mailman.qth.net/pipermail/1000mp/2007-April/020755.html
>>
>> There is a fascinating article describing IMD tests on the IC-7800 by
>> DC4KU
>> in CQ-DL, August 2005 (in German). In these tests, IP3 at 2 kHz offset
>> degrades by an astounding 16 dB when switching from the 15 kHz to the
>> 6 kHz
>> roofing filter. This degradation is due to passive IMD in the filter,
>> and
>> possibly also to IMD in the filter driver amplifiers caused by
>> mismatch when
>> the filter is excited outside its passband. I can send you an
>> English-language summary of the relevant part privately, if you wish.
>>
>> It is highly significant that professional receivers manufactured by the
>> likes of R&S, Rockwell-Collins, Racal and Harris have a single roofing
>> filter. This filter is typically 12 to 16 kHz wide, to pass
>> multi-channel
>> ISB, VFT (multiplexed teletype) and high-speed crypto, all of which have
>> extremely stringent in-band IMD requirements. To quote a British
>> engineer
>> who used to design shipboard HF receivers for the Royal Navy:
>>
>> The up-converting architecture, with a roofing filter at a first IF
>> above
>> the highest RF frequency, allows the designer to limit the bandwidth
>> presented to the first IF chain and second mixer. The bandwidth of this
>> filter is a trade-off. Its 3 dB BW must be sufficient to pass the widest
>> emission the receiver is required to handle, but not so narrow that
>> IMD and
>> temperature-drift effects in the filter become a concern.
>>
>> Cheers for now, 73,
>> Adam VA7OJ/AB4OJ
>>
>>
>>
>> _______________________________________________
>> 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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> You must be a subscriber to post to the list.
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>

Geoffrey Mackenzie-Kennedy-2

Re: start without ANY extra roofing filters....

In reply to this post by N8LP

The IMD contributed by inductor cores used in the front-end selective
circuits is often not taken into account either, or for that matter any core
within the signal path. They can bite.

73,
Geoff
GM4ESD

Larry Phipps wrote:

> This is a subject which seems to be gaining in importance as receiver
> designs surrounding the xtal filter seem to be improving to the point
> where the filters are becoming the limiting factor in IMD performance.
>
> 73,
> Larry N8LP

>> 1. Narrower is not always better (Ten-Tec experience)
>> 2. 8-poles is not always better than 5-poles (per Inrad)
>> 3. Let IMD and BDR measurements be your guide
>>
>> More evidence below to support waiting for IMD/BDR measurements
>> before ordering any roofing filters.
>>
>> 73, Bill W4ZV
>>
>> http://mailman.qth.net/pipermail/1000mp/2007-April/020755.html
>>
>> There is a fascinating article describing IMD tests on the IC-7800 by
>> DC4KU
>> in CQ-DL, August 2005 (in German). In these tests, IP3 at 2 kHz offset
>> degrades by an astounding 16 dB when switching from the 15 kHz to the 6
>> kHz
>> roofing filter. This degradation is due to passive IMD in the filter, and
>> possibly also to IMD in the filter driver amplifiers caused by mismatch
>> when
>> the filter is excited outside its passband. I can send you an
>> English-language summary of the relevant part privately, if you wish.

<snip>

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Dave, G4AON

Re: start without ANY extra roofing filters....

In reply to this post by Bill W4ZV

Adam (VA7OJ/AB4OJ) wrote:

"It is highly significant that professional receivers manufactured by the

likes of R&S, Rockwell-Collins, Racal and Harris have a single roofing
filter. This filter is typically 12 to 16 kHz wide, to pass multi-channel
ISB, VFT (multiplexed teletype) and high-speed crypto, all of which have
extremely stringent in-band IMD requirements. To quote a British engineer
who used to design shipboard HF receivers for the Royal Navy:"

Adam

I wouldn't get too hung up on commercial/military designs, while not
wishing to start a thread running off at a tangent... a friend has a
well maintained Racal RA1792 military receiver which is hopeless
compared to even moderately good amateur gear such as his Kenwood
TS-850, I appreciate the 1792 wasn't one of the better Racal receivers
but they weren't cheap. A listening comparison - even under "non
contest" conditions quickly leads to the Racal on/off switch. Add the
generally poor sensitivity of those designs, weight and cost and there's
no way they will ever compete with a K2 or K3. I don't come across many
hams using ex military/commercial gear in preference to amateur gear...
At least not those I work on QRP CW.

73 Dave
K1, K2 and soon K3/100 (on order)
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k6dgw

Re: start without ANY extra roofing filters....

Dave G4AON wrote:
> Adam
>
> a friend has a
> well maintained Racal RA1792 military receiver which is hopeless
> compared to even moderately good amateur gear such as his Kenwood
> TS-850, I appreciate the 1792 wasn't one of the better Racal receivers
> but they weren't cheap.

If things in the Mother Country are anything close to how they are over
here in the Colonies, nothing the military buys is cheap, regardless of
how well they work ... or don't :-)

Fred K6DGW
- Northern California Contest Club
- CU in the 2007 CQP Oct 6-7
- www.cqp.org
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