[Elecraft] start without ANY extra roofing filters....

[Larry Phipps]

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
>
>
>
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