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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