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
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