Preselector

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How much of a difference should I see on 7th order vs 3rd order preselector and how is it done  digital domain or tuned circuits?
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paul

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Posted 1 year ago

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KF4HR

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It would be interesting to know what performance difference(s) could be expected between a 3rd and 7th Order receiver, and a technical description on what a 7th Order filter actually is. 

And I know we'll never get an answer on this forum, but this begs the question, should we expect a 8th, 9th, and 10th Order filter next? :) 
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k3Tim

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Filters: the order of the filter is the number of reactive elements (L + C). In S-domain analysis, it's the number of 'S' terms. The higher order the tighter the filter skirts. The priced paid is delay and group delay. 

In the ARRL Handbook, there are good details / topology and design examples filters. For 5th and 7th order filters, looking at the Chebyshev low pass:

5th order - at 7.80 Mc  the 3dB rolloff point is 9.22 Mc. 20 dB RO 12.6 Mc
7th order - at 7.46 Mc  the 3dB rolloff point is 8.97 Mc. 20 dB RO 8.97 Mc

Between 5th and 7th order filter the skirts are steeper so from 3rd to 7th there must be a much larger difference (ie. steeper as in mountain Vs mole hill).

Will one see a difference ?
   Depends on station conditions. If away from local strong stations and used a low performance antenna, probably not. If operating field day with stations nearby, or other such transmitters - very likely.

Higher order filters -  Not sure but suspect this would target a very small portion of a tiny market.

Analog or digital implementation - Not sure but strongly suspect analog. Would have to pop the covers on the 6500 to know for sure or someone with first hand knowledge can jump in.

Hope this was helpful.
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Tim - W4TME, Customer Experience Manager

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They are analog filters.
(Edited)
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paul

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Tnx  Tim for the response but I would like to see how Flex is doing it and what their specs are along with the delay. paul, wa3qpx
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Varistor

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You may want to study other tests done on the subject that can shed light on what to expect. As the filters are analog, they are bound to be fairly similar to what other top manufacturers are doing

http://www.ab4oj.com/icom/ic7700/dsel.html

Note that there is a material difference between BPF filters and digital preselector. Your ask is somewhat confusing and unclear what exactly you had in mind.