[R-390] Clevite Filters

[John Kolb]

Very good commentary on Clevite filters by Tisha.

I think, though, you meant to say phase shift or group delay, rather 
than phase noise?

I would add the nose of the ceramic filter bandpass is more rounded, 
compared to the square
shape of a mechanical filter.  Example
<http://www.jlkolb.cts.com/site/curves/LF-C2A.PDF>
That can be corrected for with a little bit of treble boost in the audio
chain.

John

At 06:02 AM 12/16/2008, Tisha Hayes wrote:

>Clevite filters are an interesting footnote in the history of the 
>R-390A. At the time of their conception they were more expensive 
>than the Collins mechanical filter.
>
>The downsides; The input/output impedances and capacitances were 
>completely different than the mechanical filters. Insertion losses 
>were higher, all of this resulted in circuit changes to interconnect 
>to the Clevite filter very differently than the mechanical filter. 
>The shape factor was not as good as the Collins mechanical filter so 
>IMD was worse.
>
>Because of the differences in electrical specifications the Clevite 
>filter required a different impedance matching network. As a result, 
>you cannot mix and match Clevite's with mechanical filters in the same radio.
>
>The upside; The Clevite filter did not have as much phase noise as a 
>mechanical filter. This made it very useful for installations where 
>direction finding was important as you could get a much better 
>bearing on the transmitting site.
>
>What that means now: The Clevite filter was a very early crystal 
>filter. Over time, ceramic filters became much better with better 
>shape factors approaching those of mechanical filters. Insertion 
>losses decreased and the ceramic filter is now available in more 
>appropriate circuit impedances.
>
>Today you can find multi-pole ceramic filters with better shape 
>factors than the Collins mechanical filter. Their ultimate rejection 
>is not as good because of crosstalk between the input and output 
>connections. Notice how the R-390A mechanical filters have a 
>mounting flange between the top and the bottom. This was to improve 
>filter action by isolating the leads with a ground plane (the 
>chassis and the mounting flange).
>
>If the R-390A was a new design today (with tubes), it would use 9 to 
>11 pole ceramic filters with the electro/mechanical seperation 
>between the inputs and outputs for better isolation. For DF 
>applications, more advanced techniques like doppler shift , 
>interferometery and advanced phase analysis would make for a much 
>better DF platform.
>
>That is my opinion,
>
>Ms. Tisha Hayes
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