[Premium-Rx] Siemens CHR 531 and Phase Linear Filters

[George Georgevits]

Greetings to all,

Just to add my $0.02 worth, I always thought the phase linearity of a filter
was not dependent on how it was physically realised (LC, active, mechanical,
quartz crystal etc.) but rather on its design response type, ie. whether it
is a Butterworth, Bessel, Tschebyshev, Elliptical, to put a name to the most
common types of filter designs? Each filter design type can then be applied
to a low pass, high pass, band pass and band reject response, as the
application requires.

Each of these design responses has its own magnitude and phase response
characteristics. Bessel filters have the most linear phase response, but the
poorest magnitude cutoff characteristics. Consequently, they require many
more cascaded stages (or orders, as they are called) to achieve the same
cutoff slope as an equivalent elliptic function filter (which, by the way,
has the worst phase response of the types mentioned above).

Regards,
George Georgevits


  -----Original Message-----
  From: premium-rx-bounces at ml.skirrow.org
[mailto:premium-rx-bounces at ml.skirrow.org]On Behalf Of Michael O'Beirne
  Sent: Saturday, 15 January 2005 9:42 AM
  To: Premium-Radio
  Subject: [Premium-Rx] Siemens CHR 531


  Hi everyone,

  I don't know the Siemens CHR 531 but in general terms, 30kHz is a popular
final IF with many German receivers, typically my Siemens E311 from the
1960s (it cost DM15,000 new in those days!!!!) and the R&S test receiver ESH
3 which requires a second mortgage on the family home to purchase.

  At this low frequency it is possible to create a good IF response using
just LC pot cores.  A lot of old Drake ham receivers used 50kHz for the
final selectivity for this reason.

  These LC filters have the added advantage that they usually have very good
phase linearity and in consequence they do not ring, unlike many tight
mechanical and crystal filters.  Thus they could be used for DFing whereas
most other filters are useless in that application.

  I believe that Telefunken used to make (and perhaps still make) excellent
mechanical filters at 30kHz.

  As for the drop in response at MW, this is presumably because a filter has
been interposed in the RF stage to remove the effect of some crushing MW
signals.  A typical example is the Collins KWM380 transceiver. It has a
reasonable receiver but it loses sensitivity at MW.

  I'd stick with the RA6790.

  Regards and 73s
  Michael
  G8MOB
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